20 files changed, 18165 insertions, 0 deletions

diff --git a/hardware/src/libusb/config.h b/hardware/src/libusb/config.h
new file mode 100644
index 0000000..59f3463
--- /dev/null
+++ b/hardware/src/libusb/config.h
@@ -0,0 +1,37 @@
+/* config.h.  Manually generated for Xcode.  */
+
+#include <AvailabilityMacros.h>
+
+/* Define to the attribute for default visibility. */
+#define DEFAULT_VISIBILITY __attribute__ ((visibility ("default")))
+
+/* Define to 1 to enable message logging. */
+#define ENABLE_LOGGING 1
+
+/* On 10.12 and later, use newly available clock_*() functions */
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 101200
+/* Define to 1 if you have the `clock_gettime' function. */
+#define HAVE_CLOCK_GETTIME 1
+#endif
+
+/* On 10.6 and later, use newly available pthread_threadid_np() function */
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
+/* Define to 1 if you have the 'pthread_threadid_np' function. */
+#define HAVE_PTHREAD_THREADID_NP 1
+#endif
+
+/* Define to 1 if the system has the type `nfds_t'. */
+#define HAVE_NFDS_T 1
+
+/* Define to 1 if you have the <sys/time.h> header file. */
+#define HAVE_SYS_TIME_H 1
+
+/* Define to 1 if compiling for a POSIX platform. */
+#define PLATFORM_POSIX 1
+
+/* Define to the attribute for enabling parameter checks on printf-like
+   functions. */
+#define PRINTF_FORMAT(a, b) __attribute__ ((__format__ (__printf__, a, b)))
+
+/* Enable GNU extensions. */
+#define _GNU_SOURCE 1
diff --git a/hardware/src/libusb/core.c b/hardware/src/libusb/core.c
new file mode 100644
index 0000000..7893ac2
--- /dev/null
+++ b/hardware/src/libusb/core.c
@@ -0,0 +1,2737 @@
+/* -*- Mode: C; indent-tabs-mode:t ; c-basic-offset:8 -*- */
+/*
+ * Core functions for libusb
+ * Copyright © 2012-2013 Nathan Hjelm <hjelmn@cs.unm.edu>
+ * Copyright © 2007-2008 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+#include "version.h"
+
+#ifdef __ANDROID__
+#include <android/log.h>
+#endif
+#include <stdio.h>
+#include <string.h>
+#ifdef HAVE_SYSLOG
+#include <syslog.h>
+#endif
+
+static const struct libusb_version libusb_version_internal =
+	{ LIBUSB_MAJOR, LIBUSB_MINOR, LIBUSB_MICRO, LIBUSB_NANO,
+	  LIBUSB_RC, "http://libusb.info" };
+static struct timespec timestamp_origin;
+#if defined(ENABLE_LOGGING) && !defined(USE_SYSTEM_LOGGING_FACILITY)
+static libusb_log_cb log_handler;
+#endif
+
+struct libusb_context *usbi_default_context;
+static int default_context_refcnt;
+static usbi_mutex_static_t default_context_lock = USBI_MUTEX_INITIALIZER;
+static struct usbi_option default_context_options[LIBUSB_OPTION_MAX];
+
+
+usbi_mutex_static_t active_contexts_lock = USBI_MUTEX_INITIALIZER;
+struct list_head active_contexts_list;
+
+/**
+ * \mainpage libusb-1.0 API Reference
+ *
+ * \section intro Introduction
+ *
+ * libusb is an open source library that allows you to communicate with USB
+ * devices from user space. For more info, see the
+ * <a href="http://libusb.info">libusb homepage</a>.
+ *
+ * This documentation is aimed at application developers wishing to
+ * communicate with USB peripherals from their own software. After reviewing
+ * this documentation, feedback and questions can be sent to the
+ * <a href="http://mailing-list.libusb.info">libusb-devel mailing list</a>.
+ *
+ * This documentation assumes knowledge of how to operate USB devices from
+ * a software standpoint (descriptors, configurations, interfaces, endpoints,
+ * control/bulk/interrupt/isochronous transfers, etc). Full information
+ * can be found in the <a href="http://www.usb.org/developers/docs/">USB 3.0
+ * Specification</a> which is available for free download. You can probably
+ * find less verbose introductions by searching the web.
+ *
+ * \section API Application Programming Interface (API)
+ *
+ * See the \ref libusb_api page for a complete list of the libusb functions.
+ *
+ * \section features Library features
+ *
+ * - All transfer types supported (control/bulk/interrupt/isochronous)
+ * - 2 transfer interfaces:
+ *    -# Synchronous (simple)
+ *    -# Asynchronous (more complicated, but more powerful)
+ * - Thread safe (although the asynchronous interface means that you
+ *   usually won't need to thread)
+ * - Lightweight with lean API
+ * - Compatible with libusb-0.1 through the libusb-compat-0.1 translation layer
+ * - Hotplug support (on some platforms). See \ref libusb_hotplug.
+ *
+ * \section gettingstarted Getting Started
+ *
+ * To begin reading the API documentation, start with the Modules page which
+ * links to the different categories of libusb's functionality.
+ *
+ * One decision you will have to make is whether to use the synchronous
+ * or the asynchronous data transfer interface. The \ref libusb_io documentation
+ * provides some insight into this topic.
+ *
+ * Some example programs can be found in the libusb source distribution under
+ * the "examples" subdirectory. The libusb homepage includes a list of
+ * real-life project examples which use libusb.
+ *
+ * \section errorhandling Error handling
+ *
+ * libusb functions typically return 0 on success or a negative error code
+ * on failure. These negative error codes relate to LIBUSB_ERROR constants
+ * which are listed on the \ref libusb_misc "miscellaneous" documentation page.
+ *
+ * \section msglog Debug message logging
+ *
+ * libusb uses stderr for all logging. By default, logging is set to NONE,
+ * which means that no output will be produced. However, unless the library
+ * has been compiled with logging disabled, then any application calls to
+ * libusb_set_option(ctx, LIBUSB_OPTION_LOG_LEVEL, level), or the setting of the
+ * environmental variable LIBUSB_DEBUG outside of the application, can result
+ * in logging being produced. Your application should therefore not close
+ * stderr, but instead direct it to the null device if its output is
+ * undesirable.
+ *
+ * The libusb_set_option(ctx, LIBUSB_OPTION_LOG_LEVEL, level) function can be
+ * used to enable logging of certain messages. Under standard configuration,
+ * libusb doesn't really log much so you are advised to use this function
+ * to enable all error/warning/ informational messages. It will help debug
+ * problems with your software.
+ *
+ * The logged messages are unstructured. There is no one-to-one correspondence
+ * between messages being logged and success or failure return codes from
+ * libusb functions. There is no format to the messages, so you should not
+ * try to capture or parse them. They are not and will not be localized.
+ * These messages are not intended to being passed to your application user;
+ * instead, you should interpret the error codes returned from libusb functions
+ * and provide appropriate notification to the user. The messages are simply
+ * there to aid you as a programmer, and if you're confused because you're
+ * getting a strange error code from a libusb function, enabling message
+ * logging may give you a suitable explanation.
+ *
+ * The LIBUSB_DEBUG environment variable can be used to enable message logging
+ * at run-time. This environment variable should be set to a log level number,
+ * which is interpreted the same as the
+ * libusb_set_option(ctx, LIBUSB_OPTION_LOG_LEVEL, level) parameter. When this
+ * environment variable is set, the message logging verbosity level is fixed
+ * and libusb_set_option(ctx, LIBUSB_OPTION_LOG_LEVEL, level) effectively does
+ * nothing.
+ *
+ * libusb can be compiled without any logging functions, useful for embedded
+ * systems. In this case, libusb_set_option(ctx, LIBUSB_OPTION_LOG_LEVEL, level)
+ * and the LIBUSB_DEBUG environment variable have no effects.
+ *
+ * libusb can also be compiled with verbose debugging messages always. When
+ * the library is compiled in this way, all messages of all verbosities are
+ * always logged. libusb_set_option(ctx, LIBUSB_OPTION_LOG_LEVEL, level) and
+ * the LIBUSB_DEBUG environment variable have no effects.
+ *
+ * \section remarks Other remarks
+ *
+ * libusb does have imperfections. The \ref libusb_caveats "caveats" page attempts
+ * to document these.
+ */
+
+/**
+ * \page libusb_caveats Caveats
+ *
+ * \section threadsafety Thread safety
+ *
+ * libusb is designed to be completely thread-safe, but as with any API it
+ * cannot prevent a user from sabotaging themselves, either intentionally or
+ * otherwise.
+ *
+ * Observe the following general guidelines:
+ *
+ * - Calls to functions that release a resource (e.g. libusb_close(),
+ *   libusb_free_config_descriptor()) should not be called concurrently on
+ *   the same resource. This is no different than concurrently calling free()
+ *   on the same allocated pointer.
+ * - Each individual \ref libusb_transfer should be prepared by a single
+ *   thread. In other words, no two threads should ever be concurrently
+ *   filling out the fields of a \ref libusb_transfer. You can liken this to
+ *   calling sprintf() with the same destination buffer from multiple threads.
+ *   The results will likely not be what you want unless the input parameters
+ *   are all the same, but its best to avoid this situation entirely.
+ * - Both the \ref libusb_transfer structure and its associated data buffer
+ *   should not be accessed between the time the transfer is submitted and the
+ *   time the completion callback is invoked. You can think of "ownership" of
+ *   these things as being transferred to libusb while the transfer is active.
+ * - The various "setter" functions (e.g. libusb_set_log_cb(),
+ *   libusb_set_pollfd_notifiers()) should not be called concurrently on the
+ *   resource. Though doing so will not lead to any undefined behavior, it
+ *   will likely produce results that the application does not expect.
+ *
+ * Rules for multiple threads and asynchronous I/O are detailed
+ * \ref libusb_mtasync "here".
+ *
+ * \section fork Fork considerations
+ *
+ * libusb is <em>not</em> designed to work across fork() calls. Depending on
+ * the platform, there may be resources in the parent process that are not
+ * available to the child (e.g. the hotplug monitor thread on Linux). In
+ * addition, since the parent and child will share libusb's internal file
+ * descriptors, using libusb in any way from the child could cause the parent
+ * process's \ref libusb_context to get into an inconsistent state.
+ *
+ * On Linux, libusb's file descriptors will be marked as CLOEXEC, which means
+ * that it is safe to fork() and exec() without worrying about the child
+ * process needing to clean up state or having access to these file descriptors.
+ * Other platforms may not be so forgiving, so consider yourself warned!
+ *
+ * \section devresets Device resets
+ *
+ * The libusb_reset_device() function allows you to reset a device. If your
+ * program has to call such a function, it should obviously be aware that
+ * the reset will cause device state to change (e.g. register values may be
+ * reset).
+ *
+ * The problem is that any other program could reset the device your program
+ * is working with, at any time. libusb does not offer a mechanism to inform
+ * you when this has happened, so if someone else resets your device it will
+ * not be clear to your own program why the device state has changed.
+ *
+ * Ultimately, this is a limitation of writing drivers in user space.
+ * Separation from the USB stack in the underlying kernel makes it difficult
+ * for the operating system to deliver such notifications to your program.
+ * The Linux kernel USB stack allows such reset notifications to be delivered
+ * to in-kernel USB drivers, but it is not clear how such notifications could
+ * be delivered to second-class drivers that live in user space.
+ *
+ * \section blockonly Blocking-only functionality
+ *
+ * The functionality listed below is only available through synchronous,
+ * blocking functions. There are no asynchronous/non-blocking alternatives,
+ * and no clear ways of implementing these.
+ *
+ * - Configuration activation (libusb_set_configuration())
+ * - Interface/alternate setting activation (libusb_set_interface_alt_setting())
+ * - Releasing of interfaces (libusb_release_interface())
+ * - Clearing of halt/stall condition (libusb_clear_halt())
+ * - Device resets (libusb_reset_device())
+ *
+ * \section configsel Configuration selection and handling
+ *
+ * When libusb presents a device handle to an application, there is a chance
+ * that the corresponding device may be in unconfigured state. For devices
+ * with multiple configurations, there is also a chance that the configuration
+ * currently selected is not the one that the application wants to use.
+ *
+ * The obvious solution is to add a call to libusb_set_configuration() early
+ * on during your device initialization routines, but there are caveats to
+ * be aware of:
+ * -# If the device is already in the desired configuration, calling
+ *    libusb_set_configuration() using the same configuration value will cause
+ *    a lightweight device reset. This may not be desirable behaviour.
+ * -# In the case where the desired configuration is already active, libusb
+ *    may not even be able to perform a lightweight device reset. For example,
+ *    take my USB keyboard with fingerprint reader: I'm interested in driving
+ *    the fingerprint reader interface through libusb, but the kernel's
+ *    USB-HID driver will almost always have claimed the keyboard interface.
+ *    Because the kernel has claimed an interface, it is not even possible to
+ *    perform the lightweight device reset, so libusb_set_configuration() will
+ *    fail. (Luckily the device in question only has a single configuration.)
+ * -# libusb will be unable to set a configuration if other programs or
+ *    drivers have claimed interfaces. In particular, this means that kernel
+ *    drivers must be detached from all the interfaces before
+ *    libusb_set_configuration() may succeed.
+ *
+ * One solution to some of the above problems is to consider the currently
+ * active configuration. If the configuration we want is already active, then
+ * we don't have to select any configuration:
+\code
+cfg = -1;
+libusb_get_configuration(dev, &cfg);
+if (cfg != desired)
+	libusb_set_configuration(dev, desired);
+\endcode
+ *
+ * This is probably suitable for most scenarios, but is inherently racy:
+ * another application or driver may change the selected configuration
+ * <em>after</em> the libusb_get_configuration() call.
+ *
+ * Even in cases where libusb_set_configuration() succeeds, consider that other
+ * applications or drivers may change configuration after your application
+ * calls libusb_set_configuration().
+ *
+ * One possible way to lock your device into a specific configuration is as
+ * follows:
+ * -# Set the desired configuration (or use the logic above to realise that
+ *    it is already in the desired configuration)
+ * -# Claim the interface that you wish to use
+ * -# Check that the currently active configuration is the one that you want
+ *    to use.
+ *
+ * The above method works because once an interface is claimed, no application
+ * or driver is able to select another configuration.
+ *
+ * \section earlycomp Early transfer completion
+ *
+ * NOTE: This section is currently Linux-centric. I am not sure if any of these
+ * considerations apply to Darwin or other platforms.
+ *
+ * When a transfer completes early (i.e. when less data is received/sent in
+ * any one packet than the transfer buffer allows for) then libusb is designed
+ * to terminate the transfer immediately, not transferring or receiving any
+ * more data unless other transfers have been queued by the user.
+ *
+ * On legacy platforms, libusb is unable to do this in all situations. After
+ * the incomplete packet occurs, "surplus" data may be transferred. For recent
+ * versions of libusb, this information is kept (the data length of the
+ * transfer is updated) and, for device-to-host transfers, any surplus data was
+ * added to the buffer. Still, this is not a nice solution because it loses the
+ * information about the end of the short packet, and the user probably wanted
+ * that surplus data to arrive in the next logical transfer.
+ *
+ * \section zlp Zero length packets
+ *
+ * - libusb is able to send a packet of zero length to an endpoint simply by
+ * submitting a transfer of zero length.
+ * - The \ref libusb_transfer_flags::LIBUSB_TRANSFER_ADD_ZERO_PACKET
+ * "LIBUSB_TRANSFER_ADD_ZERO_PACKET" flag is currently supported on Linux,
+ * Darwin and Windows (WinUSB).
+ */
+
+/**
+ * \page libusb_contexts Contexts
+ *
+ * It is possible that libusb may be used simultaneously from two independent
+ * libraries linked into the same executable. For example, if your application
+ * has a plugin-like system which allows the user to dynamically load a range
+ * of modules into your program, it is feasible that two independently
+ * developed modules may both use libusb.
+ *
+ * libusb is written to allow for these multiple user scenarios. The two
+ * "instances" of libusb will not interfere: libusb_set_option() calls
+ * from one user will not affect the same settings for other users, other
+ * users can continue using libusb after one of them calls libusb_exit(), etc.
+ *
+ * This is made possible through libusb's <em>context</em> concept. When you
+ * call libusb_init(), you are (optionally) given a context. You can then pass
+ * this context pointer back into future libusb functions.
+ *
+ * In order to keep things simple for more simplistic applications, it is
+ * legal to pass NULL to all functions requiring a context pointer (as long as
+ * you're sure no other code will attempt to use libusb from the same process).
+ * When you pass NULL, the default context will be used. The default context
+ * is created the first time a process calls libusb_init() when no other
+ * context is alive. Contexts are destroyed during libusb_exit().
+ *
+ * The default context is reference-counted and can be shared. That means that
+ * if libusb_init(NULL) is called twice within the same process, the two
+ * users end up sharing the same context. The deinitialization and freeing of
+ * the default context will only happen when the last user calls libusb_exit().
+ * In other words, the default context is created and initialized when its
+ * reference count goes from 0 to 1, and is deinitialized and destroyed when
+ * its reference count goes from 1 to 0.
+ *
+ * You may be wondering why only a subset of libusb functions require a
+ * context pointer in their function definition. Internally, libusb stores
+ * context pointers in other objects (e.g. libusb_device instances) and hence
+ * can infer the context from those objects.
+ */
+
+ /**
+  * \page libusb_api Application Programming Interface
+  *
+  * This is the complete list of libusb functions, structures and
+  * enumerations in alphabetical order.
+  *
+  * \section Functions
+  * - libusb_alloc_streams()
+  * - libusb_alloc_transfer()
+  * - libusb_attach_kernel_driver()
+  * - libusb_bulk_transfer()
+  * - libusb_cancel_transfer()
+  * - libusb_claim_interface()
+  * - libusb_clear_halt()
+  * - libusb_close()
+  * - libusb_control_transfer()
+  * - libusb_control_transfer_get_data()
+  * - libusb_control_transfer_get_setup()
+  * - libusb_cpu_to_le16()
+  * - libusb_detach_kernel_driver()
+  * - libusb_dev_mem_alloc()
+  * - libusb_dev_mem_free()
+  * - libusb_error_name()
+  * - libusb_event_handler_active()
+  * - libusb_event_handling_ok()
+  * - libusb_exit()
+  * - libusb_fill_bulk_stream_transfer()
+  * - libusb_fill_bulk_transfer()
+  * - libusb_fill_control_setup()
+  * - libusb_fill_control_transfer()
+  * - libusb_fill_interrupt_transfer()
+  * - libusb_fill_iso_transfer()
+  * - libusb_free_bos_descriptor()
+  * - libusb_free_config_descriptor()
+  * - libusb_free_container_id_descriptor()
+  * - libusb_free_device_list()
+  * - libusb_free_pollfds()
+  * - libusb_free_ss_endpoint_companion_descriptor()
+  * - libusb_free_ss_usb_device_capability_descriptor()
+  * - libusb_free_streams()
+  * - libusb_free_transfer()
+  * - libusb_free_usb_2_0_extension_descriptor()
+  * - libusb_get_active_config_descriptor()
+  * - libusb_get_bos_descriptor()
+  * - libusb_get_bus_number()
+  * - libusb_get_config_descriptor()
+  * - libusb_get_config_descriptor_by_value()
+  * - libusb_get_configuration()
+  * - libusb_get_container_id_descriptor()
+  * - libusb_get_descriptor()
+  * - libusb_get_device()
+  * - libusb_get_device_address()
+  * - libusb_get_device_descriptor()
+  * - libusb_get_device_list()
+  * - libusb_get_device_speed()
+  * - libusb_get_iso_packet_buffer()
+  * - libusb_get_iso_packet_buffer_simple()
+  * - libusb_get_max_iso_packet_size()
+  * - libusb_get_max_packet_size()
+  * - libusb_get_next_timeout()
+  * - libusb_get_parent()
+  * - libusb_get_pollfds()
+  * - libusb_get_port_number()
+  * - libusb_get_port_numbers()
+  * - libusb_get_port_path()
+  * - libusb_get_ss_endpoint_companion_descriptor()
+  * - libusb_get_ss_usb_device_capability_descriptor()
+  * - libusb_get_string_descriptor()
+  * - libusb_get_string_descriptor_ascii()
+  * - libusb_get_usb_2_0_extension_descriptor()
+  * - libusb_get_version()
+  * - libusb_handle_events()
+  * - libusb_handle_events_completed()
+  * - libusb_handle_events_locked()
+  * - libusb_handle_events_timeout()
+  * - libusb_handle_events_timeout_completed()
+  * - libusb_has_capability()
+  * - libusb_hotplug_deregister_callback()
+  * - libusb_hotplug_register_callback()
+  * - libusb_init()
+  * - libusb_interrupt_event_handler()
+  * - libusb_interrupt_transfer()
+  * - libusb_kernel_driver_active()
+  * - libusb_lock_events()
+  * - libusb_lock_event_waiters()
+  * - libusb_open()
+  * - libusb_open_device_with_vid_pid()
+  * - libusb_pollfds_handle_timeouts()
+  * - libusb_ref_device()
+  * - libusb_release_interface()
+  * - libusb_reset_device()
+  * - libusb_set_auto_detach_kernel_driver()
+  * - libusb_set_configuration()
+  * - libusb_set_debug()
+  * - libusb_set_log_cb()
+  * - libusb_set_interface_alt_setting()
+  * - libusb_set_iso_packet_lengths()
+  * - libusb_set_option()
+  * - libusb_setlocale()
+  * - libusb_set_pollfd_notifiers()
+  * - libusb_strerror()
+  * - libusb_submit_transfer()
+  * - libusb_transfer_get_stream_id()
+  * - libusb_transfer_set_stream_id()
+  * - libusb_try_lock_events()
+  * - libusb_unlock_events()
+  * - libusb_unlock_event_waiters()
+  * - libusb_unref_device()
+  * - libusb_wait_for_event()
+  * - libusb_wrap_sys_device()
+  *
+  * \section Structures
+  * - libusb_bos_descriptor
+  * - libusb_bos_dev_capability_descriptor
+  * - libusb_config_descriptor
+  * - libusb_container_id_descriptor
+  * - \ref libusb_context
+  * - libusb_control_setup
+  * - \ref libusb_device
+  * - libusb_device_descriptor
+  * - \ref libusb_device_handle
+  * - libusb_endpoint_descriptor
+  * - libusb_interface
+  * - libusb_interface_descriptor
+  * - libusb_iso_packet_descriptor
+  * - libusb_pollfd
+  * - libusb_ss_endpoint_companion_descriptor
+  * - libusb_ss_usb_device_capability_descriptor
+  * - libusb_transfer
+  * - libusb_usb_2_0_extension_descriptor
+  * - libusb_version
+  *
+  * \section Enums
+  * - \ref libusb_bos_type
+  * - \ref libusb_capability
+  * - \ref libusb_class_code
+  * - \ref libusb_descriptor_type
+  * - \ref libusb_endpoint_direction
+  * - \ref libusb_endpoint_transfer_type
+  * - \ref libusb_error
+  * - \ref libusb_iso_sync_type
+  * - \ref libusb_iso_usage_type
+  * - \ref libusb_log_level
+  * - \ref libusb_option
+  * - \ref libusb_request_recipient
+  * - \ref libusb_request_type
+  * - \ref libusb_speed
+  * - \ref libusb_ss_usb_device_capability_attributes
+  * - \ref libusb_standard_request
+  * - \ref libusb_supported_speed
+  * - \ref libusb_transfer_flags
+  * - \ref libusb_transfer_status
+  * - \ref libusb_transfer_type
+  * - \ref libusb_usb_2_0_extension_attributes
+  */
+
+/**
+ * @defgroup libusb_lib Library initialization/deinitialization
+ * This page details how to initialize and deinitialize libusb. Initialization
+ * must be performed before using any libusb functionality, and similarly you
+ * must not call any libusb functions after deinitialization.
+ */
+
+/**
+ * @defgroup libusb_dev Device handling and enumeration
+ * The functionality documented below is designed to help with the following
+ * operations:
+ * - Enumerating the USB devices currently attached to the system
+ * - Choosing a device to operate from your software
+ * - Opening and closing the chosen device
+ *
+ * \section nutshell In a nutshell...
+ *
+ * The description below really makes things sound more complicated than they
+ * actually are. The following sequence of function calls will be suitable
+ * for almost all scenarios and does not require you to have such a deep
+ * understanding of the resource management issues:
+ * \code
+// discover devices
+libusb_device **list;
+libusb_device *found = NULL;
+ssize_t cnt = libusb_get_device_list(NULL, &list);
+ssize_t i = 0;
+int err = 0;
+if (cnt < 0)
+	error();
+
+for (i = 0; i < cnt; i++) {
+	libusb_device *device = list[i];
+	if (is_interesting(device)) {
+		found = device;
+		break;
+	}
+}
+
+if (found) {
+	libusb_device_handle *handle;
+
+	err = libusb_open(found, &handle);
+	if (err)
+		error();
+	// etc
+}
+
+libusb_free_device_list(list, 1);
+\endcode
+ *
+ * The two important points:
+ * - You asked libusb_free_device_list() to unreference the devices (2nd
+ *   parameter)
+ * - You opened the device before freeing the list and unreferencing the
+ *   devices
+ *
+ * If you ended up with a handle, you can now proceed to perform I/O on the
+ * device.
+ *
+ * \section devshandles Devices and device handles
+ * libusb has a concept of a USB device, represented by the
+ * \ref libusb_device opaque type. A device represents a USB device that
+ * is currently or was previously connected to the system. Using a reference
+ * to a device, you can determine certain information about the device (e.g.
+ * you can read the descriptor data).
+ *
+ * The libusb_get_device_list() function can be used to obtain a list of
+ * devices currently connected to the system. This is known as device
+ * discovery. Devices can also be discovered with the hotplug mechanism,
+ * whereby a callback function registered with libusb_hotplug_register_callback()
+ * will be called when a device of interest is connected or disconnected.
+ *
+ * Just because you have a reference to a device does not mean it is
+ * necessarily usable. The device may have been unplugged, you may not have
+ * permission to operate such device, or another program or driver may be
+ * using the device.
+ *
+ * When you've found a device that you'd like to operate, you must ask
+ * libusb to open the device using the libusb_open() function. Assuming
+ * success, libusb then returns you a <em>device handle</em>
+ * (a \ref libusb_device_handle pointer). All "real" I/O operations then
+ * operate on the handle rather than the original device pointer.
+ *
+ * \section devref Device discovery and reference counting
+ *
+ * Device discovery (i.e. calling libusb_get_device_list()) returns a
+ * freshly-allocated list of devices. The list itself must be freed when
+ * you are done with it. libusb also needs to know when it is OK to free
+ * the contents of the list - the devices themselves.
+ *
+ * To handle these issues, libusb provides you with two separate items:
+ * - A function to free the list itself
+ * - A reference counting system for the devices inside
+ *
+ * New devices presented by the libusb_get_device_list() function all have a
+ * reference count of 1. You can increase and decrease reference count using
+ * libusb_ref_device() and libusb_unref_device(). A device is destroyed when
+ * its reference count reaches 0.
+ *
+ * With the above information in mind, the process of opening a device can
+ * be viewed as follows:
+ * -# Discover devices using libusb_get_device_list() or libusb_hotplug_register_callback().
+ * -# Choose the device that you want to operate, and call libusb_open().
+ * -# Unref all devices in the discovered device list.
+ * -# Free the discovered device list.
+ *
+ * The order is important - you must not unreference the device before
+ * attempting to open it, because unreferencing it may destroy the device.
+ *
+ * For convenience, the libusb_free_device_list() function includes a
+ * parameter to optionally unreference all the devices in the list before
+ * freeing the list itself. This combines steps 3 and 4 above.
+ *
+ * As an implementation detail, libusb_open() actually adds a reference to
+ * the device in question. This is because the device remains available
+ * through the handle via libusb_get_device(). The reference is deleted during
+ * libusb_close().
+ */
+
+/** @defgroup libusb_misc Miscellaneous */
+
+/* we traverse usbfs without knowing how many devices we are going to find.
+ * so we create this discovered_devs model which is similar to a linked-list
+ * which grows when required. it can be freed once discovery has completed,
+ * eliminating the need for a list node in the libusb_device structure
+ * itself. */
+#define DISCOVERED_DEVICES_SIZE_STEP 16
+
+static struct discovered_devs *discovered_devs_alloc(void)
+{
+	struct discovered_devs *ret =
+		malloc(sizeof(*ret) + (sizeof(void *) * DISCOVERED_DEVICES_SIZE_STEP));
+
+	if (ret) {
+		ret->len = 0;
+		ret->capacity = DISCOVERED_DEVICES_SIZE_STEP;
+	}
+	return ret;
+}
+
+static void discovered_devs_free(struct discovered_devs *discdevs)
+{
+	size_t i;
+
+	for (i = 0; i < discdevs->len; i++)
+		libusb_unref_device(discdevs->devices[i]);
+
+	free(discdevs);
+}
+
+/* append a device to the discovered devices collection. may realloc itself,
+ * returning new discdevs. returns NULL on realloc failure. */
+struct discovered_devs *discovered_devs_append(
+	struct discovered_devs *discdevs, struct libusb_device *dev)
+{
+	size_t len = discdevs->len;
+	size_t capacity;
+	struct discovered_devs *new_discdevs;
+
+	/* if there is space, just append the device */
+	if (len < discdevs->capacity) {
+		discdevs->devices[len] = libusb_ref_device(dev);
+		discdevs->len++;
+		return discdevs;
+	}
+
+	/* exceeded capacity, need to grow */
+	usbi_dbg(DEVICE_CTX(dev), "need to increase capacity");
+	capacity = discdevs->capacity + DISCOVERED_DEVICES_SIZE_STEP;
+	/* can't use usbi_reallocf here because in failure cases it would
+	 * free the existing discdevs without unreferencing its devices. */
+	new_discdevs = realloc(discdevs,
+		sizeof(*discdevs) + (sizeof(void *) * capacity));
+	if (!new_discdevs) {
+		discovered_devs_free(discdevs);
+		return NULL;
+	}
+
+	discdevs = new_discdevs;
+	discdevs->capacity = capacity;
+	discdevs->devices[len] = libusb_ref_device(dev);
+	discdevs->len++;
+
+	return discdevs;
+}
+
+/* Allocate a new device with a specific session ID. The returned device has
+ * a reference count of 1. */
+struct libusb_device *usbi_alloc_device(struct libusb_context *ctx,
+	unsigned long session_id)
+{
+	size_t priv_size = usbi_backend.device_priv_size;
+	struct libusb_device *dev = calloc(1, PTR_ALIGN(sizeof(*dev)) + priv_size);
+
+	if (!dev)
+		return NULL;
+
+	usbi_atomic_store(&dev->refcnt, 1);
+
+	dev->ctx = ctx;
+	dev->session_data = session_id;
+	dev->speed = LIBUSB_SPEED_UNKNOWN;
+
+	if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG))
+		usbi_connect_device(dev);
+
+	return dev;
+}
+
+void usbi_connect_device(struct libusb_device *dev)
+{
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+
+	usbi_atomic_store(&dev->attached, 1);
+
+	usbi_mutex_lock(&dev->ctx->usb_devs_lock);
+	list_add(&dev->list, &dev->ctx->usb_devs);
+	usbi_mutex_unlock(&dev->ctx->usb_devs_lock);
+
+	usbi_hotplug_notification(ctx, dev, LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED);
+}
+
+void usbi_disconnect_device(struct libusb_device *dev)
+{
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+
+	usbi_atomic_store(&dev->attached, 0);
+
+	usbi_mutex_lock(&ctx->usb_devs_lock);
+	list_del(&dev->list);
+	usbi_mutex_unlock(&ctx->usb_devs_lock);
+
+	usbi_hotplug_notification(ctx, dev, LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT);
+}
+
+/* Perform some final sanity checks on a newly discovered device. If this
+ * function fails (negative return code), the device should not be added
+ * to the discovered device list. */
+int usbi_sanitize_device(struct libusb_device *dev)
+{
+	uint8_t num_configurations;
+
+	if (dev->device_descriptor.bLength != LIBUSB_DT_DEVICE_SIZE ||
+	    dev->device_descriptor.bDescriptorType != LIBUSB_DT_DEVICE) {
+		usbi_err(DEVICE_CTX(dev), "invalid device descriptor");
+		return LIBUSB_ERROR_IO;
+	}
+
+	num_configurations = dev->device_descriptor.bNumConfigurations;
+	if (num_configurations > USB_MAXCONFIG) {
+		usbi_err(DEVICE_CTX(dev), "too many configurations");
+		return LIBUSB_ERROR_IO;
+	} else if (0 == num_configurations) {
+		usbi_dbg(DEVICE_CTX(dev), "zero configurations, maybe an unauthorized device");
+	}
+
+	return 0;
+}
+
+/* Examine libusb's internal list of known devices, looking for one with
+ * a specific session ID. Returns the matching device if it was found, and
+ * NULL otherwise. */
+struct libusb_device *usbi_get_device_by_session_id(struct libusb_context *ctx,
+	unsigned long session_id)
+{
+	struct libusb_device *dev;
+	struct libusb_device *ret = NULL;
+
+	usbi_mutex_lock(&ctx->usb_devs_lock);
+	for_each_device(ctx, dev) {
+		if (dev->session_data == session_id) {
+			ret = libusb_ref_device(dev);
+			break;
+		}
+	}
+	usbi_mutex_unlock(&ctx->usb_devs_lock);
+
+	return ret;
+}
+
+/** @ingroup libusb_dev
+ * Returns a list of USB devices currently attached to the system. This is
+ * your entry point into finding a USB device to operate.
+ *
+ * You are expected to unreference all the devices when you are done with
+ * them, and then free the list with libusb_free_device_list(). Note that
+ * libusb_free_device_list() can unref all the devices for you. Be careful
+ * not to unreference a device you are about to open until after you have
+ * opened it.
+ *
+ * This return value of this function indicates the number of devices in
+ * the resultant list. The list is actually one element larger, as it is
+ * NULL-terminated.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param list output location for a list of devices. Must be later freed with
+ * libusb_free_device_list().
+ * \returns the number of devices in the outputted list, or any
+ * \ref libusb_error according to errors encountered by the backend.
+ */
+ssize_t API_EXPORTED libusb_get_device_list(libusb_context *ctx,
+	libusb_device ***list)
+{
+	struct discovered_devs *discdevs = discovered_devs_alloc();
+	struct libusb_device **ret;
+	int r = 0;
+	ssize_t i, len;
+
+	usbi_dbg(ctx, " ");
+
+	if (!discdevs)
+		return LIBUSB_ERROR_NO_MEM;
+
+	ctx = usbi_get_context(ctx);
+
+	if (libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG)) {
+		/* backend provides hotplug support */
+		struct libusb_device *dev;
+
+		if (usbi_backend.hotplug_poll)
+			usbi_backend.hotplug_poll();
+
+		usbi_mutex_lock(&ctx->usb_devs_lock);
+		for_each_device(ctx, dev) {
+			discdevs = discovered_devs_append(discdevs, dev);
+
+			if (!discdevs) {
+				r = LIBUSB_ERROR_NO_MEM;
+				break;
+			}
+		}
+		usbi_mutex_unlock(&ctx->usb_devs_lock);
+	} else {
+		/* backend does not provide hotplug support */
+		r = usbi_backend.get_device_list(ctx, &discdevs);
+	}
+
+	if (r < 0) {
+		len = r;
+		goto out;
+	}
+
+	/* convert discovered_devs into a list */
+	len = (ssize_t)discdevs->len;
+	ret = calloc((size_t)len + 1, sizeof(struct libusb_device *));
+	if (!ret) {
+		len = LIBUSB_ERROR_NO_MEM;
+		goto out;
+	}
+
+	ret[len] = NULL;
+	for (i = 0; i < len; i++) {
+		struct libusb_device *dev = discdevs->devices[i];
+		ret[i] = libusb_ref_device(dev);
+	}
+	*list = ret;
+
+out:
+	if (discdevs)
+		discovered_devs_free(discdevs);
+	return len;
+}
+
+/** \ingroup libusb_dev
+ * Frees a list of devices previously discovered using
+ * libusb_get_device_list(). If the unref_devices parameter is set, the
+ * reference count of each device in the list is decremented by 1.
+ * \param list the list to free
+ * \param unref_devices whether to unref the devices in the list
+ */
+void API_EXPORTED libusb_free_device_list(libusb_device **list,
+	int unref_devices)
+{
+	if (!list)
+		return;
+
+	if (unref_devices) {
+		int i = 0;
+		struct libusb_device *dev;
+
+		while ((dev = list[i++]) != NULL)
+			libusb_unref_device(dev);
+	}
+	free(list);
+}
+
+/** \ingroup libusb_dev
+ * Get the number of the bus that a device is connected to.
+ * \param dev a device
+ * \returns the bus number
+ */
+uint8_t API_EXPORTED libusb_get_bus_number(libusb_device *dev)
+{
+	return dev->bus_number;
+}
+
+/** \ingroup libusb_dev
+ * Get the number of the port that a device is connected to.
+ * Unless the OS does something funky, or you are hot-plugging USB extension cards,
+ * the port number returned by this call is usually guaranteed to be uniquely tied
+ * to a physical port, meaning that different devices plugged on the same physical
+ * port should return the same port number.
+ *
+ * But outside of this, there is no guarantee that the port number returned by this
+ * call will remain the same, or even match the order in which ports have been
+ * numbered by the HUB/HCD manufacturer.
+ *
+ * \param dev a device
+ * \returns the port number (0 if not available)
+ */
+uint8_t API_EXPORTED libusb_get_port_number(libusb_device *dev)
+{
+	return dev->port_number;
+}
+
+/** \ingroup libusb_dev
+ * Get the list of all port numbers from root for the specified device
+ *
+ * Since version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102
+ * \param dev a device
+ * \param port_numbers the array that should contain the port numbers
+ * \param port_numbers_len the maximum length of the array. As per the USB 3.0
+ * specs, the current maximum limit for the depth is 7.
+ * \returns the number of elements filled
+ * \returns LIBUSB_ERROR_OVERFLOW if the array is too small
+ */
+int API_EXPORTED libusb_get_port_numbers(libusb_device *dev,
+	uint8_t *port_numbers, int port_numbers_len)
+{
+	int i = port_numbers_len;
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+
+	if (port_numbers_len <= 0)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	// HCDs can be listed as devices with port #0
+	while((dev) && (dev->port_number != 0)) {
+		if (--i < 0) {
+			usbi_warn(ctx, "port numbers array is too small");
+			return LIBUSB_ERROR_OVERFLOW;
+		}
+		port_numbers[i] = dev->port_number;
+		dev = dev->parent_dev;
+	}
+	if (i < port_numbers_len)
+		memmove(port_numbers, &port_numbers[i], port_numbers_len - i);
+	return port_numbers_len - i;
+}
+
+/** \ingroup libusb_dev
+ * \deprecated Please use \ref libusb_get_port_numbers() instead.
+ */
+int API_EXPORTED libusb_get_port_path(libusb_context *ctx, libusb_device *dev,
+	uint8_t *port_numbers, uint8_t port_numbers_len)
+{
+	UNUSED(ctx);
+
+	return libusb_get_port_numbers(dev, port_numbers, port_numbers_len);
+}
+
+/** \ingroup libusb_dev
+ * Get the the parent from the specified device.
+ * \param dev a device
+ * \returns the device parent or NULL if not available
+ * You should issue a \ref libusb_get_device_list() before calling this
+ * function and make sure that you only access the parent before issuing
+ * \ref libusb_free_device_list(). The reason is that libusb currently does
+ * not maintain a permanent list of device instances, and therefore can
+ * only guarantee that parents are fully instantiated within a
+ * libusb_get_device_list() - libusb_free_device_list() block.
+ */
+DEFAULT_VISIBILITY
+libusb_device * LIBUSB_CALL libusb_get_parent(libusb_device *dev)
+{
+	return dev->parent_dev;
+}
+
+/** \ingroup libusb_dev
+ * Get the address of the device on the bus it is connected to.
+ * \param dev a device
+ * \returns the device address
+ */
+uint8_t API_EXPORTED libusb_get_device_address(libusb_device *dev)
+{
+	return dev->device_address;
+}
+
+/** \ingroup libusb_dev
+ * Get the negotiated connection speed for a device.
+ * \param dev a device
+ * \returns a \ref libusb_speed code, where LIBUSB_SPEED_UNKNOWN means that
+ * the OS doesn't know or doesn't support returning the negotiated speed.
+ */
+int API_EXPORTED libusb_get_device_speed(libusb_device *dev)
+{
+	return dev->speed;
+}
+
+static const struct libusb_endpoint_descriptor *find_endpoint(
+	struct libusb_config_descriptor *config, unsigned char endpoint)
+{
+	int iface_idx;
+	for (iface_idx = 0; iface_idx < config->bNumInterfaces; iface_idx++) {
+		const struct libusb_interface *iface = &config->interface[iface_idx];
+		int altsetting_idx;
+
+		for (altsetting_idx = 0; altsetting_idx < iface->num_altsetting;
+				altsetting_idx++) {
+			const struct libusb_interface_descriptor *altsetting
+				= &iface->altsetting[altsetting_idx];
+			int ep_idx;
+
+			for (ep_idx = 0; ep_idx < altsetting->bNumEndpoints; ep_idx++) {
+				const struct libusb_endpoint_descriptor *ep =
+					&altsetting->endpoint[ep_idx];
+				if (ep->bEndpointAddress == endpoint)
+					return ep;
+			}
+		}
+	}
+	return NULL;
+}
+
+/** \ingroup libusb_dev
+ * Convenience function to retrieve the wMaxPacketSize value for a particular
+ * endpoint in the active device configuration.
+ *
+ * This function was originally intended to be of assistance when setting up
+ * isochronous transfers, but a design mistake resulted in this function
+ * instead. It simply returns the wMaxPacketSize value without considering
+ * its contents. If you're dealing with isochronous transfers, you probably
+ * want libusb_get_max_iso_packet_size() instead.
+ *
+ * \param dev a device
+ * \param endpoint address of the endpoint in question
+ * \returns the wMaxPacketSize value
+ * \returns LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
+ * \returns LIBUSB_ERROR_OTHER on other failure
+ */
+int API_EXPORTED libusb_get_max_packet_size(libusb_device *dev,
+	unsigned char endpoint)
+{
+	struct libusb_config_descriptor *config;
+	const struct libusb_endpoint_descriptor *ep;
+	int r;
+
+	r = libusb_get_active_config_descriptor(dev, &config);
+	if (r < 0) {
+		usbi_err(DEVICE_CTX(dev),
+			"could not retrieve active config descriptor");
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	ep = find_endpoint(config, endpoint);
+	if (!ep) {
+		r = LIBUSB_ERROR_NOT_FOUND;
+		goto out;
+	}
+
+	r = ep->wMaxPacketSize;
+
+out:
+	libusb_free_config_descriptor(config);
+	return r;
+}
+
+/** \ingroup libusb_dev
+ * Calculate the maximum packet size which a specific endpoint is capable is
+ * sending or receiving in the duration of 1 microframe
+ *
+ * Only the active configuration is examined. The calculation is based on the
+ * wMaxPacketSize field in the endpoint descriptor as described in section
+ * 9.6.6 in the USB 2.0 specifications.
+ *
+ * If acting on an isochronous or interrupt endpoint, this function will
+ * multiply the value found in bits 0:10 by the number of transactions per
+ * microframe (determined by bits 11:12). Otherwise, this function just
+ * returns the numeric value found in bits 0:10. For USB 3.0 device, it
+ * will attempts to retrieve the Endpoint Companion Descriptor to return
+ * wBytesPerInterval.
+ *
+ * This function is useful for setting up isochronous transfers, for example
+ * you might pass the return value from this function to
+ * libusb_set_iso_packet_lengths() in order to set the length field of every
+ * isochronous packet in a transfer.
+ *
+ * Since v1.0.3.
+ *
+ * \param dev a device
+ * \param endpoint address of the endpoint in question
+ * \returns the maximum packet size which can be sent/received on this endpoint
+ * \returns LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
+ * \returns LIBUSB_ERROR_OTHER on other failure
+ */
+int API_EXPORTED libusb_get_max_iso_packet_size(libusb_device *dev,
+	unsigned char endpoint)
+{
+	struct libusb_config_descriptor *config;
+	const struct libusb_endpoint_descriptor *ep;
+	struct libusb_ss_endpoint_companion_descriptor *ss_ep_cmp;
+	enum libusb_endpoint_transfer_type ep_type;
+	uint16_t val;
+	int r;
+	int speed;
+
+	r = libusb_get_active_config_descriptor(dev, &config);
+	if (r < 0) {
+		usbi_err(DEVICE_CTX(dev),
+			"could not retrieve active config descriptor");
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	ep = find_endpoint(config, endpoint);
+	if (!ep) {
+		r = LIBUSB_ERROR_NOT_FOUND;
+		goto out;
+	}
+
+	speed = libusb_get_device_speed(dev);
+	if (speed >= LIBUSB_SPEED_SUPER) {
+		r = libusb_get_ss_endpoint_companion_descriptor(dev->ctx, ep, &ss_ep_cmp);
+		if (r == LIBUSB_SUCCESS) {
+			r = ss_ep_cmp->wBytesPerInterval;
+			libusb_free_ss_endpoint_companion_descriptor(ss_ep_cmp);
+		}
+	}
+
+	/* If the device isn't a SuperSpeed device or retrieving the SS endpoint didn't worked. */
+	if (speed < LIBUSB_SPEED_SUPER || r < 0) {
+		val = ep->wMaxPacketSize;
+		ep_type = (enum libusb_endpoint_transfer_type) (ep->bmAttributes & 0x3);
+
+		r = val & 0x07ff;
+		if (ep_type == LIBUSB_ENDPOINT_TRANSFER_TYPE_ISOCHRONOUS
+		    || ep_type == LIBUSB_ENDPOINT_TRANSFER_TYPE_INTERRUPT)
+			r *= (1 + ((val >> 11) & 3));
+	}
+
+out:
+	libusb_free_config_descriptor(config);
+	return r;
+}
+
+/** \ingroup libusb_dev
+ * Increment the reference count of a device.
+ * \param dev the device to reference
+ * \returns the same device
+ */
+DEFAULT_VISIBILITY
+libusb_device * LIBUSB_CALL libusb_ref_device(libusb_device *dev)
+{
+	long refcnt;
+
+	refcnt = usbi_atomic_inc(&dev->refcnt);
+	assert(refcnt >= 2);
+
+	return dev;
+}
+
+/** \ingroup libusb_dev
+ * Decrement the reference count of a device. If the decrement operation
+ * causes the reference count to reach zero, the device shall be destroyed.
+ * \param dev the device to unreference
+ */
+void API_EXPORTED libusb_unref_device(libusb_device *dev)
+{
+	long refcnt;
+
+	if (!dev)
+		return;
+
+	refcnt = usbi_atomic_dec(&dev->refcnt);
+	assert(refcnt >= 0);
+
+	if (refcnt == 0) {
+		usbi_dbg(DEVICE_CTX(dev), "destroy device %d.%d", dev->bus_number, dev->device_address);
+
+		libusb_unref_device(dev->parent_dev);
+
+		if (usbi_backend.destroy_device)
+			usbi_backend.destroy_device(dev);
+
+		if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG)) {
+			/* backend does not support hotplug */
+			usbi_disconnect_device(dev);
+		}
+
+		free(dev);
+	}
+}
+
+/** \ingroup libusb_dev
+ * Wrap a platform-specific system device handle and obtain a libusb device
+ * handle for the underlying device. The handle allows you to use libusb to
+ * perform I/O on the device in question.
+ *
+ * Call libusb_set_option(NULL, LIBUSB_OPTION_NO_DEVICE_DISCOVERY) before
+ * libusb_init() if you want to skip enumeration of USB devices. In particular,
+ * this might be needed on Android if you don't have authority to access USB
+ * devices in general.
+ *
+ * On Linux, the system device handle must be a valid file descriptor opened
+ * on the device node.
+ *
+ * The system device handle must remain open until libusb_close() is called.
+ * The system device handle will not be closed by libusb_close().
+ *
+ * Internally, this function creates a temporary device and makes it
+ * available to you through libusb_get_device(). This device is destroyed
+ * during libusb_close(). The device shall not be opened through libusb_open().
+ *
+ * This is a non-blocking function; no requests are sent over the bus.
+ *
+ * Since version 1.0.23, \ref LIBUSB_API_VERSION >= 0x01000107
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param sys_dev the platform-specific system device handle
+ * \param dev_handle output location for the returned device handle pointer. Only
+ * populated when the return code is 0.
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NO_MEM on memory allocation failure
+ * \returns LIBUSB_ERROR_ACCESS if the user has insufficient permissions
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED if the operation is not supported on this
+ * platform
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_wrap_sys_device(libusb_context *ctx, intptr_t sys_dev,
+	libusb_device_handle **dev_handle)
+{
+	struct libusb_device_handle *_dev_handle;
+	size_t priv_size = usbi_backend.device_handle_priv_size;
+	int r;
+
+	usbi_dbg(ctx, "wrap_sys_device 0x%" PRIxPTR, (uintptr_t)sys_dev);
+
+	ctx = usbi_get_context(ctx);
+
+	if (!usbi_backend.wrap_sys_device)
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+
+	_dev_handle = calloc(1, PTR_ALIGN(sizeof(*_dev_handle)) + priv_size);
+	if (!_dev_handle)
+		return LIBUSB_ERROR_NO_MEM;
+
+	usbi_mutex_init(&_dev_handle->lock);
+
+	r = usbi_backend.wrap_sys_device(ctx, _dev_handle, sys_dev);
+	if (r < 0) {
+		usbi_dbg(ctx, "wrap_sys_device 0x%" PRIxPTR " returns %d", (uintptr_t)sys_dev, r);
+		usbi_mutex_destroy(&_dev_handle->lock);
+		free(_dev_handle);
+		return r;
+	}
+
+	usbi_mutex_lock(&ctx->open_devs_lock);
+	list_add(&_dev_handle->list, &ctx->open_devs);
+	usbi_mutex_unlock(&ctx->open_devs_lock);
+	*dev_handle = _dev_handle;
+
+	return 0;
+}
+
+/** \ingroup libusb_dev
+ * Open a device and obtain a device handle. A handle allows you to perform
+ * I/O on the device in question.
+ *
+ * Internally, this function adds a reference to the device and makes it
+ * available to you through libusb_get_device(). This reference is removed
+ * during libusb_close().
+ *
+ * This is a non-blocking function; no requests are sent over the bus.
+ *
+ * \param dev the device to open
+ * \param dev_handle output location for the returned device handle pointer. Only
+ * populated when the return code is 0.
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NO_MEM on memory allocation failure
+ * \returns LIBUSB_ERROR_ACCESS if the user has insufficient permissions
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_open(libusb_device *dev,
+	libusb_device_handle **dev_handle)
+{
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+	struct libusb_device_handle *_dev_handle;
+	size_t priv_size = usbi_backend.device_handle_priv_size;
+	int r;
+
+	usbi_dbg(DEVICE_CTX(dev), "open %d.%d", dev->bus_number, dev->device_address);
+
+	if (!usbi_atomic_load(&dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	_dev_handle = calloc(1, PTR_ALIGN(sizeof(*_dev_handle)) + priv_size);
+	if (!_dev_handle)
+		return LIBUSB_ERROR_NO_MEM;
+
+	usbi_mutex_init(&_dev_handle->lock);
+
+	_dev_handle->dev = libusb_ref_device(dev);
+
+	r = usbi_backend.open(_dev_handle);
+	if (r < 0) {
+		usbi_dbg(DEVICE_CTX(dev), "open %d.%d returns %d", dev->bus_number, dev->device_address, r);
+		libusb_unref_device(dev);
+		usbi_mutex_destroy(&_dev_handle->lock);
+		free(_dev_handle);
+		return r;
+	}
+
+	usbi_mutex_lock(&ctx->open_devs_lock);
+	list_add(&_dev_handle->list, &ctx->open_devs);
+	usbi_mutex_unlock(&ctx->open_devs_lock);
+	*dev_handle = _dev_handle;
+
+	return 0;
+}
+
+/** \ingroup libusb_dev
+ * Convenience function for finding a device with a particular
+ * <tt>idVendor</tt>/<tt>idProduct</tt> combination. This function is intended
+ * for those scenarios where you are using libusb to knock up a quick test
+ * application - it allows you to avoid calling libusb_get_device_list() and
+ * worrying about traversing/freeing the list.
+ *
+ * This function has limitations and is hence not intended for use in real
+ * applications: if multiple devices have the same IDs it will only
+ * give you the first one, etc.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param vendor_id the idVendor value to search for
+ * \param product_id the idProduct value to search for
+ * \returns a device handle for the first found device, or NULL on error
+ * or if the device could not be found. */
+DEFAULT_VISIBILITY
+libusb_device_handle * LIBUSB_CALL libusb_open_device_with_vid_pid(
+	libusb_context *ctx, uint16_t vendor_id, uint16_t product_id)
+{
+	struct libusb_device **devs;
+	struct libusb_device *found = NULL;
+	struct libusb_device *dev;
+	struct libusb_device_handle *dev_handle = NULL;
+	size_t i = 0;
+	int r;
+
+	if (libusb_get_device_list(ctx, &devs) < 0)
+		return NULL;
+
+	while ((dev = devs[i++]) != NULL) {
+		struct libusb_device_descriptor desc;
+		r = libusb_get_device_descriptor(dev, &desc);
+		if (r < 0)
+			goto out;
+		if (desc.idVendor == vendor_id && desc.idProduct == product_id) {
+			found = dev;
+			break;
+		}
+	}
+
+	if (found) {
+		r = libusb_open(found, &dev_handle);
+		if (r < 0)
+			dev_handle = NULL;
+	}
+
+out:
+	libusb_free_device_list(devs, 1);
+	return dev_handle;
+}
+
+static void do_close(struct libusb_context *ctx,
+	struct libusb_device_handle *dev_handle)
+{
+	struct usbi_transfer *itransfer;
+	struct usbi_transfer *tmp;
+
+	/* remove any transfers in flight that are for this device */
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+
+	/* safe iteration because transfers may be being deleted */
+	for_each_transfer_safe(ctx, itransfer, tmp) {
+		struct libusb_transfer *transfer =
+			USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+
+		if (transfer->dev_handle != dev_handle)
+			continue;
+
+		usbi_mutex_lock(&itransfer->lock);
+		if (!(itransfer->state_flags & USBI_TRANSFER_DEVICE_DISAPPEARED)) {
+			usbi_err(ctx, "Device handle closed while transfer was still being processed, but the device is still connected as far as we know");
+
+			if (itransfer->state_flags & USBI_TRANSFER_CANCELLING)
+				usbi_warn(ctx, "A cancellation for an in-flight transfer hasn't completed but closing the device handle");
+			else
+				usbi_err(ctx, "A cancellation hasn't even been scheduled on the transfer for which the device is closing");
+		}
+		usbi_mutex_unlock(&itransfer->lock);
+
+		/* remove from the list of in-flight transfers and make sure
+		 * we don't accidentally use the device handle in the future
+		 * (or that such accesses will be easily caught and identified as a crash)
+		 */
+		list_del(&itransfer->list);
+		transfer->dev_handle = NULL;
+
+		/* it is up to the user to free up the actual transfer struct.  this is
+		 * just making sure that we don't attempt to process the transfer after
+		 * the device handle is invalid
+		 */
+		usbi_dbg(ctx, "Removed transfer %p from the in-flight list because device handle %p closed",
+			 transfer, dev_handle);
+	}
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	usbi_mutex_lock(&ctx->open_devs_lock);
+	list_del(&dev_handle->list);
+	usbi_mutex_unlock(&ctx->open_devs_lock);
+
+	usbi_backend.close(dev_handle);
+	libusb_unref_device(dev_handle->dev);
+	usbi_mutex_destroy(&dev_handle->lock);
+	free(dev_handle);
+}
+
+/** \ingroup libusb_dev
+ * Close a device handle. Should be called on all open handles before your
+ * application exits.
+ *
+ * Internally, this function destroys the reference that was added by
+ * libusb_open() on the given device.
+ *
+ * This is a non-blocking function; no requests are sent over the bus.
+ *
+ * \param dev_handle the device handle to close
+ */
+void API_EXPORTED libusb_close(libusb_device_handle *dev_handle)
+{
+	struct libusb_context *ctx;
+	unsigned int event_flags;
+	int handling_events;
+
+	if (!dev_handle)
+		return;
+	ctx = HANDLE_CTX(dev_handle);
+	usbi_dbg(ctx, " ");
+
+	handling_events = usbi_handling_events(ctx);
+
+	/* Similarly to libusb_open(), we want to interrupt all event handlers
+	 * at this point. More importantly, we want to perform the actual close of
+	 * the device while holding the event handling lock (preventing any other
+	 * thread from doing event handling) because we will be removing a file
+	 * descriptor from the polling loop. If this is being called by the current
+	 * event handler, we can bypass the interruption code because we already
+	 * hold the event handling lock. */
+
+	if (!handling_events) {
+		/* Record that we are closing a device.
+		 * Only signal an event if there are no prior pending events. */
+		usbi_mutex_lock(&ctx->event_data_lock);
+		event_flags = ctx->event_flags;
+		if (!ctx->device_close++)
+			ctx->event_flags |= USBI_EVENT_DEVICE_CLOSE;
+		if (!event_flags)
+			usbi_signal_event(&ctx->event);
+		usbi_mutex_unlock(&ctx->event_data_lock);
+
+		/* take event handling lock */
+		libusb_lock_events(ctx);
+	}
+
+	/* Close the device */
+	do_close(ctx, dev_handle);
+
+	if (!handling_events) {
+		/* We're done with closing this device.
+		 * Clear the event pipe if there are no further pending events. */
+		usbi_mutex_lock(&ctx->event_data_lock);
+		if (!--ctx->device_close)
+			ctx->event_flags &= ~USBI_EVENT_DEVICE_CLOSE;
+		if (!ctx->event_flags)
+			usbi_clear_event(&ctx->event);
+		usbi_mutex_unlock(&ctx->event_data_lock);
+
+		/* Release event handling lock and wake up event waiters */
+		libusb_unlock_events(ctx);
+	}
+}
+
+/** \ingroup libusb_dev
+ * Get the underlying device for a device handle. This function does not modify
+ * the reference count of the returned device, so do not feel compelled to
+ * unreference it when you are done.
+ * \param dev_handle a device handle
+ * \returns the underlying device
+ */
+DEFAULT_VISIBILITY
+libusb_device * LIBUSB_CALL libusb_get_device(libusb_device_handle *dev_handle)
+{
+	return dev_handle->dev;
+}
+
+/** \ingroup libusb_dev
+ * Determine the bConfigurationValue of the currently active configuration.
+ *
+ * You could formulate your own control request to obtain this information,
+ * but this function has the advantage that it may be able to retrieve the
+ * information from operating system caches (no I/O involved).
+ *
+ * If the OS does not cache this information, then this function will block
+ * while a control transfer is submitted to retrieve the information.
+ *
+ * This function will return a value of 0 in the <tt>config</tt> output
+ * parameter if the device is in unconfigured state.
+ *
+ * \param dev_handle a device handle
+ * \param config output location for the bConfigurationValue of the active
+ * configuration (only valid for return code 0)
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_get_configuration(libusb_device_handle *dev_handle,
+	int *config)
+{
+	int r = LIBUSB_ERROR_NOT_SUPPORTED;
+	uint8_t tmp = 0;
+	struct libusb_context *ctx = HANDLE_CTX(dev_handle);
+
+	usbi_dbg(ctx, " ");
+	if (usbi_backend.get_configuration)
+		r = usbi_backend.get_configuration(dev_handle, &tmp);
+
+	if (r == LIBUSB_ERROR_NOT_SUPPORTED) {
+		usbi_dbg(ctx, "falling back to control message");
+		r = libusb_control_transfer(dev_handle, LIBUSB_ENDPOINT_IN,
+			LIBUSB_REQUEST_GET_CONFIGURATION, 0, 0, &tmp, 1, 1000);
+		if (r == 1) {
+			r = 0;
+		} else if (r == 0) {
+			usbi_err(ctx, "zero bytes returned in ctrl transfer?");
+			r = LIBUSB_ERROR_IO;
+		} else {
+			usbi_dbg(ctx, "control failed, error %d", r);
+		}
+	}
+
+	if (r == 0) {
+		usbi_dbg(ctx, "active config %u", tmp);
+		*config = (int)tmp;
+	}
+
+	return r;
+}
+
+/** \ingroup libusb_dev
+ * Set the active configuration for a device.
+ *
+ * The operating system may or may not have already set an active
+ * configuration on the device. It is up to your application to ensure the
+ * correct configuration is selected before you attempt to claim interfaces
+ * and perform other operations.
+ *
+ * If you call this function on a device already configured with the selected
+ * configuration, then this function will act as a lightweight device reset:
+ * it will issue a SET_CONFIGURATION request using the current configuration,
+ * causing most USB-related device state to be reset (altsetting reset to zero,
+ * endpoint halts cleared, toggles reset).
+ *
+ * Not all backends support setting the configuration from user space, which
+ * will be indicated by the return code LIBUSB_ERROR_NOT_SUPPORTED. As this
+ * suggests that the platform is handling the device configuration itself,
+ * this error should generally be safe to ignore.
+ *
+ * You cannot change/reset configuration if your application has claimed
+ * interfaces. It is advised to set the desired configuration before claiming
+ * interfaces.
+ *
+ * Alternatively you can call libusb_release_interface() first. Note if you
+ * do things this way you must ensure that auto_detach_kernel_driver for
+ * <tt>dev</tt> is 0, otherwise the kernel driver will be re-attached when you
+ * release the interface(s).
+ *
+ * You cannot change/reset configuration if other applications or drivers have
+ * claimed interfaces.
+ *
+ * A configuration value of -1 will put the device in unconfigured state.
+ * The USB specifications state that a configuration value of 0 does this,
+ * however buggy devices exist which actually have a configuration 0.
+ *
+ * You should always use this function rather than formulating your own
+ * SET_CONFIGURATION control request. This is because the underlying operating
+ * system needs to know when such changes happen.
+ *
+ * This is a blocking function.
+ *
+ * \param dev_handle a device handle
+ * \param configuration the bConfigurationValue of the configuration you
+ * wish to activate, or -1 if you wish to put the device in an unconfigured
+ * state
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the requested configuration does not exist
+ * \returns LIBUSB_ERROR_BUSY if interfaces are currently claimed
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED if setting or changing the configuration
+ * is not supported by the backend
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns another LIBUSB_ERROR code on other failure
+ * \see libusb_set_auto_detach_kernel_driver()
+ */
+int API_EXPORTED libusb_set_configuration(libusb_device_handle *dev_handle,
+	int configuration)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "configuration %d", configuration);
+	if (configuration < -1 || configuration > (int)UINT8_MAX)
+		return LIBUSB_ERROR_INVALID_PARAM;
+	return usbi_backend.set_configuration(dev_handle, configuration);
+}
+
+/** \ingroup libusb_dev
+ * Claim an interface on a given device handle. You must claim the interface
+ * you wish to use before you can perform I/O on any of its endpoints.
+ *
+ * It is legal to attempt to claim an already-claimed interface, in which
+ * case libusb just returns 0 without doing anything.
+ *
+ * If auto_detach_kernel_driver is set to 1 for <tt>dev</tt>, the kernel driver
+ * will be detached if necessary, on failure the detach error is returned.
+ *
+ * Claiming of interfaces is a purely logical operation; it does not cause
+ * any requests to be sent over the bus. Interface claiming is used to
+ * instruct the underlying operating system that your application wishes
+ * to take ownership of the interface.
+ *
+ * This is a non-blocking function.
+ *
+ * \param dev_handle a device handle
+ * \param interface_number the <tt>bInterfaceNumber</tt> of the interface you
+ * wish to claim
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the requested interface does not exist
+ * \returns LIBUSB_ERROR_BUSY if another program or driver has claimed the
+ * interface
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns a LIBUSB_ERROR code on other failure
+ * \see libusb_set_auto_detach_kernel_driver()
+ */
+int API_EXPORTED libusb_claim_interface(libusb_device_handle *dev_handle,
+	int interface_number)
+{
+	int r = 0;
+
+	usbi_dbg(HANDLE_CTX(dev_handle), "interface %d", interface_number);
+	if (interface_number < 0 || interface_number >= USB_MAXINTERFACES)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	usbi_mutex_lock(&dev_handle->lock);
+	if (dev_handle->claimed_interfaces & (1U << interface_number))
+		goto out;
+
+	r = usbi_backend.claim_interface(dev_handle, (uint8_t)interface_number);
+	if (r == 0)
+		dev_handle->claimed_interfaces |= 1U << interface_number;
+
+out:
+	usbi_mutex_unlock(&dev_handle->lock);
+	return r;
+}
+
+/** \ingroup libusb_dev
+ * Release an interface previously claimed with libusb_claim_interface(). You
+ * should release all claimed interfaces before closing a device handle.
+ *
+ * This is a blocking function. A SET_INTERFACE control request will be sent
+ * to the device, resetting interface state to the first alternate setting.
+ *
+ * If auto_detach_kernel_driver is set to 1 for <tt>dev</tt>, the kernel
+ * driver will be re-attached after releasing the interface.
+ *
+ * \param dev_handle a device handle
+ * \param interface_number the <tt>bInterfaceNumber</tt> of the
+ * previously-claimed interface
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the interface was not claimed
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns another LIBUSB_ERROR code on other failure
+ * \see libusb_set_auto_detach_kernel_driver()
+ */
+int API_EXPORTED libusb_release_interface(libusb_device_handle *dev_handle,
+	int interface_number)
+{
+	int r;
+
+	usbi_dbg(HANDLE_CTX(dev_handle), "interface %d", interface_number);
+	if (interface_number < 0 || interface_number >= USB_MAXINTERFACES)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	usbi_mutex_lock(&dev_handle->lock);
+	if (!(dev_handle->claimed_interfaces & (1U << interface_number))) {
+		r = LIBUSB_ERROR_NOT_FOUND;
+		goto out;
+	}
+
+	r = usbi_backend.release_interface(dev_handle, (uint8_t)interface_number);
+	if (r == 0)
+		dev_handle->claimed_interfaces &= ~(1U << interface_number);
+
+out:
+	usbi_mutex_unlock(&dev_handle->lock);
+	return r;
+}
+
+/** \ingroup libusb_dev
+ * Activate an alternate setting for an interface. The interface must have
+ * been previously claimed with libusb_claim_interface().
+ *
+ * You should always use this function rather than formulating your own
+ * SET_INTERFACE control request. This is because the underlying operating
+ * system needs to know when such changes happen.
+ *
+ * This is a blocking function.
+ *
+ * \param dev_handle a device handle
+ * \param interface_number the <tt>bInterfaceNumber</tt> of the
+ * previously-claimed interface
+ * \param alternate_setting the <tt>bAlternateSetting</tt> of the alternate
+ * setting to activate
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the interface was not claimed, or the
+ * requested alternate setting does not exist
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_set_interface_alt_setting(libusb_device_handle *dev_handle,
+	int interface_number, int alternate_setting)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "interface %d altsetting %d",
+		interface_number, alternate_setting);
+	if (interface_number < 0 || interface_number >= USB_MAXINTERFACES)
+		return LIBUSB_ERROR_INVALID_PARAM;
+	if (alternate_setting < 0 || alternate_setting > (int)UINT8_MAX)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	if (!usbi_atomic_load(&dev_handle->dev->attached)) {
+		usbi_mutex_unlock(&dev_handle->lock);
+		return LIBUSB_ERROR_NO_DEVICE;
+	}
+
+	usbi_mutex_lock(&dev_handle->lock);
+	if (!(dev_handle->claimed_interfaces & (1U << interface_number))) {
+		usbi_mutex_unlock(&dev_handle->lock);
+		return LIBUSB_ERROR_NOT_FOUND;
+	}
+	usbi_mutex_unlock(&dev_handle->lock);
+
+	return usbi_backend.set_interface_altsetting(dev_handle,
+		(uint8_t)interface_number, (uint8_t)alternate_setting);
+}
+
+/** \ingroup libusb_dev
+ * Clear the halt/stall condition for an endpoint. Endpoints with halt status
+ * are unable to receive or transmit data until the halt condition is stalled.
+ *
+ * You should cancel all pending transfers before attempting to clear the halt
+ * condition.
+ *
+ * This is a blocking function.
+ *
+ * \param dev_handle a device handle
+ * \param endpoint the endpoint to clear halt status
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_clear_halt(libusb_device_handle *dev_handle,
+	unsigned char endpoint)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "endpoint 0x%x", endpoint);
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	return usbi_backend.clear_halt(dev_handle, endpoint);
+}
+
+/** \ingroup libusb_dev
+ * Perform a USB port reset to reinitialize a device. The system will attempt
+ * to restore the previous configuration and alternate settings after the
+ * reset has completed.
+ *
+ * If the reset fails, the descriptors change, or the previous state cannot be
+ * restored, the device will appear to be disconnected and reconnected. This
+ * means that the device handle is no longer valid (you should close it) and
+ * rediscover the device. A return code of LIBUSB_ERROR_NOT_FOUND indicates
+ * when this is the case.
+ *
+ * This is a blocking function which usually incurs a noticeable delay.
+ *
+ * \param dev_handle a handle of the device to reset
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if re-enumeration is required, or if the
+ * device has been disconnected
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_reset_device(libusb_device_handle *dev_handle)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), " ");
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	if (usbi_backend.reset_device)
+		return usbi_backend.reset_device(dev_handle);
+	else
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+/** \ingroup libusb_asyncio
+ * Allocate up to num_streams usb bulk streams on the specified endpoints. This
+ * function takes an array of endpoints rather then a single endpoint because
+ * some protocols require that endpoints are setup with similar stream ids.
+ * All endpoints passed in must belong to the same interface.
+ *
+ * Note this function may return less streams then requested. Also note that the
+ * same number of streams are allocated for each endpoint in the endpoint array.
+ *
+ * Stream id 0 is reserved, and should not be used to communicate with devices.
+ * If libusb_alloc_streams() returns with a value of N, you may use stream ids
+ * 1 to N.
+ *
+ * Since version 1.0.19, \ref LIBUSB_API_VERSION >= 0x01000103
+ *
+ * \param dev_handle a device handle
+ * \param num_streams number of streams to try to allocate
+ * \param endpoints array of endpoints to allocate streams on
+ * \param num_endpoints length of the endpoints array
+ * \returns number of streams allocated, or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_alloc_streams(libusb_device_handle *dev_handle,
+	uint32_t num_streams, unsigned char *endpoints, int num_endpoints)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "streams %u eps %d", (unsigned)num_streams, num_endpoints);
+
+	if (!num_streams || !endpoints || num_endpoints <= 0)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	if (usbi_backend.alloc_streams)
+		return usbi_backend.alloc_streams(dev_handle, num_streams, endpoints,
+						   num_endpoints);
+	else
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+/** \ingroup libusb_asyncio
+ * Free usb bulk streams allocated with libusb_alloc_streams().
+ *
+ * Note streams are automatically free-ed when releasing an interface.
+ *
+ * Since version 1.0.19, \ref LIBUSB_API_VERSION >= 0x01000103
+ *
+ * \param dev_handle a device handle
+ * \param endpoints array of endpoints to free streams on
+ * \param num_endpoints length of the endpoints array
+ * \returns LIBUSB_SUCCESS, or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_free_streams(libusb_device_handle *dev_handle,
+	unsigned char *endpoints, int num_endpoints)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "eps %d", num_endpoints);
+
+	if (!endpoints || num_endpoints <= 0)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	if (usbi_backend.free_streams)
+		return usbi_backend.free_streams(dev_handle, endpoints,
+						  num_endpoints);
+	else
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+/** \ingroup libusb_asyncio
+ * Attempts to allocate a block of persistent DMA memory suitable for transfers
+ * against the given device. If successful, will return a block of memory
+ * that is suitable for use as "buffer" in \ref libusb_transfer against this
+ * device. Using this memory instead of regular memory means that the host
+ * controller can use DMA directly into the buffer to increase performance, and
+ * also that transfers can no longer fail due to kernel memory fragmentation.
+ *
+ * Note that this means you should not modify this memory (or even data on
+ * the same cache lines) when a transfer is in progress, although it is legal
+ * to have several transfers going on within the same memory block.
+ *
+ * Will return NULL on failure. Many systems do not support such zero-copy
+ * and will always return NULL. Memory allocated with this function must be
+ * freed with \ref libusb_dev_mem_free. Specifically, this means that the
+ * flag \ref LIBUSB_TRANSFER_FREE_BUFFER cannot be used to free memory allocated
+ * with this function.
+ *
+ * Since version 1.0.21, \ref LIBUSB_API_VERSION >= 0x01000105
+ *
+ * \param dev_handle a device handle
+ * \param length size of desired data buffer
+ * \returns a pointer to the newly allocated memory, or NULL on failure
+ */
+DEFAULT_VISIBILITY
+unsigned char * LIBUSB_CALL libusb_dev_mem_alloc(libusb_device_handle *dev_handle,
+        size_t length)
+{
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return NULL;
+
+	if (usbi_backend.dev_mem_alloc)
+		return usbi_backend.dev_mem_alloc(dev_handle, length);
+	else
+		return NULL;
+}
+
+/** \ingroup libusb_asyncio
+ * Free device memory allocated with libusb_dev_mem_alloc().
+ *
+ * \param dev_handle a device handle
+ * \param buffer pointer to the previously allocated memory
+ * \param length size of previously allocated memory
+ * \returns LIBUSB_SUCCESS, or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_dev_mem_free(libusb_device_handle *dev_handle,
+	unsigned char *buffer, size_t length)
+{
+	if (usbi_backend.dev_mem_free)
+		return usbi_backend.dev_mem_free(dev_handle, buffer, length);
+	else
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+/** \ingroup libusb_dev
+ * Determine if a kernel driver is active on an interface. If a kernel driver
+ * is active, you cannot claim the interface, and libusb will be unable to
+ * perform I/O.
+ *
+ * This functionality is not available on Windows.
+ *
+ * \param dev_handle a device handle
+ * \param interface_number the interface to check
+ * \returns 0 if no kernel driver is active
+ * \returns 1 if a kernel driver is active
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED on platforms where the functionality
+ * is not available
+ * \returns another LIBUSB_ERROR code on other failure
+ * \see libusb_detach_kernel_driver()
+ */
+int API_EXPORTED libusb_kernel_driver_active(libusb_device_handle *dev_handle,
+	int interface_number)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "interface %d", interface_number);
+
+	if (interface_number < 0 || interface_number >= USB_MAXINTERFACES)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	if (usbi_backend.kernel_driver_active)
+		return usbi_backend.kernel_driver_active(dev_handle, (uint8_t)interface_number);
+	else
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+/** \ingroup libusb_dev
+ * Detach a kernel driver from an interface. If successful, you will then be
+ * able to claim the interface and perform I/O.
+ *
+ * This functionality is not available on Windows.
+ *
+ * Note that libusb itself also talks to the device through a special kernel
+ * driver, if this driver is already attached to the device, this call will
+ * not detach it and return LIBUSB_ERROR_NOT_FOUND.
+ *
+ * \param dev_handle a device handle
+ * \param interface_number the interface to detach the driver from
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED on platforms where the functionality
+ * is not available
+ * \returns another LIBUSB_ERROR code on other failure
+ * \see libusb_kernel_driver_active()
+ */
+int API_EXPORTED libusb_detach_kernel_driver(libusb_device_handle *dev_handle,
+	int interface_number)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "interface %d", interface_number);
+
+	if (interface_number < 0 || interface_number >= USB_MAXINTERFACES)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	if (usbi_backend.detach_kernel_driver)
+		return usbi_backend.detach_kernel_driver(dev_handle, (uint8_t)interface_number);
+	else
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+/** \ingroup libusb_dev
+ * Re-attach an interface's kernel driver, which was previously detached
+ * using libusb_detach_kernel_driver().
+ *
+ * This functionality is not available on Windows.
+ *
+ * \param dev_handle a device handle
+ * \param interface_number the interface to attach the driver from
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED on platforms where the functionality
+ * is not available
+ * \returns LIBUSB_ERROR_BUSY if the driver cannot be attached because the
+ * interface is claimed by a program or driver
+ * \returns another LIBUSB_ERROR code on other failure
+ * \see libusb_kernel_driver_active()
+ */
+int API_EXPORTED libusb_attach_kernel_driver(libusb_device_handle *dev_handle,
+	int interface_number)
+{
+	usbi_dbg(HANDLE_CTX(dev_handle), "interface %d", interface_number);
+
+	if (interface_number < 0 || interface_number >= USB_MAXINTERFACES)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	if (!usbi_atomic_load(&dev_handle->dev->attached))
+		return LIBUSB_ERROR_NO_DEVICE;
+
+	if (usbi_backend.attach_kernel_driver)
+		return usbi_backend.attach_kernel_driver(dev_handle, (uint8_t)interface_number);
+	else
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+/** \ingroup libusb_dev
+ * Enable/disable libusb's automatic kernel driver detachment. When this is
+ * enabled libusb will automatically detach the kernel driver on an interface
+ * when claiming the interface, and attach it when releasing the interface.
+ *
+ * Automatic kernel driver detachment is disabled on newly opened device
+ * handles by default.
+ *
+ * On platforms which do not have LIBUSB_CAP_SUPPORTS_DETACH_KERNEL_DRIVER
+ * this function will return LIBUSB_ERROR_NOT_SUPPORTED, and libusb will
+ * continue as if this function was never called.
+ *
+ * \param dev_handle a device handle
+ * \param enable whether to enable or disable auto kernel driver detachment
+ *
+ * \returns LIBUSB_SUCCESS on success
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED on platforms where the functionality
+ * is not available
+ * \see libusb_claim_interface()
+ * \see libusb_release_interface()
+ * \see libusb_set_configuration()
+ */
+int API_EXPORTED libusb_set_auto_detach_kernel_driver(
+	libusb_device_handle *dev_handle, int enable)
+{
+	if (!(usbi_backend.caps & USBI_CAP_SUPPORTS_DETACH_KERNEL_DRIVER))
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+
+	dev_handle->auto_detach_kernel_driver = enable;
+	return LIBUSB_SUCCESS;
+}
+
+/** \ingroup libusb_lib
+ * \deprecated Use libusb_set_option() instead using the
+ * \ref LIBUSB_OPTION_LOG_LEVEL option.
+ */
+void API_EXPORTED libusb_set_debug(libusb_context *ctx, int level)
+{
+#if defined(ENABLE_LOGGING) && !defined(ENABLE_DEBUG_LOGGING)
+	ctx = usbi_get_context(ctx);
+	if (!ctx->debug_fixed) {
+		level = CLAMP(level, LIBUSB_LOG_LEVEL_NONE, LIBUSB_LOG_LEVEL_DEBUG);
+		ctx->debug = (enum libusb_log_level)level;
+	}
+#else
+	UNUSED(ctx);
+	UNUSED(level);
+#endif
+}
+
+/** \ingroup libusb_lib
+ * Set log handler.
+ *
+ * libusb will redirect its log messages to the provided callback function.
+ * libusb supports redirection of per context and global log messages.
+ * Log messages sent to the context will be sent to the global log handler too.
+ *
+ * If libusb is compiled without message logging or USE_SYSTEM_LOGGING_FACILITY
+ * is defined then global callback function will never be called.
+ * If ENABLE_DEBUG_LOGGING is defined then per context callback function will
+ * never be called.
+ *
+ * Since version 1.0.23, \ref LIBUSB_API_VERSION >= 0x01000107
+ *
+ * \param ctx context on which to assign log handler, or NULL for the default
+ * context. Parameter ignored if only LIBUSB_LOG_CB_GLOBAL mode is requested.
+ * \param cb pointer to the callback function, or NULL to stop log
+ * messages redirection
+ * \param mode mode of callback function operation. Several modes can be
+ * selected for a single callback function, see \ref libusb_log_cb_mode for
+ * a description.
+ * \see libusb_log_cb, libusb_log_cb_mode
+ */
+void API_EXPORTED libusb_set_log_cb(libusb_context *ctx, libusb_log_cb cb,
+	int mode)
+{
+#if defined(ENABLE_LOGGING) && (!defined(ENABLE_DEBUG_LOGGING) || !defined(USE_SYSTEM_LOGGING_FACILITY))
+#if !defined(USE_SYSTEM_LOGGING_FACILITY)
+	if (mode & LIBUSB_LOG_CB_GLOBAL)
+		log_handler = cb;
+#endif
+#if !defined(ENABLE_DEBUG_LOGGING)
+	if (mode & LIBUSB_LOG_CB_CONTEXT) {
+		ctx = usbi_get_context(ctx);
+		ctx->log_handler = cb;
+	}
+#else
+	UNUSED(ctx);
+#endif
+#else
+	UNUSED(ctx);
+	UNUSED(cb);
+	UNUSED(mode);
+#endif
+}
+
+/** \ingroup libusb_lib
+ * Set an option in the library.
+ *
+ * Use this function to configure a specific option within the library.
+ *
+ * Some options require one or more arguments to be provided. Consult each
+ * option's documentation for specific requirements.
+ *
+ * If the context ctx is NULL, the option will be added to a list of default
+ * options that will be applied to all subsequently created contexts.
+ *
+ * Since version 1.0.22, \ref LIBUSB_API_VERSION >= 0x01000106
+ *
+ * \param ctx context on which to operate
+ * \param option which option to set
+ * \param ... any required arguments for the specified option
+ *
+ * \returns LIBUSB_SUCCESS on success
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the option or arguments are invalid
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED if the option is valid but not supported
+ * on this platform
+ * \returns LIBUSB_ERROR_NOT_FOUND if LIBUSB_OPTION_USE_USBDK is valid on this platform but UsbDk is not available
+ */
+int API_EXPORTED libusb_set_option(libusb_context *ctx,
+	enum libusb_option option, ...)
+{
+	int arg = 0, r = LIBUSB_SUCCESS;
+	va_list ap;
+
+	va_start(ap, option);
+	if (LIBUSB_OPTION_LOG_LEVEL == option) {
+		arg = va_arg(ap, int);
+		if (arg < LIBUSB_LOG_LEVEL_NONE || arg > LIBUSB_LOG_LEVEL_DEBUG) {
+			r = LIBUSB_ERROR_INVALID_PARAM;
+		}
+	}
+	va_end(ap);
+
+	if (LIBUSB_SUCCESS != r) {
+		return r;
+	}
+
+	if (option >= LIBUSB_OPTION_MAX) {
+		return LIBUSB_ERROR_INVALID_PARAM;
+	}
+
+	if (NULL == ctx) {
+		usbi_mutex_static_lock(&default_context_lock);
+		default_context_options[option].is_set = 1;
+		if (LIBUSB_OPTION_LOG_LEVEL == option) {
+			default_context_options[option].arg.ival = arg;
+		}
+		usbi_mutex_static_unlock(&default_context_lock);
+	}
+
+	ctx = usbi_get_context(ctx);
+	if (NULL == ctx) {
+		return LIBUSB_SUCCESS;
+	}
+
+	switch (option) {
+	case LIBUSB_OPTION_LOG_LEVEL:
+#if defined(ENABLE_LOGGING) && !defined(ENABLE_DEBUG_LOGGING)
+		if (!ctx->debug_fixed)
+			ctx->debug = (enum libusb_log_level)arg;
+#endif
+		break;
+
+		/* Handle all backend-specific options here */
+	case LIBUSB_OPTION_USE_USBDK:
+	case LIBUSB_OPTION_NO_DEVICE_DISCOVERY:
+		if (usbi_backend.set_option)
+			return usbi_backend.set_option(ctx, option, ap);
+
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+		break;
+
+	case LIBUSB_OPTION_MAX:
+	default:
+		return LIBUSB_ERROR_INVALID_PARAM;
+	}
+
+	return LIBUSB_SUCCESS;;
+}
+
+#if defined(ENABLE_LOGGING) && !defined(ENABLE_DEBUG_LOGGING)
+/* returns the log level as defined in the LIBUSB_DEBUG environment variable.
+ * if LIBUSB_DEBUG is not present or not a number, returns LIBUSB_LOG_LEVEL_NONE.
+ * value is clamped to ensure it is within the valid range of possibilities.
+ */
+static enum libusb_log_level get_env_debug_level(void)
+{
+	const char *dbg = getenv("LIBUSB_DEBUG");
+	enum libusb_log_level level;
+	if (dbg) {
+		int dbg_level = atoi(dbg);
+		dbg_level = CLAMP(dbg_level, LIBUSB_LOG_LEVEL_NONE, LIBUSB_LOG_LEVEL_DEBUG);
+		level = (enum libusb_log_level)dbg_level;
+	} else {
+		level = LIBUSB_LOG_LEVEL_NONE;
+	}
+	return level;
+}
+#endif
+
+/** \ingroup libusb_lib
+ * Initialize libusb. This function must be called before calling any other
+ * libusb function.
+ *
+ * If you do not provide an output location for a context pointer, a default
+ * context will be created. If there was already a default context, it will
+ * be reused (and nothing will be initialized/reinitialized).
+ *
+ * \param ctx Optional output location for context pointer.
+ * Only valid on return code 0.
+ * \returns 0 on success, or a LIBUSB_ERROR code on failure
+ * \see libusb_contexts
+ */
+int API_EXPORTED libusb_init(libusb_context **ctx)
+{
+	size_t priv_size = usbi_backend.context_priv_size;
+	struct libusb_context *_ctx;
+	int r;
+
+	usbi_mutex_static_lock(&default_context_lock);
+
+	if (!ctx && usbi_default_context) {
+		usbi_dbg(usbi_default_context, "reusing default context");
+		default_context_refcnt++;
+		usbi_mutex_static_unlock(&default_context_lock);
+		return 0;
+	}
+
+	/* check for first init */
+	if (!active_contexts_list.next) {
+		list_init(&active_contexts_list);
+		usbi_get_monotonic_time(&timestamp_origin);
+	}
+
+	_ctx = calloc(1, PTR_ALIGN(sizeof(*_ctx)) + priv_size);
+	if (!_ctx) {
+		usbi_mutex_static_unlock(&default_context_lock);
+		return LIBUSB_ERROR_NO_MEM;
+	}
+
+#if defined(ENABLE_LOGGING) && !defined(ENABLE_DEBUG_LOGGING)
+	if (NULL == ctx && default_context_options[LIBUSB_OPTION_LOG_LEVEL].is_set) {
+		_ctx->debug = default_context_options[LIBUSB_OPTION_LOG_LEVEL].arg.ival;
+	} else {
+		_ctx->debug = get_env_debug_level();
+	}
+	if (_ctx->debug != LIBUSB_LOG_LEVEL_NONE)
+		_ctx->debug_fixed = 1;
+#endif
+
+	usbi_mutex_init(&_ctx->usb_devs_lock);
+	usbi_mutex_init(&_ctx->open_devs_lock);
+	list_init(&_ctx->usb_devs);
+	list_init(&_ctx->open_devs);
+
+	/* apply default options to all new contexts */
+	for (enum libusb_option option = 0 ; option < LIBUSB_OPTION_MAX ; option++) {
+		if (LIBUSB_OPTION_LOG_LEVEL == option || !default_context_options[option].is_set) {
+			continue;
+		}
+		r = libusb_set_option(_ctx, option);
+		if (LIBUSB_SUCCESS != r)
+			goto err_free_ctx;
+	}
+
+	/* default context must be initialized before calling usbi_dbg */
+	if (!ctx) {
+		usbi_default_context = _ctx;
+		default_context_refcnt = 1;
+		usbi_dbg(usbi_default_context, "created default context");
+	}
+
+	usbi_dbg(_ctx, "libusb v%u.%u.%u.%u%s", libusb_version_internal.major, libusb_version_internal.minor,
+		libusb_version_internal.micro, libusb_version_internal.nano, libusb_version_internal.rc);
+
+	r = usbi_io_init(_ctx);
+	if (r < 0)
+		goto err_free_ctx;
+
+	usbi_mutex_static_lock(&active_contexts_lock);
+	list_add(&_ctx->list, &active_contexts_list);
+	usbi_mutex_static_unlock(&active_contexts_lock);
+
+	usbi_hotplug_init(_ctx);
+
+	if (usbi_backend.init) {
+		r = usbi_backend.init(_ctx);
+		if (r)
+			goto err_io_exit;
+	}
+
+	if (ctx)
+		*ctx = _ctx;
+
+	usbi_mutex_static_unlock(&default_context_lock);
+
+	return 0;
+
+err_io_exit:
+	usbi_mutex_static_lock(&active_contexts_lock);
+	list_del(&_ctx->list);
+	usbi_mutex_static_unlock(&active_contexts_lock);
+
+	usbi_hotplug_exit(_ctx);
+	usbi_io_exit(_ctx);
+
+err_free_ctx:
+	if (!ctx) {
+		/* clear default context that was not fully initialized */
+		usbi_default_context = NULL;
+		default_context_refcnt = 0;
+	}
+
+	usbi_mutex_destroy(&_ctx->open_devs_lock);
+	usbi_mutex_destroy(&_ctx->usb_devs_lock);
+
+	free(_ctx);
+
+	usbi_mutex_static_unlock(&default_context_lock);
+
+	return r;
+}
+
+/** \ingroup libusb_lib
+ * Deinitialize libusb. Should be called after closing all open devices and
+ * before your application terminates.
+ * \param ctx the context to deinitialize, or NULL for the default context
+ */
+void API_EXPORTED libusb_exit(libusb_context *ctx)
+{
+	struct libusb_context *_ctx;
+	struct libusb_device *dev;
+
+	usbi_mutex_static_lock(&default_context_lock);
+
+	/* if working with default context, only actually do the deinitialization
+	 * if we're the last user */
+	if (!ctx) {
+		if (!usbi_default_context) {
+			usbi_dbg(ctx, "no default context, not initialized?");
+			usbi_mutex_static_unlock(&default_context_lock);
+			return;
+		}
+
+		if (--default_context_refcnt > 0) {
+			usbi_dbg(ctx, "not destroying default context");
+			usbi_mutex_static_unlock(&default_context_lock);
+			return;
+		}
+
+		usbi_dbg(ctx, "destroying default context");
+		_ctx = usbi_default_context;
+	} else {
+		usbi_dbg(ctx, " ");
+		_ctx = ctx;
+	}
+
+	usbi_mutex_static_lock(&active_contexts_lock);
+	list_del(&_ctx->list);
+	usbi_mutex_static_unlock(&active_contexts_lock);
+
+	if (usbi_backend.exit)
+		usbi_backend.exit(_ctx);
+
+	if (!ctx)
+		usbi_default_context = NULL;
+
+	usbi_mutex_static_unlock(&default_context_lock);
+
+	/* Don't bother with locking after this point because unless there is
+	 * an application bug, nobody will be accessing the context. */
+
+	usbi_hotplug_exit(_ctx);
+	usbi_io_exit(_ctx);
+
+	for_each_device(_ctx, dev) {
+		usbi_warn(_ctx, "device %d.%d still referenced",
+			dev->bus_number, dev->device_address);
+	}
+
+	if (!list_empty(&_ctx->open_devs))
+		usbi_warn(_ctx, "application left some devices open");
+
+	usbi_mutex_destroy(&_ctx->open_devs_lock);
+	usbi_mutex_destroy(&_ctx->usb_devs_lock);
+
+	free(_ctx);
+}
+
+/** \ingroup libusb_misc
+ * Check at runtime if the loaded library has a given capability.
+ * This call should be performed after \ref libusb_init(), to ensure the
+ * backend has updated its capability set.
+ *
+ * \param capability the \ref libusb_capability to check for
+ * \returns nonzero if the running library has the capability, 0 otherwise
+ */
+int API_EXPORTED libusb_has_capability(uint32_t capability)
+{
+	switch (capability) {
+	case LIBUSB_CAP_HAS_CAPABILITY:
+		return 1;
+	case LIBUSB_CAP_HAS_HOTPLUG:
+		return !(usbi_backend.get_device_list);
+	case LIBUSB_CAP_HAS_HID_ACCESS:
+		return (usbi_backend.caps & USBI_CAP_HAS_HID_ACCESS);
+	case LIBUSB_CAP_SUPPORTS_DETACH_KERNEL_DRIVER:
+		return (usbi_backend.caps & USBI_CAP_SUPPORTS_DETACH_KERNEL_DRIVER);
+	}
+	return 0;
+}
+
+#ifdef ENABLE_LOGGING
+
+/* this is defined in libusbi.h if needed */
+#ifdef LIBUSB_PRINTF_WIN32
+/*
+ * Prior to VS2015, Microsoft did not provide the snprintf() function and
+ * provided a vsnprintf() that did not guarantee NUL-terminated output.
+ * Microsoft did provide a _snprintf() function, but again it did not
+ * guarantee NULL-terminated output.
+ *
+ * The below implementations guarantee NUL-terminated output and are
+ * C99 compliant.
+ */
+
+int usbi_snprintf(char *str, size_t size, const char *format, ...)
+{
+	va_list args;
+	int ret;
+
+	va_start(args, format);
+	ret = usbi_vsnprintf(str, size, format, args);
+	va_end(args);
+
+	return ret;
+}
+
+int usbi_vsnprintf(char *str, size_t size, const char *format, va_list args)
+{
+	int ret;
+
+	ret = _vsnprintf(str, size, format, args);
+	if (ret < 0 || ret == (int)size) {
+		/* Output is truncated, ensure buffer is NUL-terminated and
+		 * determine how many characters would have been written. */
+		str[size - 1] = '\0';
+		if (ret < 0)
+			ret = _vsnprintf(NULL, 0, format, args);
+	}
+
+	return ret;
+}
+#endif /* LIBUSB_PRINTF_WIN32 */
+
+static void log_str(enum libusb_log_level level, const char *str)
+{
+#if defined(USE_SYSTEM_LOGGING_FACILITY)
+#if defined(__ANDROID__)
+	int priority;
+	switch (level) {
+	case LIBUSB_LOG_LEVEL_NONE: return;	/* Impossible, but keeps compiler happy */
+	case LIBUSB_LOG_LEVEL_ERROR: priority = ANDROID_LOG_ERROR; break;
+	case LIBUSB_LOG_LEVEL_WARNING: priority = ANDROID_LOG_WARN; break;
+	case LIBUSB_LOG_LEVEL_INFO: priority = ANDROID_LOG_INFO; break;
+	case LIBUSB_LOG_LEVEL_DEBUG: priority = ANDROID_LOG_DEBUG; break;
+	default: priority = ANDROID_LOG_UNKNOWN;
+	}
+	__android_log_write(priority, "libusb", str);
+#elif defined(_WIN32)
+	UNUSED(level);
+	OutputDebugStringA(str);
+#elif defined(HAVE_SYSLOG)
+	int syslog_level;
+	switch (level) {
+	case LIBUSB_LOG_LEVEL_NONE: return;	/* Impossible, but keeps compiler happy */
+	case LIBUSB_LOG_LEVEL_ERROR: syslog_level = LOG_ERR; break;
+	case LIBUSB_LOG_LEVEL_WARNING: syslog_level = LOG_WARNING; break;
+	case LIBUSB_LOG_LEVEL_INFO: syslog_level = LOG_INFO; break;
+	case LIBUSB_LOG_LEVEL_DEBUG: syslog_level = LOG_DEBUG; break;
+	default: syslog_level = LOG_INFO;
+	}
+	syslog(syslog_level, "%s", str);
+#else /* All of gcc, Clang, Xcode seem to use #warning */
+#warning System logging is not supported on this platform. Logging to stderr will be used instead.
+	UNUSED(level);
+	fputs(str, stderr);
+#endif
+#else
+	/* Global log handler */
+	if (log_handler)
+		log_handler(NULL, level, str);
+	else
+		fputs(str, stderr);
+#endif /* USE_SYSTEM_LOGGING_FACILITY */
+}
+
+static void log_v(struct libusb_context *ctx, enum libusb_log_level level,
+	const char *function, const char *format, va_list args)
+{
+	const char *prefix;
+	char buf[USBI_MAX_LOG_LEN];
+	int global_debug, header_len, text_len;
+	static int has_debug_header_been_displayed = 0;
+
+#ifdef ENABLE_DEBUG_LOGGING
+	global_debug = 1;
+	UNUSED(ctx);
+#else
+	enum libusb_log_level ctx_level;
+
+	ctx = usbi_get_context(ctx);
+	if (ctx)
+		ctx_level = ctx->debug;
+	else
+		ctx_level = get_env_debug_level();
+
+	if (ctx_level < level)
+		return;
+
+	global_debug = (ctx_level == LIBUSB_LOG_LEVEL_DEBUG);
+#endif
+
+	switch (level) {
+	case LIBUSB_LOG_LEVEL_NONE:	/* Impossible, but keeps compiler happy */
+		return;
+	case LIBUSB_LOG_LEVEL_ERROR:
+		prefix = "error";
+		break;
+	case LIBUSB_LOG_LEVEL_WARNING:
+		prefix = "warning";
+		break;
+	case LIBUSB_LOG_LEVEL_INFO:
+		prefix = "info";
+		break;
+	case LIBUSB_LOG_LEVEL_DEBUG:
+		prefix = "debug";
+		break;
+	default:
+		prefix = "unknown";
+		break;
+	}
+
+	if (global_debug) {
+		struct timespec timestamp;
+
+		if (!has_debug_header_been_displayed) {
+			has_debug_header_been_displayed = 1;
+			log_str(LIBUSB_LOG_LEVEL_DEBUG, "[timestamp] [threadID] facility level [function call] <message>" USBI_LOG_LINE_END);
+			log_str(LIBUSB_LOG_LEVEL_DEBUG, "--------------------------------------------------------------------------------" USBI_LOG_LINE_END);
+		}
+
+		usbi_get_monotonic_time(&timestamp);
+		TIMESPEC_SUB(&timestamp, &timestamp_origin, &timestamp);
+
+		header_len = snprintf(buf, sizeof(buf),
+			"[%2ld.%06ld] [%08x] libusb: %s [%s] ",
+			(long)timestamp.tv_sec, (long)(timestamp.tv_nsec / 1000L), usbi_get_tid(), prefix, function);
+	} else {
+		header_len = snprintf(buf, sizeof(buf),
+			"libusb: %s [%s] ", prefix, function);
+	}
+
+	if (header_len < 0 || header_len >= (int)sizeof(buf)) {
+		/* Somehow snprintf() failed to write to the buffer,
+		 * remove the header so something useful is output. */
+		header_len = 0;
+	}
+
+	text_len = vsnprintf(buf + header_len, sizeof(buf) - (size_t)header_len,
+		format, args);
+	if (text_len < 0 || text_len + header_len >= (int)sizeof(buf)) {
+		/* Truncated log output. On some platforms a -1 return value means
+		 * that the output was truncated. */
+		text_len = (int)sizeof(buf) - header_len;
+	}
+	if (header_len + text_len + (int)sizeof(USBI_LOG_LINE_END) >= (int)sizeof(buf)) {
+		/* Need to truncate the text slightly to fit on the terminator. */
+		text_len -= (header_len + text_len + (int)sizeof(USBI_LOG_LINE_END)) - (int)sizeof(buf);
+	}
+	strcpy(buf + header_len + text_len, USBI_LOG_LINE_END);
+
+	log_str(level, buf);
+
+	/* Per-context log handler */
+#ifndef ENABLE_DEBUG_LOGGING
+	if (ctx && ctx->log_handler)
+		ctx->log_handler(ctx, level, buf);
+#endif
+}
+
+void usbi_log(struct libusb_context *ctx, enum libusb_log_level level,
+	const char *function, const char *format, ...)
+{
+	va_list args;
+
+	va_start(args, format);
+	log_v(ctx, level, function, format, args);
+	va_end(args);
+}
+
+#endif /* ENABLE_LOGGING */
+
+/** \ingroup libusb_misc
+ * Returns a constant NULL-terminated string with the ASCII name of a libusb
+ * error or transfer status code. The caller must not free() the returned
+ * string.
+ *
+ * \param error_code The \ref libusb_error or libusb_transfer_status code to
+ * return the name of.
+ * \returns The error name, or the string **UNKNOWN** if the value of
+ * error_code is not a known error / status code.
+ */
+DEFAULT_VISIBILITY const char * LIBUSB_CALL libusb_error_name(int error_code)
+{
+	switch (error_code) {
+	case LIBUSB_ERROR_IO:
+		return "LIBUSB_ERROR_IO";
+	case LIBUSB_ERROR_INVALID_PARAM:
+		return "LIBUSB_ERROR_INVALID_PARAM";
+	case LIBUSB_ERROR_ACCESS:
+		return "LIBUSB_ERROR_ACCESS";
+	case LIBUSB_ERROR_NO_DEVICE:
+		return "LIBUSB_ERROR_NO_DEVICE";
+	case LIBUSB_ERROR_NOT_FOUND:
+		return "LIBUSB_ERROR_NOT_FOUND";
+	case LIBUSB_ERROR_BUSY:
+		return "LIBUSB_ERROR_BUSY";
+	case LIBUSB_ERROR_TIMEOUT:
+		return "LIBUSB_ERROR_TIMEOUT";
+	case LIBUSB_ERROR_OVERFLOW:
+		return "LIBUSB_ERROR_OVERFLOW";
+	case LIBUSB_ERROR_PIPE:
+		return "LIBUSB_ERROR_PIPE";
+	case LIBUSB_ERROR_INTERRUPTED:
+		return "LIBUSB_ERROR_INTERRUPTED";
+	case LIBUSB_ERROR_NO_MEM:
+		return "LIBUSB_ERROR_NO_MEM";
+	case LIBUSB_ERROR_NOT_SUPPORTED:
+		return "LIBUSB_ERROR_NOT_SUPPORTED";
+	case LIBUSB_ERROR_OTHER:
+		return "LIBUSB_ERROR_OTHER";
+
+	case LIBUSB_TRANSFER_ERROR:
+		return "LIBUSB_TRANSFER_ERROR";
+	case LIBUSB_TRANSFER_TIMED_OUT:
+		return "LIBUSB_TRANSFER_TIMED_OUT";
+	case LIBUSB_TRANSFER_CANCELLED:
+		return "LIBUSB_TRANSFER_CANCELLED";
+	case LIBUSB_TRANSFER_STALL:
+		return "LIBUSB_TRANSFER_STALL";
+	case LIBUSB_TRANSFER_NO_DEVICE:
+		return "LIBUSB_TRANSFER_NO_DEVICE";
+	case LIBUSB_TRANSFER_OVERFLOW:
+		return "LIBUSB_TRANSFER_OVERFLOW";
+
+	case 0:
+		return "LIBUSB_SUCCESS / LIBUSB_TRANSFER_COMPLETED";
+	default:
+		return "**UNKNOWN**";
+	}
+}
+
+/** \ingroup libusb_misc
+ * Returns a pointer to const struct libusb_version with the version
+ * (major, minor, micro, nano and rc) of the running library.
+ */
+DEFAULT_VISIBILITY
+const struct libusb_version * LIBUSB_CALL libusb_get_version(void)
+{
+	return &libusb_version_internal;
+}
diff --git a/hardware/src/libusb/descriptor.c b/hardware/src/libusb/descriptor.c
new file mode 100644
index 0000000..253ef1c
--- /dev/null
+++ b/hardware/src/libusb/descriptor.c
@@ -0,0 +1,1139 @@
+/* -*- Mode: C; indent-tabs-mode:t ; c-basic-offset:8 -*- */
+/*
+ * USB descriptor handling functions for libusb
+ * Copyright © 2007 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+
+#include <string.h>
+
+#define DESC_HEADER_LENGTH	2
+
+/** @defgroup libusb_desc USB descriptors
+ * This page details how to examine the various standard USB descriptors
+ * for detected devices
+ */
+
+#define READ_LE16(p) ((uint16_t)	\
+	(((uint16_t)((p)[1]) << 8) |	\
+	 ((uint16_t)((p)[0]))))
+
+#define READ_LE32(p) ((uint32_t)	\
+	(((uint32_t)((p)[3]) << 24) |	\
+	 ((uint32_t)((p)[2]) << 16) |	\
+	 ((uint32_t)((p)[1]) <<  8) |	\
+	 ((uint32_t)((p)[0]))))
+
+static void parse_descriptor(const void *source, const char *descriptor, void *dest)
+{
+	const uint8_t *sp = source;
+	uint8_t *dp = dest;
+	char field_type;
+
+	while (*descriptor) {
+		field_type = *descriptor++;
+		switch (field_type) {
+		case 'b':	/* 8-bit byte */
+			*dp++ = *sp++;
+			break;
+		case 'w':	/* 16-bit word, convert from little endian to CPU */
+			dp += ((uintptr_t)dp & 1);	/* Align to 16-bit word boundary */
+
+			*((uint16_t *)dp) = READ_LE16(sp);
+			sp += 2;
+			dp += 2;
+			break;
+		case 'd':	/* 32-bit word, convert from little endian to CPU */
+			dp += 4 - ((uintptr_t)dp & 3);	/* Align to 32-bit word boundary */
+
+			*((uint32_t *)dp) = READ_LE32(sp);
+			sp += 4;
+			dp += 4;
+			break;
+		case 'u':	/* 16 byte UUID */
+			memcpy(dp, sp, 16);
+			sp += 16;
+			dp += 16;
+			break;
+		}
+	}
+}
+
+static void clear_endpoint(struct libusb_endpoint_descriptor *endpoint)
+{
+	free((void *)endpoint->extra);
+}
+
+static int parse_endpoint(struct libusb_context *ctx,
+	struct libusb_endpoint_descriptor *endpoint, const uint8_t *buffer, int size)
+{
+	const struct usbi_descriptor_header *header;
+	const uint8_t *begin;
+	void *extra;
+	int parsed = 0;
+	int len;
+
+	if (size < DESC_HEADER_LENGTH) {
+		usbi_err(ctx, "short endpoint descriptor read %d/%d",
+			 size, DESC_HEADER_LENGTH);
+		return LIBUSB_ERROR_IO;
+	}
+
+	header = (const struct usbi_descriptor_header *)buffer;
+	if (header->bDescriptorType != LIBUSB_DT_ENDPOINT) {
+		usbi_err(ctx, "unexpected descriptor 0x%x (expected 0x%x)",
+			header->bDescriptorType, LIBUSB_DT_ENDPOINT);
+		return parsed;
+	} else if (header->bLength < LIBUSB_DT_ENDPOINT_SIZE) {
+		usbi_err(ctx, "invalid endpoint bLength (%u)", header->bLength);
+		return LIBUSB_ERROR_IO;
+	} else if (header->bLength > size) {
+		usbi_warn(ctx, "short endpoint descriptor read %d/%u",
+			  size, header->bLength);
+		return parsed;
+	}
+
+	if (header->bLength >= LIBUSB_DT_ENDPOINT_AUDIO_SIZE)
+		parse_descriptor(buffer, "bbbbwbbb", endpoint);
+	else
+		parse_descriptor(buffer, "bbbbwb", endpoint);
+
+	buffer += header->bLength;
+	size -= header->bLength;
+	parsed += header->bLength;
+
+	/* Skip over the rest of the Class Specific or Vendor Specific */
+	/*  descriptors */
+	begin = buffer;
+	while (size >= DESC_HEADER_LENGTH) {
+		header = (const struct usbi_descriptor_header *)buffer;
+		if (header->bLength < DESC_HEADER_LENGTH) {
+			usbi_err(ctx, "invalid extra ep desc len (%u)",
+				 header->bLength);
+			return LIBUSB_ERROR_IO;
+		} else if (header->bLength > size) {
+			usbi_warn(ctx, "short extra ep desc read %d/%u",
+				  size, header->bLength);
+			return parsed;
+		}
+
+		/* If we find another "proper" descriptor then we're done  */
+		if (header->bDescriptorType == LIBUSB_DT_ENDPOINT ||
+		    header->bDescriptorType == LIBUSB_DT_INTERFACE ||
+		    header->bDescriptorType == LIBUSB_DT_CONFIG ||
+		    header->bDescriptorType == LIBUSB_DT_DEVICE)
+			break;
+
+		usbi_dbg(ctx, "skipping descriptor 0x%x", header->bDescriptorType);
+		buffer += header->bLength;
+		size -= header->bLength;
+		parsed += header->bLength;
+	}
+
+	/* Copy any unknown descriptors into a storage area for drivers */
+	/*  to later parse */
+	len = (int)(buffer - begin);
+	if (len <= 0)
+		return parsed;
+
+	extra = malloc((size_t)len);
+	if (!extra)
+		return LIBUSB_ERROR_NO_MEM;
+
+	memcpy(extra, begin, len);
+	endpoint->extra = extra;
+	endpoint->extra_length = len;
+
+	return parsed;
+}
+
+static void clear_interface(struct libusb_interface *usb_interface)
+{
+	int i;
+
+	if (usb_interface->altsetting) {
+		for (i = 0; i < usb_interface->num_altsetting; i++) {
+			struct libusb_interface_descriptor *ifp =
+				(struct libusb_interface_descriptor *)
+				usb_interface->altsetting + i;
+
+			free((void *)ifp->extra);
+			if (ifp->endpoint) {
+				uint8_t j;
+
+				for (j = 0; j < ifp->bNumEndpoints; j++)
+					clear_endpoint((struct libusb_endpoint_descriptor *)
+						       ifp->endpoint + j);
+			}
+			free((void *)ifp->endpoint);
+		}
+	}
+	free((void *)usb_interface->altsetting);
+	usb_interface->altsetting = NULL;
+}
+
+static int parse_interface(libusb_context *ctx,
+	struct libusb_interface *usb_interface, const uint8_t *buffer, int size)
+{
+	int len;
+	int r;
+	int parsed = 0;
+	int interface_number = -1;
+	const struct usbi_descriptor_header *header;
+	const struct usbi_interface_descriptor *if_desc;
+	struct libusb_interface_descriptor *ifp;
+	const uint8_t *begin;
+
+	while (size >= LIBUSB_DT_INTERFACE_SIZE) {
+		struct libusb_interface_descriptor *altsetting;
+
+		altsetting = realloc((void *)usb_interface->altsetting,
+			sizeof(*altsetting) * (size_t)(usb_interface->num_altsetting + 1));
+		if (!altsetting) {
+			r = LIBUSB_ERROR_NO_MEM;
+			goto err;
+		}
+		usb_interface->altsetting = altsetting;
+
+		ifp = altsetting + usb_interface->num_altsetting;
+		parse_descriptor(buffer, "bbbbbbbbb", ifp);
+		if (ifp->bDescriptorType != LIBUSB_DT_INTERFACE) {
+			usbi_err(ctx, "unexpected descriptor 0x%x (expected 0x%x)",
+				 ifp->bDescriptorType, LIBUSB_DT_INTERFACE);
+			return parsed;
+		} else if (ifp->bLength < LIBUSB_DT_INTERFACE_SIZE) {
+			usbi_err(ctx, "invalid interface bLength (%u)",
+				 ifp->bLength);
+			r = LIBUSB_ERROR_IO;
+			goto err;
+		} else if (ifp->bLength > size) {
+			usbi_warn(ctx, "short intf descriptor read %d/%u",
+				 size, ifp->bLength);
+			return parsed;
+		} else if (ifp->bNumEndpoints > USB_MAXENDPOINTS) {
+			usbi_err(ctx, "too many endpoints (%u)", ifp->bNumEndpoints);
+			r = LIBUSB_ERROR_IO;
+			goto err;
+		}
+
+		usb_interface->num_altsetting++;
+		ifp->extra = NULL;
+		ifp->extra_length = 0;
+		ifp->endpoint = NULL;
+
+		if (interface_number == -1)
+			interface_number = ifp->bInterfaceNumber;
+
+		/* Skip over the interface */
+		buffer += ifp->bLength;
+		parsed += ifp->bLength;
+		size -= ifp->bLength;
+
+		begin = buffer;
+
+		/* Skip over any interface, class or vendor descriptors */
+		while (size >= DESC_HEADER_LENGTH) {
+			header = (const struct usbi_descriptor_header *)buffer;
+			if (header->bLength < DESC_HEADER_LENGTH) {
+				usbi_err(ctx,
+					 "invalid extra intf desc len (%u)",
+					 header->bLength);
+				r = LIBUSB_ERROR_IO;
+				goto err;
+			} else if (header->bLength > size) {
+				usbi_warn(ctx,
+					  "short extra intf desc read %d/%u",
+					  size, header->bLength);
+				return parsed;
+			}
+
+			/* If we find another "proper" descriptor then we're done */
+			if (header->bDescriptorType == LIBUSB_DT_INTERFACE ||
+			    header->bDescriptorType == LIBUSB_DT_ENDPOINT ||
+			    header->bDescriptorType == LIBUSB_DT_CONFIG ||
+			    header->bDescriptorType == LIBUSB_DT_DEVICE)
+				break;
+
+			buffer += header->bLength;
+			parsed += header->bLength;
+			size -= header->bLength;
+		}
+
+		/* Copy any unknown descriptors into a storage area for */
+		/*  drivers to later parse */
+		len = (int)(buffer - begin);
+		if (len > 0) {
+			void *extra = malloc((size_t)len);
+
+			if (!extra) {
+				r = LIBUSB_ERROR_NO_MEM;
+				goto err;
+			}
+
+			memcpy(extra, begin, len);
+			ifp->extra = extra;
+			ifp->extra_length = len;
+		}
+
+		if (ifp->bNumEndpoints > 0) {
+			struct libusb_endpoint_descriptor *endpoint;
+			uint8_t i;
+
+			endpoint = calloc(ifp->bNumEndpoints, sizeof(*endpoint));
+			if (!endpoint) {
+				r = LIBUSB_ERROR_NO_MEM;
+				goto err;
+			}
+
+			ifp->endpoint = endpoint;
+			for (i = 0; i < ifp->bNumEndpoints; i++) {
+				r = parse_endpoint(ctx, endpoint + i, buffer, size);
+				if (r < 0)
+					goto err;
+				if (r == 0) {
+					ifp->bNumEndpoints = i;
+					break;
+				}
+
+				buffer += r;
+				parsed += r;
+				size -= r;
+			}
+		}
+
+		/* We check to see if it's an alternate to this one */
+		if_desc = (const struct usbi_interface_descriptor *)buffer;
+		if (size < LIBUSB_DT_INTERFACE_SIZE ||
+		    if_desc->bDescriptorType != LIBUSB_DT_INTERFACE ||
+		    if_desc->bInterfaceNumber != interface_number)
+			return parsed;
+	}
+
+	return parsed;
+err:
+	clear_interface(usb_interface);
+	return r;
+}
+
+static void clear_configuration(struct libusb_config_descriptor *config)
+{
+	uint8_t i;
+
+	if (config->interface) {
+		for (i = 0; i < config->bNumInterfaces; i++)
+			clear_interface((struct libusb_interface *)
+					config->interface + i);
+	}
+	free((void *)config->interface);
+	free((void *)config->extra);
+}
+
+static int parse_configuration(struct libusb_context *ctx,
+	struct libusb_config_descriptor *config, const uint8_t *buffer, int size)
+{
+	uint8_t i;
+	int r;
+	const struct usbi_descriptor_header *header;
+	struct libusb_interface *usb_interface;
+
+	if (size < LIBUSB_DT_CONFIG_SIZE) {
+		usbi_err(ctx, "short config descriptor read %d/%d",
+			 size, LIBUSB_DT_CONFIG_SIZE);
+		return LIBUSB_ERROR_IO;
+	}
+
+	parse_descriptor(buffer, "bbwbbbbb", config);
+	if (config->bDescriptorType != LIBUSB_DT_CONFIG) {
+		usbi_err(ctx, "unexpected descriptor 0x%x (expected 0x%x)",
+			 config->bDescriptorType, LIBUSB_DT_CONFIG);
+		return LIBUSB_ERROR_IO;
+	} else if (config->bLength < LIBUSB_DT_CONFIG_SIZE) {
+		usbi_err(ctx, "invalid config bLength (%u)", config->bLength);
+		return LIBUSB_ERROR_IO;
+	} else if (config->bLength > size) {
+		usbi_err(ctx, "short config descriptor read %d/%u",
+			 size, config->bLength);
+		return LIBUSB_ERROR_IO;
+	} else if (config->bNumInterfaces > USB_MAXINTERFACES) {
+		usbi_err(ctx, "too many interfaces (%u)", config->bNumInterfaces);
+		return LIBUSB_ERROR_IO;
+	}
+
+	usb_interface = calloc(config->bNumInterfaces, sizeof(*usb_interface));
+	if (!usb_interface)
+		return LIBUSB_ERROR_NO_MEM;
+
+	config->interface = usb_interface;
+
+	buffer += config->bLength;
+	size -= config->bLength;
+
+	for (i = 0; i < config->bNumInterfaces; i++) {
+		int len;
+		const uint8_t *begin;
+
+		/* Skip over the rest of the Class Specific or Vendor */
+		/*  Specific descriptors */
+		begin = buffer;
+		while (size >= DESC_HEADER_LENGTH) {
+			header = (const struct usbi_descriptor_header *)buffer;
+			if (header->bLength < DESC_HEADER_LENGTH) {
+				usbi_err(ctx,
+					 "invalid extra config desc len (%u)",
+					 header->bLength);
+				r = LIBUSB_ERROR_IO;
+				goto err;
+			} else if (header->bLength > size) {
+				usbi_warn(ctx,
+					  "short extra config desc read %d/%u",
+					  size, header->bLength);
+				config->bNumInterfaces = i;
+				return size;
+			}
+
+			/* If we find another "proper" descriptor then we're done */
+			if (header->bDescriptorType == LIBUSB_DT_ENDPOINT ||
+			    header->bDescriptorType == LIBUSB_DT_INTERFACE ||
+			    header->bDescriptorType == LIBUSB_DT_CONFIG ||
+			    header->bDescriptorType == LIBUSB_DT_DEVICE)
+				break;
+
+			usbi_dbg(ctx, "skipping descriptor 0x%x", header->bDescriptorType);
+			buffer += header->bLength;
+			size -= header->bLength;
+		}
+
+		/* Copy any unknown descriptors into a storage area for */
+		/*  drivers to later parse */
+		len = (int)(buffer - begin);
+		if (len > 0) {
+			uint8_t *extra = realloc((void *)config->extra,
+						 (size_t)(config->extra_length + len));
+
+			if (!extra) {
+				r = LIBUSB_ERROR_NO_MEM;
+				goto err;
+			}
+
+			memcpy(extra + config->extra_length, begin, len);
+			config->extra = extra;
+			config->extra_length += len;
+		}
+
+		r = parse_interface(ctx, usb_interface + i, buffer, size);
+		if (r < 0)
+			goto err;
+		if (r == 0) {
+			config->bNumInterfaces = i;
+			break;
+		}
+
+		buffer += r;
+		size -= r;
+	}
+
+	return size;
+
+err:
+	clear_configuration(config);
+	return r;
+}
+
+static int raw_desc_to_config(struct libusb_context *ctx,
+	const uint8_t *buf, int size, struct libusb_config_descriptor **config)
+{
+	struct libusb_config_descriptor *_config = calloc(1, sizeof(*_config));
+	int r;
+
+	if (!_config)
+		return LIBUSB_ERROR_NO_MEM;
+
+	r = parse_configuration(ctx, _config, buf, size);
+	if (r < 0) {
+		usbi_err(ctx, "parse_configuration failed with error %d", r);
+		free(_config);
+		return r;
+	} else if (r > 0) {
+		usbi_warn(ctx, "still %d bytes of descriptor data left", r);
+	}
+
+	*config = _config;
+	return LIBUSB_SUCCESS;
+}
+
+static int get_active_config_descriptor(struct libusb_device *dev,
+	uint8_t *buffer, size_t size)
+{
+	int r = usbi_backend.get_active_config_descriptor(dev, buffer, size);
+
+	if (r < 0)
+		return r;
+
+	if (r < LIBUSB_DT_CONFIG_SIZE) {
+		usbi_err(DEVICE_CTX(dev), "short config descriptor read %d/%d",
+			 r, LIBUSB_DT_CONFIG_SIZE);
+		return LIBUSB_ERROR_IO;
+	} else if (r != (int)size) {
+		usbi_warn(DEVICE_CTX(dev), "short config descriptor read %d/%d",
+			 r, (int)size);
+	}
+
+	return r;
+}
+
+static int get_config_descriptor(struct libusb_device *dev, uint8_t config_idx,
+	uint8_t *buffer, size_t size)
+{
+	int r = usbi_backend.get_config_descriptor(dev, config_idx, buffer, size);
+
+	if (r < 0)
+		return r;
+	if (r < LIBUSB_DT_CONFIG_SIZE) {
+		usbi_err(DEVICE_CTX(dev), "short config descriptor read %d/%d",
+			 r, LIBUSB_DT_CONFIG_SIZE);
+		return LIBUSB_ERROR_IO;
+	} else if (r != (int)size) {
+		usbi_warn(DEVICE_CTX(dev), "short config descriptor read %d/%d",
+			 r, (int)size);
+	}
+
+	return r;
+}
+
+/** \ingroup libusb_desc
+ * Get the USB device descriptor for a given device.
+ *
+ * This is a non-blocking function; the device descriptor is cached in memory.
+ *
+ * Note since libusb-1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102, this
+ * function always succeeds.
+ *
+ * \param dev the device
+ * \param desc output location for the descriptor data
+ * \returns 0 on success or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_get_device_descriptor(libusb_device *dev,
+	struct libusb_device_descriptor *desc)
+{
+	usbi_dbg(DEVICE_CTX(dev), " ");
+	static_assert(sizeof(dev->device_descriptor) == LIBUSB_DT_DEVICE_SIZE,
+		      "struct libusb_device_descriptor is not expected size");
+	*desc = dev->device_descriptor;
+	return 0;
+}
+
+/** \ingroup libusb_desc
+ * Get the USB configuration descriptor for the currently active configuration.
+ * This is a non-blocking function which does not involve any requests being
+ * sent to the device.
+ *
+ * \param dev a device
+ * \param config output location for the USB configuration descriptor. Only
+ * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
+ * after use.
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the device is in unconfigured state
+ * \returns another LIBUSB_ERROR code on error
+ * \see libusb_get_config_descriptor
+ */
+int API_EXPORTED libusb_get_active_config_descriptor(libusb_device *dev,
+	struct libusb_config_descriptor **config)
+{
+	union usbi_config_desc_buf _config;
+	uint16_t config_len;
+	uint8_t *buf;
+	int r;
+
+	r = get_active_config_descriptor(dev, _config.buf, sizeof(_config.buf));
+	if (r < 0)
+		return r;
+
+	config_len = libusb_le16_to_cpu(_config.desc.wTotalLength);
+	buf = malloc(config_len);
+	if (!buf)
+		return LIBUSB_ERROR_NO_MEM;
+
+	r = get_active_config_descriptor(dev, buf, config_len);
+	if (r >= 0)
+		r = raw_desc_to_config(DEVICE_CTX(dev), buf, r, config);
+
+	free(buf);
+	return r;
+}
+
+/** \ingroup libusb_desc
+ * Get a USB configuration descriptor based on its index.
+ * This is a non-blocking function which does not involve any requests being
+ * sent to the device.
+ *
+ * \param dev a device
+ * \param config_index the index of the configuration you wish to retrieve
+ * \param config output location for the USB configuration descriptor. Only
+ * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
+ * after use.
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
+ * \returns another LIBUSB_ERROR code on error
+ * \see libusb_get_active_config_descriptor()
+ * \see libusb_get_config_descriptor_by_value()
+ */
+int API_EXPORTED libusb_get_config_descriptor(libusb_device *dev,
+	uint8_t config_index, struct libusb_config_descriptor **config)
+{
+	union usbi_config_desc_buf _config;
+	uint16_t config_len;
+	uint8_t *buf;
+	int r;
+
+	usbi_dbg(DEVICE_CTX(dev), "index %u", config_index);
+	if (config_index >= dev->device_descriptor.bNumConfigurations)
+		return LIBUSB_ERROR_NOT_FOUND;
+
+	r = get_config_descriptor(dev, config_index, _config.buf, sizeof(_config.buf));
+	if (r < 0)
+		return r;
+
+	config_len = libusb_le16_to_cpu(_config.desc.wTotalLength);
+	buf = malloc(config_len);
+	if (!buf)
+		return LIBUSB_ERROR_NO_MEM;
+
+	r = get_config_descriptor(dev, config_index, buf, config_len);
+	if (r >= 0)
+		r = raw_desc_to_config(DEVICE_CTX(dev), buf, r, config);
+
+	free(buf);
+	return r;
+}
+
+/** \ingroup libusb_desc
+ * Get a USB configuration descriptor with a specific bConfigurationValue.
+ * This is a non-blocking function which does not involve any requests being
+ * sent to the device.
+ *
+ * \param dev a device
+ * \param bConfigurationValue the bConfigurationValue of the configuration you
+ * wish to retrieve
+ * \param config output location for the USB configuration descriptor. Only
+ * valid if 0 was returned. Must be freed with libusb_free_config_descriptor()
+ * after use.
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
+ * \returns another LIBUSB_ERROR code on error
+ * \see libusb_get_active_config_descriptor()
+ * \see libusb_get_config_descriptor()
+ */
+int API_EXPORTED libusb_get_config_descriptor_by_value(libusb_device *dev,
+	uint8_t bConfigurationValue, struct libusb_config_descriptor **config)
+{
+	uint8_t idx;
+	int r;
+
+	if (usbi_backend.get_config_descriptor_by_value) {
+		void *buf;
+
+		r = usbi_backend.get_config_descriptor_by_value(dev,
+			bConfigurationValue, &buf);
+		if (r < 0)
+			return r;
+
+		return raw_desc_to_config(DEVICE_CTX(dev), buf, r, config);
+	}
+
+	usbi_dbg(DEVICE_CTX(dev), "value %u", bConfigurationValue);
+	for (idx = 0; idx < dev->device_descriptor.bNumConfigurations; idx++) {
+		union usbi_config_desc_buf _config;
+
+		r = get_config_descriptor(dev, idx, _config.buf, sizeof(_config.buf));
+		if (r < 0)
+			return r;
+
+		if (_config.desc.bConfigurationValue == bConfigurationValue)
+			return libusb_get_config_descriptor(dev, idx, config);
+	}
+
+	return LIBUSB_ERROR_NOT_FOUND;
+}
+
+/** \ingroup libusb_desc
+ * Free a configuration descriptor obtained from
+ * libusb_get_active_config_descriptor() or libusb_get_config_descriptor().
+ * It is safe to call this function with a NULL config parameter, in which
+ * case the function simply returns.
+ *
+ * \param config the configuration descriptor to free
+ */
+void API_EXPORTED libusb_free_config_descriptor(
+	struct libusb_config_descriptor *config)
+{
+	if (!config)
+		return;
+
+	clear_configuration(config);
+	free(config);
+}
+
+/** \ingroup libusb_desc
+ * Get an endpoints superspeed endpoint companion descriptor (if any)
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param endpoint endpoint descriptor from which to get the superspeed
+ * endpoint companion descriptor
+ * \param ep_comp output location for the superspeed endpoint companion
+ * descriptor. Only valid if 0 was returned. Must be freed with
+ * libusb_free_ss_endpoint_companion_descriptor() after use.
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
+ * \returns another LIBUSB_ERROR code on error
+ */
+int API_EXPORTED libusb_get_ss_endpoint_companion_descriptor(
+	libusb_context *ctx,
+	const struct libusb_endpoint_descriptor *endpoint,
+	struct libusb_ss_endpoint_companion_descriptor **ep_comp)
+{
+	struct usbi_descriptor_header *header;
+	const uint8_t *buffer = endpoint->extra;
+	int size = endpoint->extra_length;
+
+	*ep_comp = NULL;
+
+	while (size >= DESC_HEADER_LENGTH) {
+		header = (struct usbi_descriptor_header *)buffer;
+		if (header->bDescriptorType != LIBUSB_DT_SS_ENDPOINT_COMPANION) {
+			if (header->bLength < DESC_HEADER_LENGTH) {
+				usbi_err(ctx, "invalid descriptor length %u",
+					 header->bLength);
+				return LIBUSB_ERROR_IO;
+			}
+			buffer += header->bLength;
+			size -= header->bLength;
+			continue;
+		} else if (header->bLength < LIBUSB_DT_SS_ENDPOINT_COMPANION_SIZE) {
+			usbi_err(ctx, "invalid ss-ep-comp-desc length %u",
+				 header->bLength);
+			return LIBUSB_ERROR_IO;
+		} else if (header->bLength > size) {
+			usbi_err(ctx, "short ss-ep-comp-desc read %d/%u",
+				 size, header->bLength);
+			return LIBUSB_ERROR_IO;
+		}
+
+		*ep_comp = malloc(sizeof(**ep_comp));
+		if (!*ep_comp)
+			return LIBUSB_ERROR_NO_MEM;
+		parse_descriptor(buffer, "bbbbw", *ep_comp);
+		return LIBUSB_SUCCESS;
+	}
+	return LIBUSB_ERROR_NOT_FOUND;
+}
+
+/** \ingroup libusb_desc
+ * Free a superspeed endpoint companion descriptor obtained from
+ * libusb_get_ss_endpoint_companion_descriptor().
+ * It is safe to call this function with a NULL ep_comp parameter, in which
+ * case the function simply returns.
+ *
+ * \param ep_comp the superspeed endpoint companion descriptor to free
+ */
+void API_EXPORTED libusb_free_ss_endpoint_companion_descriptor(
+	struct libusb_ss_endpoint_companion_descriptor *ep_comp)
+{
+	free(ep_comp);
+}
+
+static int parse_bos(struct libusb_context *ctx,
+	struct libusb_bos_descriptor **bos,
+	const uint8_t *buffer, int size)
+{
+	struct libusb_bos_descriptor *_bos;
+	const struct usbi_bos_descriptor *bos_desc;
+	const struct usbi_descriptor_header *header;
+	uint8_t i;
+
+	if (size < LIBUSB_DT_BOS_SIZE) {
+		usbi_err(ctx, "short bos descriptor read %d/%d",
+			 size, LIBUSB_DT_BOS_SIZE);
+		return LIBUSB_ERROR_IO;
+	}
+
+	bos_desc = (const struct usbi_bos_descriptor *)buffer;
+	if (bos_desc->bDescriptorType != LIBUSB_DT_BOS) {
+		usbi_err(ctx, "unexpected descriptor 0x%x (expected 0x%x)",
+			 bos_desc->bDescriptorType, LIBUSB_DT_BOS);
+		return LIBUSB_ERROR_IO;
+	} else if (bos_desc->bLength < LIBUSB_DT_BOS_SIZE) {
+		usbi_err(ctx, "invalid bos bLength (%u)", bos_desc->bLength);
+		return LIBUSB_ERROR_IO;
+	} else if (bos_desc->bLength > size) {
+		usbi_err(ctx, "short bos descriptor read %d/%u",
+			 size, bos_desc->bLength);
+		return LIBUSB_ERROR_IO;
+	}
+
+	_bos = calloc(1, sizeof(*_bos) + bos_desc->bNumDeviceCaps * sizeof(void *));
+	if (!_bos)
+		return LIBUSB_ERROR_NO_MEM;
+
+	parse_descriptor(buffer, "bbwb", _bos);
+	buffer += _bos->bLength;
+	size -= _bos->bLength;
+
+	/* Get the device capability descriptors */
+	for (i = 0; i < _bos->bNumDeviceCaps; i++) {
+		if (size < LIBUSB_DT_DEVICE_CAPABILITY_SIZE) {
+			usbi_warn(ctx, "short dev-cap descriptor read %d/%d",
+				  size, LIBUSB_DT_DEVICE_CAPABILITY_SIZE);
+			break;
+		}
+		header = (const struct usbi_descriptor_header *)buffer;
+		if (header->bDescriptorType != LIBUSB_DT_DEVICE_CAPABILITY) {
+			usbi_warn(ctx, "unexpected descriptor 0x%x (expected 0x%x)",
+				  header->bDescriptorType, LIBUSB_DT_DEVICE_CAPABILITY);
+			break;
+		} else if (header->bLength < LIBUSB_DT_DEVICE_CAPABILITY_SIZE) {
+			usbi_err(ctx, "invalid dev-cap bLength (%u)",
+				 header->bLength);
+			libusb_free_bos_descriptor(_bos);
+			return LIBUSB_ERROR_IO;
+		} else if (header->bLength > size) {
+			usbi_warn(ctx, "short dev-cap descriptor read %d/%u",
+				  size, header->bLength);
+			break;
+		}
+
+		_bos->dev_capability[i] = malloc(header->bLength);
+		if (!_bos->dev_capability[i]) {
+			libusb_free_bos_descriptor(_bos);
+			return LIBUSB_ERROR_NO_MEM;
+		}
+		memcpy(_bos->dev_capability[i], buffer, header->bLength);
+		buffer += header->bLength;
+		size -= header->bLength;
+	}
+	_bos->bNumDeviceCaps = i;
+	*bos = _bos;
+
+	return LIBUSB_SUCCESS;
+}
+
+/** \ingroup libusb_desc
+ * Get a Binary Object Store (BOS) descriptor
+ * This is a BLOCKING function, which will send requests to the device.
+ *
+ * \param dev_handle the handle of an open libusb device
+ * \param bos output location for the BOS descriptor. Only valid if 0 was returned.
+ * Must be freed with \ref libusb_free_bos_descriptor() after use.
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the device doesn't have a BOS descriptor
+ * \returns another LIBUSB_ERROR code on error
+ */
+int API_EXPORTED libusb_get_bos_descriptor(libusb_device_handle *dev_handle,
+	struct libusb_bos_descriptor **bos)
+{
+	union usbi_bos_desc_buf _bos;
+	uint16_t bos_len;
+	uint8_t *bos_data;
+	int r;
+	struct libusb_context *ctx = HANDLE_CTX(dev_handle);
+
+	/* Read the BOS. This generates 2 requests on the bus,
+	 * one for the header, and one for the full BOS */
+	r = libusb_get_descriptor(dev_handle, LIBUSB_DT_BOS, 0, _bos.buf, sizeof(_bos.buf));
+	if (r < 0) {
+		if (r != LIBUSB_ERROR_PIPE)
+			usbi_err(ctx, "failed to read BOS (%d)", r);
+		return r;
+	}
+	if (r < LIBUSB_DT_BOS_SIZE) {
+		usbi_err(ctx, "short BOS read %d/%d",
+			 r, LIBUSB_DT_BOS_SIZE);
+		return LIBUSB_ERROR_IO;
+	}
+
+	bos_len = libusb_le16_to_cpu(_bos.desc.wTotalLength);
+	usbi_dbg(ctx, "found BOS descriptor: size %u bytes, %u capabilities",
+		 bos_len, _bos.desc.bNumDeviceCaps);
+	bos_data = calloc(1, bos_len);
+	if (!bos_data)
+		return LIBUSB_ERROR_NO_MEM;
+
+	r = libusb_get_descriptor(dev_handle, LIBUSB_DT_BOS, 0, bos_data, bos_len);
+	if (r >= 0) {
+		if (r != (int)bos_len)
+			usbi_warn(ctx, "short BOS read %d/%u", r, bos_len);
+		r = parse_bos(HANDLE_CTX(dev_handle), bos, bos_data, r);
+	} else {
+		usbi_err(ctx, "failed to read BOS (%d)", r);
+	}
+
+	free(bos_data);
+	return r;
+}
+
+/** \ingroup libusb_desc
+ * Free a BOS descriptor obtained from libusb_get_bos_descriptor().
+ * It is safe to call this function with a NULL bos parameter, in which
+ * case the function simply returns.
+ *
+ * \param bos the BOS descriptor to free
+ */
+void API_EXPORTED libusb_free_bos_descriptor(struct libusb_bos_descriptor *bos)
+{
+	uint8_t i;
+
+	if (!bos)
+		return;
+
+	for (i = 0; i < bos->bNumDeviceCaps; i++)
+		free(bos->dev_capability[i]);
+	free(bos);
+}
+
+/** \ingroup libusb_desc
+ * Get an USB 2.0 Extension descriptor
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param dev_cap Device Capability descriptor with a bDevCapabilityType of
+ * \ref libusb_capability_type::LIBUSB_BT_USB_2_0_EXTENSION
+ * LIBUSB_BT_USB_2_0_EXTENSION
+ * \param usb_2_0_extension output location for the USB 2.0 Extension
+ * descriptor. Only valid if 0 was returned. Must be freed with
+ * libusb_free_usb_2_0_extension_descriptor() after use.
+ * \returns 0 on success
+ * \returns a LIBUSB_ERROR code on error
+ */
+int API_EXPORTED libusb_get_usb_2_0_extension_descriptor(
+	libusb_context *ctx,
+	struct libusb_bos_dev_capability_descriptor *dev_cap,
+	struct libusb_usb_2_0_extension_descriptor **usb_2_0_extension)
+{
+	struct libusb_usb_2_0_extension_descriptor *_usb_2_0_extension;
+
+	if (dev_cap->bDevCapabilityType != LIBUSB_BT_USB_2_0_EXTENSION) {
+		usbi_err(ctx, "unexpected bDevCapabilityType 0x%x (expected 0x%x)",
+			 dev_cap->bDevCapabilityType,
+			 LIBUSB_BT_USB_2_0_EXTENSION);
+		return LIBUSB_ERROR_INVALID_PARAM;
+	} else if (dev_cap->bLength < LIBUSB_BT_USB_2_0_EXTENSION_SIZE) {
+		usbi_err(ctx, "short dev-cap descriptor read %u/%d",
+			 dev_cap->bLength, LIBUSB_BT_USB_2_0_EXTENSION_SIZE);
+		return LIBUSB_ERROR_IO;
+	}
+
+	_usb_2_0_extension = malloc(sizeof(*_usb_2_0_extension));
+	if (!_usb_2_0_extension)
+		return LIBUSB_ERROR_NO_MEM;
+
+	parse_descriptor(dev_cap, "bbbd", _usb_2_0_extension);
+
+	*usb_2_0_extension = _usb_2_0_extension;
+	return LIBUSB_SUCCESS;
+}
+
+/** \ingroup libusb_desc
+ * Free a USB 2.0 Extension descriptor obtained from
+ * libusb_get_usb_2_0_extension_descriptor().
+ * It is safe to call this function with a NULL usb_2_0_extension parameter,
+ * in which case the function simply returns.
+ *
+ * \param usb_2_0_extension the USB 2.0 Extension descriptor to free
+ */
+void API_EXPORTED libusb_free_usb_2_0_extension_descriptor(
+	struct libusb_usb_2_0_extension_descriptor *usb_2_0_extension)
+{
+	free(usb_2_0_extension);
+}
+
+/** \ingroup libusb_desc
+ * Get a SuperSpeed USB Device Capability descriptor
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param dev_cap Device Capability descriptor with a bDevCapabilityType of
+ * \ref libusb_capability_type::LIBUSB_BT_SS_USB_DEVICE_CAPABILITY
+ * LIBUSB_BT_SS_USB_DEVICE_CAPABILITY
+ * \param ss_usb_device_cap output location for the SuperSpeed USB Device
+ * Capability descriptor. Only valid if 0 was returned. Must be freed with
+ * libusb_free_ss_usb_device_capability_descriptor() after use.
+ * \returns 0 on success
+ * \returns a LIBUSB_ERROR code on error
+ */
+int API_EXPORTED libusb_get_ss_usb_device_capability_descriptor(
+	libusb_context *ctx,
+	struct libusb_bos_dev_capability_descriptor *dev_cap,
+	struct libusb_ss_usb_device_capability_descriptor **ss_usb_device_cap)
+{
+	struct libusb_ss_usb_device_capability_descriptor *_ss_usb_device_cap;
+
+	if (dev_cap->bDevCapabilityType != LIBUSB_BT_SS_USB_DEVICE_CAPABILITY) {
+		usbi_err(ctx, "unexpected bDevCapabilityType 0x%x (expected 0x%x)",
+			 dev_cap->bDevCapabilityType,
+			 LIBUSB_BT_SS_USB_DEVICE_CAPABILITY);
+		return LIBUSB_ERROR_INVALID_PARAM;
+	} else if (dev_cap->bLength < LIBUSB_BT_SS_USB_DEVICE_CAPABILITY_SIZE) {
+		usbi_err(ctx, "short dev-cap descriptor read %u/%d",
+			 dev_cap->bLength, LIBUSB_BT_SS_USB_DEVICE_CAPABILITY_SIZE);
+		return LIBUSB_ERROR_IO;
+	}
+
+	_ss_usb_device_cap = malloc(sizeof(*_ss_usb_device_cap));
+	if (!_ss_usb_device_cap)
+		return LIBUSB_ERROR_NO_MEM;
+
+	parse_descriptor(dev_cap, "bbbbwbbw", _ss_usb_device_cap);
+
+	*ss_usb_device_cap = _ss_usb_device_cap;
+	return LIBUSB_SUCCESS;
+}
+
+/** \ingroup libusb_desc
+ * Free a SuperSpeed USB Device Capability descriptor obtained from
+ * libusb_get_ss_usb_device_capability_descriptor().
+ * It is safe to call this function with a NULL ss_usb_device_cap
+ * parameter, in which case the function simply returns.
+ *
+ * \param ss_usb_device_cap the SuperSpeed USB Device Capability descriptor
+ * to free
+ */
+void API_EXPORTED libusb_free_ss_usb_device_capability_descriptor(
+	struct libusb_ss_usb_device_capability_descriptor *ss_usb_device_cap)
+{
+	free(ss_usb_device_cap);
+}
+
+/** \ingroup libusb_desc
+ * Get a Container ID descriptor
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param dev_cap Device Capability descriptor with a bDevCapabilityType of
+ * \ref libusb_capability_type::LIBUSB_BT_CONTAINER_ID
+ * LIBUSB_BT_CONTAINER_ID
+ * \param container_id output location for the Container ID descriptor.
+ * Only valid if 0 was returned. Must be freed with
+ * libusb_free_container_id_descriptor() after use.
+ * \returns 0 on success
+ * \returns a LIBUSB_ERROR code on error
+ */
+int API_EXPORTED libusb_get_container_id_descriptor(libusb_context *ctx,
+	struct libusb_bos_dev_capability_descriptor *dev_cap,
+	struct libusb_container_id_descriptor **container_id)
+{
+	struct libusb_container_id_descriptor *_container_id;
+
+	if (dev_cap->bDevCapabilityType != LIBUSB_BT_CONTAINER_ID) {
+		usbi_err(ctx, "unexpected bDevCapabilityType 0x%x (expected 0x%x)",
+			 dev_cap->bDevCapabilityType,
+			 LIBUSB_BT_CONTAINER_ID);
+		return LIBUSB_ERROR_INVALID_PARAM;
+	} else if (dev_cap->bLength < LIBUSB_BT_CONTAINER_ID_SIZE) {
+		usbi_err(ctx, "short dev-cap descriptor read %u/%d",
+			 dev_cap->bLength, LIBUSB_BT_CONTAINER_ID_SIZE);
+		return LIBUSB_ERROR_IO;
+	}
+
+	_container_id = malloc(sizeof(*_container_id));
+	if (!_container_id)
+		return LIBUSB_ERROR_NO_MEM;
+
+	parse_descriptor(dev_cap, "bbbbu", _container_id);
+
+	*container_id = _container_id;
+	return LIBUSB_SUCCESS;
+}
+
+/** \ingroup libusb_desc
+ * Free a Container ID descriptor obtained from
+ * libusb_get_container_id_descriptor().
+ * It is safe to call this function with a NULL container_id parameter,
+ * in which case the function simply returns.
+ *
+ * \param container_id the Container ID descriptor to free
+ */
+void API_EXPORTED libusb_free_container_id_descriptor(
+	struct libusb_container_id_descriptor *container_id)
+{
+	free(container_id);
+}
+
+/** \ingroup libusb_desc
+ * Retrieve a string descriptor in C style ASCII.
+ *
+ * Wrapper around libusb_get_string_descriptor(). Uses the first language
+ * supported by the device.
+ *
+ * \param dev_handle a device handle
+ * \param desc_index the index of the descriptor to retrieve
+ * \param data output buffer for ASCII string descriptor
+ * \param length size of data buffer
+ * \returns number of bytes returned in data, or LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_get_string_descriptor_ascii(libusb_device_handle *dev_handle,
+	uint8_t desc_index, unsigned char *data, int length)
+{
+	union usbi_string_desc_buf str;
+	int r, si, di;
+	uint16_t langid, wdata;
+
+	/* Asking for the zero'th index is special - it returns a string
+	 * descriptor that contains all the language IDs supported by the
+	 * device. Typically there aren't many - often only one. Language
+	 * IDs are 16 bit numbers, and they start at the third byte in the
+	 * descriptor. There's also no point in trying to read descriptor 0
+	 * with this function. See USB 2.0 specification section 9.6.7 for
+	 * more information.
+	 */
+
+	if (desc_index == 0)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	r = libusb_get_string_descriptor(dev_handle, 0, 0, str.buf, 4);
+	if (r < 0)
+		return r;
+	else if (r != 4 || str.desc.bLength < 4)
+		return LIBUSB_ERROR_IO;
+	else if (str.desc.bDescriptorType != LIBUSB_DT_STRING)
+		return LIBUSB_ERROR_IO;
+	else if (str.desc.bLength & 1)
+		usbi_warn(HANDLE_CTX(dev_handle), "suspicious bLength %u for language ID string descriptor", str.desc.bLength);
+
+	langid = libusb_le16_to_cpu(str.desc.wData[0]);
+	r = libusb_get_string_descriptor(dev_handle, desc_index, langid, str.buf, sizeof(str.buf));
+	if (r < 0)
+		return r;
+	else if (r < DESC_HEADER_LENGTH || str.desc.bLength > r)
+		return LIBUSB_ERROR_IO;
+	else if (str.desc.bDescriptorType != LIBUSB_DT_STRING)
+		return LIBUSB_ERROR_IO;
+	else if ((str.desc.bLength & 1) || str.desc.bLength != r)
+		usbi_warn(HANDLE_CTX(dev_handle), "suspicious bLength %u for string descriptor (read %d)", str.desc.bLength, r);
+
+	di = 0;
+	for (si = 2; si < str.desc.bLength; si += 2) {
+		if (di >= (length - 1))
+			break;
+
+		wdata = libusb_le16_to_cpu(str.desc.wData[di]);
+		if (wdata < 0x80)
+			data[di++] = (unsigned char)wdata;
+		else
+			data[di++] = '?'; /* non-ASCII */
+	}
+
+	data[di] = 0;
+	return di;
+}
diff --git a/hardware/src/libusb/hotplug.c b/hardware/src/libusb/hotplug.c
new file mode 100644
index 0000000..402e693
--- /dev/null
+++ b/hardware/src/libusb/hotplug.c
@@ -0,0 +1,466 @@
+/* -*- Mode: C; indent-tabs-mode:t ; c-basic-offset:8 -*- */
+/*
+ * Hotplug functions for libusb
+ * Copyright © 2012-2021 Nathan Hjelm <hjelmn@mac.com>
+ * Copyright © 2012-2013 Peter Stuge <peter@stuge.se>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+
+/**
+ * @defgroup libusb_hotplug Device hotplug event notification
+ * This page details how to use the libusb hotplug interface, where available.
+ *
+ * Be mindful that not all platforms currently implement hotplug notification and
+ * that you should first call on \ref libusb_has_capability() with parameter
+ * \ref LIBUSB_CAP_HAS_HOTPLUG to confirm that hotplug support is available.
+ *
+ * \page libusb_hotplug Device hotplug event notification
+ *
+ * \section hotplug_intro Introduction
+ *
+ * Version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102, has added support
+ * for hotplug events on <b>some</b> platforms (you should test if your platform
+ * supports hotplug notification by calling \ref libusb_has_capability() with
+ * parameter \ref LIBUSB_CAP_HAS_HOTPLUG).
+ *
+ * This interface allows you to request notification for the arrival and departure
+ * of matching USB devices.
+ *
+ * To receive hotplug notification you register a callback by calling
+ * \ref libusb_hotplug_register_callback(). This function will optionally return
+ * a callback handle that can be passed to \ref libusb_hotplug_deregister_callback().
+ *
+ * A callback function must return an int (0 or 1) indicating whether the callback is
+ * expecting additional events. Returning 0 will rearm the callback and 1 will cause
+ * the callback to be deregistered. Note that when callbacks are called from
+ * libusb_hotplug_register_callback() because of the \ref LIBUSB_HOTPLUG_ENUMERATE
+ * flag, the callback return value is ignored. In other words, you cannot cause a
+ * callback to be deregistered by returning 1 when it is called from
+ * libusb_hotplug_register_callback().
+ *
+ * Callbacks for a particular context are automatically deregistered by libusb_exit().
+ *
+ * As of 1.0.16 there are two supported hotplug events:
+ *  - LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED: A device has arrived and is ready to use
+ *  - LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT: A device has left and is no longer available
+ *
+ * A hotplug event can listen for either or both of these events.
+ *
+ * Note: If you receive notification that a device has left and you have any
+ * libusb_device_handles for the device it is up to you to call libusb_close()
+ * on each device handle to free up any remaining resources associated with the device.
+ * Once a device has left any libusb_device_handle associated with the device
+ * are invalid and will remain so even if the device comes back.
+ *
+ * When handling a LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED event it is considered
+ * safe to call any libusb function that takes a libusb_device. It also safe to
+ * open a device and submit asynchronous transfers. However, most other functions
+ * that take a libusb_device_handle are <b>not</b> safe to call. Examples of such
+ * functions are any of the \ref libusb_syncio "synchronous API" functions or the blocking
+ * functions that retrieve various \ref libusb_desc "USB descriptors". These functions must
+ * be used outside of the context of the hotplug callback.
+ *
+ * When handling a LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT event the only safe function
+ * is libusb_get_device_descriptor().
+ *
+ * The following code provides an example of the usage of the hotplug interface:
+\code
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <libusb.h>
+
+static int count = 0;
+
+int hotplug_callback(struct libusb_context *ctx, struct libusb_device *dev,
+                     libusb_hotplug_event event, void *user_data) {
+  static libusb_device_handle *dev_handle = NULL;
+  struct libusb_device_descriptor desc;
+  int rc;
+
+  (void)libusb_get_device_descriptor(dev, &desc);
+
+  if (LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED == event) {
+    rc = libusb_open(dev, &dev_handle);
+    if (LIBUSB_SUCCESS != rc) {
+      printf("Could not open USB device\n");
+    }
+  } else if (LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT == event) {
+    if (dev_handle) {
+      libusb_close(dev_handle);
+      dev_handle = NULL;
+    }
+  } else {
+    printf("Unhandled event %d\n", event);
+  }
+  count++;
+
+  return 0;
+}
+
+int main (void) {
+  libusb_hotplug_callback_handle callback_handle;
+  int rc;
+
+  libusb_init(NULL);
+
+  rc = libusb_hotplug_register_callback(NULL, LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED |
+                                        LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT, 0, 0x045a, 0x5005,
+                                        LIBUSB_HOTPLUG_MATCH_ANY, hotplug_callback, NULL,
+                                        &callback_handle);
+  if (LIBUSB_SUCCESS != rc) {
+    printf("Error creating a hotplug callback\n");
+    libusb_exit(NULL);
+    return EXIT_FAILURE;
+  }
+
+  while (count < 2) {
+    libusb_handle_events_completed(NULL, NULL);
+    nanosleep(&(struct timespec){0, 10000000UL}, NULL);
+  }
+
+  libusb_hotplug_deregister_callback(NULL, callback_handle);
+  libusb_exit(NULL);
+
+  return 0;
+}
+\endcode
+ */
+
+#define VALID_HOTPLUG_EVENTS			\
+	(LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED |	\
+	 LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT)
+
+#define VALID_HOTPLUG_FLAGS			\
+	(LIBUSB_HOTPLUG_ENUMERATE)
+
+void usbi_hotplug_init(struct libusb_context *ctx)
+{
+	/* check for hotplug support */
+	if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG))
+		return;
+
+	usbi_mutex_init(&ctx->hotplug_cbs_lock);
+	list_init(&ctx->hotplug_cbs);
+	ctx->next_hotplug_cb_handle = 1;
+	usbi_atomic_store(&ctx->hotplug_ready, 1);
+}
+
+void usbi_hotplug_exit(struct libusb_context *ctx)
+{
+	struct usbi_hotplug_callback *hotplug_cb, *next_cb;
+	struct usbi_hotplug_message *msg;
+	struct libusb_device *dev, *next_dev;
+
+	/* check for hotplug support */
+	if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG))
+		return;
+
+	/* free all registered hotplug callbacks */
+	for_each_hotplug_cb_safe(ctx, hotplug_cb, next_cb) {
+		list_del(&hotplug_cb->list);
+		free(hotplug_cb);
+	}
+
+	/* free all pending hotplug messages */
+	while (!list_empty(&ctx->hotplug_msgs)) {
+		msg = list_first_entry(&ctx->hotplug_msgs, struct usbi_hotplug_message, list);
+
+		/* if the device left, the message holds a reference
+		 * and we must drop it */
+		if (msg->event == LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT)
+			libusb_unref_device(msg->device);
+
+		list_del(&msg->list);
+		free(msg);
+	}
+
+	/* free all discovered devices. due to parent references loop until no devices are freed. */
+	for_each_device_safe(ctx, dev, next_dev) {
+		/* remove the device from the usb_devs list only if there are no
+		 * references held, otherwise leave it on the list so that a
+		 * warning message will be shown */
+		if (usbi_atomic_load(&dev->refcnt) == 1) {
+			list_del(&dev->list);
+		}
+		if (dev->parent_dev && usbi_atomic_load(&dev->parent_dev->refcnt) == 1) {
+			/* the parent was before this device in the list and will be released.
+			   remove it from the list. this is safe as parent_dev can not be
+			   equal to next_dev. */
+			assert (dev->parent_dev != next_dev);
+			list_del(&dev->parent_dev->list);
+		}
+		libusb_unref_device(dev);
+	}
+
+	usbi_mutex_destroy(&ctx->hotplug_cbs_lock);
+}
+
+static int usbi_hotplug_match_cb(struct libusb_device *dev,
+	libusb_hotplug_event event, struct usbi_hotplug_callback *hotplug_cb)
+{
+	if (!(hotplug_cb->flags & event)) {
+		return 0;
+	}
+
+	if ((hotplug_cb->flags & USBI_HOTPLUG_VENDOR_ID_VALID) &&
+	    hotplug_cb->vendor_id != dev->device_descriptor.idVendor) {
+		return 0;
+	}
+
+	if ((hotplug_cb->flags & USBI_HOTPLUG_PRODUCT_ID_VALID) &&
+	    hotplug_cb->product_id != dev->device_descriptor.idProduct) {
+		return 0;
+	}
+
+	if ((hotplug_cb->flags & USBI_HOTPLUG_DEV_CLASS_VALID) &&
+	    hotplug_cb->dev_class != dev->device_descriptor.bDeviceClass) {
+		return 0;
+	}
+
+	return hotplug_cb->cb(DEVICE_CTX(dev), dev, event, hotplug_cb->user_data);
+}
+
+void usbi_hotplug_notification(struct libusb_context *ctx, struct libusb_device *dev,
+	libusb_hotplug_event event)
+{
+	struct usbi_hotplug_message *msg;
+	unsigned int event_flags;
+
+	/* Only generate a notification if hotplug is ready. This prevents hotplug
+	 * notifications from being generated during initial enumeration or if the
+	 * backend does not support hotplug. */
+	if (!usbi_atomic_load(&ctx->hotplug_ready))
+		return;
+
+	msg = calloc(1, sizeof(*msg));
+	if (!msg) {
+		usbi_err(ctx, "error allocating hotplug message");
+		return;
+	}
+
+	msg->event = event;
+	msg->device = dev;
+
+	/* Take the event data lock and add this message to the list.
+	 * Only signal an event if there are no prior pending events. */
+	usbi_mutex_lock(&ctx->event_data_lock);
+	event_flags = ctx->event_flags;
+	ctx->event_flags |= USBI_EVENT_HOTPLUG_MSG_PENDING;
+	list_add_tail(&msg->list, &ctx->hotplug_msgs);
+	if (!event_flags)
+		usbi_signal_event(&ctx->event);
+	usbi_mutex_unlock(&ctx->event_data_lock);
+}
+
+void usbi_hotplug_process(struct libusb_context *ctx, struct list_head *hotplug_msgs)
+{
+	struct usbi_hotplug_callback *hotplug_cb, *next_cb;
+	struct usbi_hotplug_message *msg;
+	int r;
+
+	usbi_mutex_lock(&ctx->hotplug_cbs_lock);
+
+	/* dispatch all pending hotplug messages */
+	while (!list_empty(hotplug_msgs)) {
+		msg = list_first_entry(hotplug_msgs, struct usbi_hotplug_message, list);
+
+		for_each_hotplug_cb_safe(ctx, hotplug_cb, next_cb) {
+			/* skip callbacks that have unregistered */
+			if (hotplug_cb->flags & USBI_HOTPLUG_NEEDS_FREE)
+				continue;
+
+			usbi_mutex_unlock(&ctx->hotplug_cbs_lock);
+			r = usbi_hotplug_match_cb(msg->device, msg->event, hotplug_cb);
+			usbi_mutex_lock(&ctx->hotplug_cbs_lock);
+
+			if (r) {
+				list_del(&hotplug_cb->list);
+				free(hotplug_cb);
+			}
+		}
+
+		/* if the device left, the message holds a reference
+		 * and we must drop it */
+		if (msg->event == LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT)
+			libusb_unref_device(msg->device);
+
+		list_del(&msg->list);
+		free(msg);
+	}
+
+	/* free any callbacks that have unregistered */
+	for_each_hotplug_cb_safe(ctx, hotplug_cb, next_cb) {
+		if (hotplug_cb->flags & USBI_HOTPLUG_NEEDS_FREE) {
+			usbi_dbg(ctx, "freeing hotplug cb %p with handle %d",
+				hotplug_cb, hotplug_cb->handle);
+			list_del(&hotplug_cb->list);
+			free(hotplug_cb);
+		}
+	}
+
+	usbi_mutex_unlock(&ctx->hotplug_cbs_lock);
+}
+
+int API_EXPORTED libusb_hotplug_register_callback(libusb_context *ctx,
+	int events, int flags,
+	int vendor_id, int product_id, int dev_class,
+	libusb_hotplug_callback_fn cb_fn, void *user_data,
+	libusb_hotplug_callback_handle *callback_handle)
+{
+	struct usbi_hotplug_callback *hotplug_cb;
+
+	/* check for sane values */
+	if (!events || (~VALID_HOTPLUG_EVENTS & events) ||
+	    (~VALID_HOTPLUG_FLAGS & flags) ||
+	    (LIBUSB_HOTPLUG_MATCH_ANY != vendor_id && (~0xffff & vendor_id)) ||
+	    (LIBUSB_HOTPLUG_MATCH_ANY != product_id && (~0xffff & product_id)) ||
+	    (LIBUSB_HOTPLUG_MATCH_ANY != dev_class && (~0xff & dev_class)) ||
+	    !cb_fn) {
+		return LIBUSB_ERROR_INVALID_PARAM;
+	}
+
+	/* check for hotplug support */
+	if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG))
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+
+	ctx = usbi_get_context(ctx);
+
+	hotplug_cb = calloc(1, sizeof(*hotplug_cb));
+	if (!hotplug_cb)
+		return LIBUSB_ERROR_NO_MEM;
+
+	hotplug_cb->flags = (uint8_t)events;
+	if (LIBUSB_HOTPLUG_MATCH_ANY != vendor_id) {
+		hotplug_cb->flags |= USBI_HOTPLUG_VENDOR_ID_VALID;
+		hotplug_cb->vendor_id = (uint16_t)vendor_id;
+	}
+	if (LIBUSB_HOTPLUG_MATCH_ANY != product_id) {
+		hotplug_cb->flags |= USBI_HOTPLUG_PRODUCT_ID_VALID;
+		hotplug_cb->product_id = (uint16_t)product_id;
+	}
+	if (LIBUSB_HOTPLUG_MATCH_ANY != dev_class) {
+		hotplug_cb->flags |= USBI_HOTPLUG_DEV_CLASS_VALID;
+		hotplug_cb->dev_class = (uint8_t)dev_class;
+	}
+	hotplug_cb->cb = cb_fn;
+	hotplug_cb->user_data = user_data;
+
+	usbi_mutex_lock(&ctx->hotplug_cbs_lock);
+
+	/* protect the handle by the context hotplug lock */
+	hotplug_cb->handle = ctx->next_hotplug_cb_handle++;
+
+	/* handle the unlikely case of overflow */
+	if (ctx->next_hotplug_cb_handle < 0)
+		ctx->next_hotplug_cb_handle = 1;
+
+	list_add(&hotplug_cb->list, &ctx->hotplug_cbs);
+
+	usbi_mutex_unlock(&ctx->hotplug_cbs_lock);
+
+	usbi_dbg(ctx, "new hotplug cb %p with handle %d", hotplug_cb, hotplug_cb->handle);
+
+	if ((flags & LIBUSB_HOTPLUG_ENUMERATE) && (events & LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED)) {
+		ssize_t i, len;
+		struct libusb_device **devs;
+
+		len = libusb_get_device_list(ctx, &devs);
+		if (len < 0) {
+			libusb_hotplug_deregister_callback(ctx, hotplug_cb->handle);
+			return (int)len;
+		}
+
+		for (i = 0; i < len; i++) {
+			usbi_hotplug_match_cb(devs[i],
+					LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED,
+					hotplug_cb);
+		}
+
+		libusb_free_device_list(devs, 1);
+	}
+
+	if (callback_handle)
+		*callback_handle = hotplug_cb->handle;
+
+	return LIBUSB_SUCCESS;
+}
+
+void API_EXPORTED libusb_hotplug_deregister_callback(libusb_context *ctx,
+	libusb_hotplug_callback_handle callback_handle)
+{
+	struct usbi_hotplug_callback *hotplug_cb;
+	int deregistered = 0;
+
+	/* check for hotplug support */
+	if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG))
+		return;
+
+	usbi_dbg(ctx, "deregister hotplug cb %d", callback_handle);
+
+	ctx = usbi_get_context(ctx);
+
+	usbi_mutex_lock(&ctx->hotplug_cbs_lock);
+	for_each_hotplug_cb(ctx, hotplug_cb) {
+		if (callback_handle == hotplug_cb->handle) {
+			/* mark this callback for deregistration */
+			hotplug_cb->flags |= USBI_HOTPLUG_NEEDS_FREE;
+			deregistered = 1;
+			break;
+		}
+	}
+	usbi_mutex_unlock(&ctx->hotplug_cbs_lock);
+
+	if (deregistered) {
+		unsigned int event_flags;
+
+		usbi_mutex_lock(&ctx->event_data_lock);
+		event_flags = ctx->event_flags;
+		ctx->event_flags |= USBI_EVENT_HOTPLUG_CB_DEREGISTERED;
+		if (!event_flags)
+			usbi_signal_event(&ctx->event);
+		usbi_mutex_unlock(&ctx->event_data_lock);
+	}
+}
+
+DEFAULT_VISIBILITY
+void * LIBUSB_CALL libusb_hotplug_get_user_data(libusb_context *ctx,
+	libusb_hotplug_callback_handle callback_handle)
+{
+	struct usbi_hotplug_callback *hotplug_cb;
+	void *user_data = NULL;
+
+	/* check for hotplug support */
+	if (!libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG))
+		return NULL;
+
+	usbi_dbg(ctx, "get hotplug cb %d user data", callback_handle);
+
+	ctx = usbi_get_context(ctx);
+
+	usbi_mutex_lock(&ctx->hotplug_cbs_lock);
+	for_each_hotplug_cb(ctx, hotplug_cb) {
+		if (callback_handle == hotplug_cb->handle) {
+			user_data = hotplug_cb->user_data;
+			break;
+		}
+	}
+	usbi_mutex_unlock(&ctx->hotplug_cbs_lock);
+
+	return user_data;
+}
diff --git a/hardware/src/libusb/io.c b/hardware/src/libusb/io.c
new file mode 100644
index 0000000..0d2ac9e
--- /dev/null
+++ b/hardware/src/libusb/io.c
@@ -0,0 +1,2829 @@
+/* -*- Mode: C; indent-tabs-mode:t ; c-basic-offset:8 -*- */
+/*
+ * I/O functions for libusb
+ * Copyright © 2007-2009 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ * Copyright © 2019 Nathan Hjelm <hjelmn@cs.umm.edu>
+ * Copyright © 2019 Google LLC. All rights reserved.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+
+/**
+ * \page libusb_io Synchronous and asynchronous device I/O
+ *
+ * \section io_intro Introduction
+ *
+ * If you're using libusb in your application, you're probably wanting to
+ * perform I/O with devices - you want to perform USB data transfers.
+ *
+ * libusb offers two separate interfaces for device I/O. This page aims to
+ * introduce the two in order to help you decide which one is more suitable
+ * for your application. You can also choose to use both interfaces in your
+ * application by considering each transfer on a case-by-case basis.
+ *
+ * Once you have read through the following discussion, you should consult the
+ * detailed API documentation pages for the details:
+ * - \ref libusb_syncio
+ * - \ref libusb_asyncio
+ *
+ * \section theory Transfers at a logical level
+ *
+ * At a logical level, USB transfers typically happen in two parts. For
+ * example, when reading data from a endpoint:
+ * -# A request for data is sent to the device
+ * -# Some time later, the incoming data is received by the host
+ *
+ * or when writing data to an endpoint:
+ *
+ * -# The data is sent to the device
+ * -# Some time later, the host receives acknowledgement from the device that
+ *    the data has been transferred.
+ *
+ * There may be an indefinite delay between the two steps. Consider a
+ * fictional USB input device with a button that the user can press. In order
+ * to determine when the button is pressed, you would likely submit a request
+ * to read data on a bulk or interrupt endpoint and wait for data to arrive.
+ * Data will arrive when the button is pressed by the user, which is
+ * potentially hours later.
+ *
+ * libusb offers both a synchronous and an asynchronous interface to performing
+ * USB transfers. The main difference is that the synchronous interface
+ * combines both steps indicated above into a single function call, whereas
+ * the asynchronous interface separates them.
+ *
+ * \section sync The synchronous interface
+ *
+ * The synchronous I/O interface allows you to perform a USB transfer with
+ * a single function call. When the function call returns, the transfer has
+ * completed and you can parse the results.
+ *
+ * If you have used libusb-0.1 before, this I/O style will seem familiar to
+ * you. libusb-0.1 only offered a synchronous interface.
+ *
+ * In our input device example, to read button presses you might write code
+ * in the following style:
+\code
+unsigned char data[4];
+int actual_length;
+int r = libusb_bulk_transfer(dev_handle, LIBUSB_ENDPOINT_IN, data, sizeof(data), &actual_length, 0);
+if (r == 0 && actual_length == sizeof(data)) {
+	// results of the transaction can now be found in the data buffer
+	// parse them here and report button press
+} else {
+	error();
+}
+\endcode
+ *
+ * The main advantage of this model is simplicity: you did everything with
+ * a single simple function call.
+ *
+ * However, this interface has its limitations. Your application will sleep
+ * inside libusb_bulk_transfer() until the transaction has completed. If it
+ * takes the user 3 hours to press the button, your application will be
+ * sleeping for that long. Execution will be tied up inside the library -
+ * the entire thread will be useless for that duration.
+ *
+ * Another issue is that by tying up the thread with that single transaction
+ * there is no possibility of performing I/O with multiple endpoints and/or
+ * multiple devices simultaneously, unless you resort to creating one thread
+ * per transaction.
+ *
+ * Additionally, there is no opportunity to cancel the transfer after the
+ * request has been submitted.
+ *
+ * For details on how to use the synchronous API, see the
+ * \ref libusb_syncio "synchronous I/O API documentation" pages.
+ *
+ * \section async The asynchronous interface
+ *
+ * Asynchronous I/O is the most significant new feature in libusb-1.0.
+ * Although it is a more complex interface, it solves all the issues detailed
+ * above.
+ *
+ * Instead of providing which functions that block until the I/O has complete,
+ * libusb's asynchronous interface presents non-blocking functions which
+ * begin a transfer and then return immediately. Your application passes a
+ * callback function pointer to this non-blocking function, which libusb will
+ * call with the results of the transaction when it has completed.
+ *
+ * Transfers which have been submitted through the non-blocking functions
+ * can be cancelled with a separate function call.
+ *
+ * The non-blocking nature of this interface allows you to be simultaneously
+ * performing I/O to multiple endpoints on multiple devices, without having
+ * to use threads.
+ *
+ * This added flexibility does come with some complications though:
+ * - In the interest of being a lightweight library, libusb does not create
+ * threads and can only operate when your application is calling into it. Your
+ * application must call into libusb from it's main loop when events are ready
+ * to be handled, or you must use some other scheme to allow libusb to
+ * undertake whatever work needs to be done.
+ * - libusb also needs to be called into at certain fixed points in time in
+ * order to accurately handle transfer timeouts.
+ * - Memory handling becomes more complex. You cannot use stack memory unless
+ * the function with that stack is guaranteed not to return until the transfer
+ * callback has finished executing.
+ * - You generally lose some linearity from your code flow because submitting
+ * the transfer request is done in a separate function from where the transfer
+ * results are handled. This becomes particularly obvious when you want to
+ * submit a second transfer based on the results of an earlier transfer.
+ *
+ * Internally, libusb's synchronous interface is expressed in terms of function
+ * calls to the asynchronous interface.
+ *
+ * For details on how to use the asynchronous API, see the
+ * \ref libusb_asyncio "asynchronous I/O API" documentation pages.
+ */
+
+
+/**
+ * \page libusb_packetoverflow Packets and overflows
+ *
+ * \section packets Packet abstraction
+ *
+ * The USB specifications describe how data is transmitted in packets, with
+ * constraints on packet size defined by endpoint descriptors. The host must
+ * not send data payloads larger than the endpoint's maximum packet size.
+ *
+ * libusb and the underlying OS abstract out the packet concept, allowing you
+ * to request transfers of any size. Internally, the request will be divided
+ * up into correctly-sized packets. You do not have to be concerned with
+ * packet sizes, but there is one exception when considering overflows.
+ *
+ * \section overflow Bulk/interrupt transfer overflows
+ *
+ * When requesting data on a bulk endpoint, libusb requires you to supply a
+ * buffer and the maximum number of bytes of data that libusb can put in that
+ * buffer. However, the size of the buffer is not communicated to the device -
+ * the device is just asked to send any amount of data.
+ *
+ * There is no problem if the device sends an amount of data that is less than
+ * or equal to the buffer size. libusb reports this condition to you through
+ * the \ref libusb_transfer::actual_length "libusb_transfer.actual_length"
+ * field.
+ *
+ * Problems may occur if the device attempts to send more data than can fit in
+ * the buffer. libusb reports LIBUSB_TRANSFER_OVERFLOW for this condition but
+ * other behaviour is largely undefined: actual_length may or may not be
+ * accurate, the chunk of data that can fit in the buffer (before overflow)
+ * may or may not have been transferred.
+ *
+ * Overflows are nasty, but can be avoided. Even though you were told to
+ * ignore packets above, think about the lower level details: each transfer is
+ * split into packets (typically small, with a maximum size of 512 bytes).
+ * Overflows can only happen if the final packet in an incoming data transfer
+ * is smaller than the actual packet that the device wants to transfer.
+ * Therefore, you will never see an overflow if your transfer buffer size is a
+ * multiple of the endpoint's packet size: the final packet will either
+ * fill up completely or will be only partially filled.
+ */
+
+/**
+ * @defgroup libusb_asyncio Asynchronous device I/O
+ *
+ * This page details libusb's asynchronous (non-blocking) API for USB device
+ * I/O. This interface is very powerful but is also quite complex - you will
+ * need to read this page carefully to understand the necessary considerations
+ * and issues surrounding use of this interface. Simplistic applications
+ * may wish to consider the \ref libusb_syncio "synchronous I/O API" instead.
+ *
+ * The asynchronous interface is built around the idea of separating transfer
+ * submission and handling of transfer completion (the synchronous model
+ * combines both of these into one). There may be a long delay between
+ * submission and completion, however the asynchronous submission function
+ * is non-blocking so will return control to your application during that
+ * potentially long delay.
+ *
+ * \section asyncabstraction Transfer abstraction
+ *
+ * For the asynchronous I/O, libusb implements the concept of a generic
+ * transfer entity for all types of I/O (control, bulk, interrupt,
+ * isochronous). The generic transfer object must be treated slightly
+ * differently depending on which type of I/O you are performing with it.
+ *
+ * This is represented by the public libusb_transfer structure type.
+ *
+ * \section asynctrf Asynchronous transfers
+ *
+ * We can view asynchronous I/O as a 5 step process:
+ * -# <b>Allocation</b>: allocate a libusb_transfer
+ * -# <b>Filling</b>: populate the libusb_transfer instance with information
+ *    about the transfer you wish to perform
+ * -# <b>Submission</b>: ask libusb to submit the transfer
+ * -# <b>Completion handling</b>: examine transfer results in the
+ *    libusb_transfer structure
+ * -# <b>Deallocation</b>: clean up resources
+ *
+ *
+ * \subsection asyncalloc Allocation
+ *
+ * This step involves allocating memory for a USB transfer. This is the
+ * generic transfer object mentioned above. At this stage, the transfer
+ * is "blank" with no details about what type of I/O it will be used for.
+ *
+ * Allocation is done with the libusb_alloc_transfer() function. You must use
+ * this function rather than allocating your own transfers.
+ *
+ * \subsection asyncfill Filling
+ *
+ * This step is where you take a previously allocated transfer and fill it
+ * with information to determine the message type and direction, data buffer,
+ * callback function, etc.
+ *
+ * You can either fill the required fields yourself or you can use the
+ * helper functions: libusb_fill_control_transfer(), libusb_fill_bulk_transfer()
+ * and libusb_fill_interrupt_transfer().
+ *
+ * \subsection asyncsubmit Submission
+ *
+ * When you have allocated a transfer and filled it, you can submit it using
+ * libusb_submit_transfer(). This function returns immediately but can be
+ * regarded as firing off the I/O request in the background.
+ *
+ * \subsection asynccomplete Completion handling
+ *
+ * After a transfer has been submitted, one of four things can happen to it:
+ *
+ * - The transfer completes (i.e. some data was transferred)
+ * - The transfer has a timeout and the timeout expires before all data is
+ * transferred
+ * - The transfer fails due to an error
+ * - The transfer is cancelled
+ *
+ * Each of these will cause the user-specified transfer callback function to
+ * be invoked. It is up to the callback function to determine which of the
+ * above actually happened and to act accordingly.
+ *
+ * The user-specified callback is passed a pointer to the libusb_transfer
+ * structure which was used to setup and submit the transfer. At completion
+ * time, libusb has populated this structure with results of the transfer:
+ * success or failure reason, number of bytes of data transferred, etc. See
+ * the libusb_transfer structure documentation for more information.
+ *
+ * <b>Important Note</b>: The user-specified callback is called from an event
+ * handling context. It is therefore important that no calls are made into
+ * libusb that will attempt to perform any event handling. Examples of such
+ * functions are any listed in the \ref libusb_syncio "synchronous API" and any of
+ * the blocking functions that retrieve \ref libusb_desc "USB descriptors".
+ *
+ * \subsection Deallocation
+ *
+ * When a transfer has completed (i.e. the callback function has been invoked),
+ * you are advised to free the transfer (unless you wish to resubmit it, see
+ * below). Transfers are deallocated with libusb_free_transfer().
+ *
+ * It is undefined behaviour to free a transfer which has not completed.
+ *
+ * \section asyncresubmit Resubmission
+ *
+ * You may be wondering why allocation, filling, and submission are all
+ * separated above where they could reasonably be combined into a single
+ * operation.
+ *
+ * The reason for separation is to allow you to resubmit transfers without
+ * having to allocate new ones every time. This is especially useful for
+ * common situations dealing with interrupt endpoints - you allocate one
+ * transfer, fill and submit it, and when it returns with results you just
+ * resubmit it for the next interrupt.
+ *
+ * \section asynccancel Cancellation
+ *
+ * Another advantage of using the asynchronous interface is that you have
+ * the ability to cancel transfers which have not yet completed. This is
+ * done by calling the libusb_cancel_transfer() function.
+ *
+ * libusb_cancel_transfer() is asynchronous/non-blocking in itself. When the
+ * cancellation actually completes, the transfer's callback function will
+ * be invoked, and the callback function should check the transfer status to
+ * determine that it was cancelled.
+ *
+ * Freeing the transfer after it has been cancelled but before cancellation
+ * has completed will result in undefined behaviour.
+ *
+ * \attention
+ * When a transfer is cancelled, some of the data may have been transferred.
+ * libusb will communicate this to you in the transfer callback.
+ * <b>Do not assume that no data was transferred.</b>
+ *
+ * \section asyncpartial Partial data transfer resulting from cancellation
+ *
+ * As noted above, some of the data may have been transferred at the time a
+ * transfer is cancelled. It is helpful to see how this is possible if you
+ * consider a bulk transfer to an endpoint with a packet size of 64 bytes.
+ * Supposing you submit a 512-byte transfer to this endpoint, the operating
+ * system will divide this transfer up into 8 separate 64-byte frames that the
+ * host controller will schedule for the device to transfer data. If this
+ * transfer is cancelled while the device is transferring data, a subset of
+ * these frames may be descheduled from the host controller before the device
+ * has the opportunity to finish transferring data to the host.
+ *
+ * What your application should do with a partial data transfer is a policy
+ * decision; there is no single answer that satisfies the needs of every
+ * application. The data that was successfully transferred should be
+ * considered entirely valid, but your application must decide what to do with
+ * the remaining data that was not transferred. Some possible actions to take
+ * are:
+ * - Resubmit another transfer for the remaining data, possibly with a shorter
+ *   timeout
+ * - Discard the partially transferred data and report an error
+ *
+ * \section asynctimeout Timeouts
+ *
+ * When a transfer times out, libusb internally notes this and attempts to
+ * cancel the transfer. As noted in \ref asyncpartial "above", it is possible
+ * that some of the data may actually have been transferred. Your application
+ * should <b>always</b> check how much data was actually transferred once the
+ * transfer completes and act accordingly.
+ *
+ * \section bulk_overflows Overflows on device-to-host bulk/interrupt endpoints
+ *
+ * If your device does not have predictable transfer sizes (or it misbehaves),
+ * your application may submit a request for data on an IN endpoint which is
+ * smaller than the data that the device wishes to send. In some circumstances
+ * this will cause an overflow, which is a nasty condition to deal with. See
+ * the \ref libusb_packetoverflow page for discussion.
+ *
+ * \section asyncctrl Considerations for control transfers
+ *
+ * The <tt>libusb_transfer</tt> structure is generic and hence does not
+ * include specific fields for the control-specific setup packet structure.
+ *
+ * In order to perform a control transfer, you must place the 8-byte setup
+ * packet at the start of the data buffer. To simplify this, you could
+ * cast the buffer pointer to type struct libusb_control_setup, or you can
+ * use the helper function libusb_fill_control_setup().
+ *
+ * The wLength field placed in the setup packet must be the length you would
+ * expect to be sent in the setup packet: the length of the payload that
+ * follows (or the expected maximum number of bytes to receive). However,
+ * the length field of the libusb_transfer object must be the length of
+ * the data buffer - i.e. it should be wLength <em>plus</em> the size of
+ * the setup packet (LIBUSB_CONTROL_SETUP_SIZE).
+ *
+ * If you use the helper functions, this is simplified for you:
+ * -# Allocate a buffer of size LIBUSB_CONTROL_SETUP_SIZE plus the size of the
+ * data you are sending/requesting.
+ * -# Call libusb_fill_control_setup() on the data buffer, using the transfer
+ * request size as the wLength value (i.e. do not include the extra space you
+ * allocated for the control setup).
+ * -# If this is a host-to-device transfer, place the data to be transferred
+ * in the data buffer, starting at offset LIBUSB_CONTROL_SETUP_SIZE.
+ * -# Call libusb_fill_control_transfer() to associate the data buffer with
+ * the transfer (and to set the remaining details such as callback and timeout).
+ *   - Note that there is no parameter to set the length field of the transfer.
+ *     The length is automatically inferred from the wLength field of the setup
+ *     packet.
+ * -# Submit the transfer.
+ *
+ * The multi-byte control setup fields (wValue, wIndex and wLength) must
+ * be given in little-endian byte order (the endianness of the USB bus).
+ * Endianness conversion is transparently handled by
+ * libusb_fill_control_setup() which is documented to accept host-endian
+ * values.
+ *
+ * Further considerations are needed when handling transfer completion in
+ * your callback function:
+ * - As you might expect, the setup packet will still be sitting at the start
+ * of the data buffer.
+ * - If this was a device-to-host transfer, the received data will be sitting
+ * at offset LIBUSB_CONTROL_SETUP_SIZE into the buffer.
+ * - The actual_length field of the transfer structure is relative to the
+ * wLength of the setup packet, rather than the size of the data buffer. So,
+ * if your wLength was 4, your transfer's <tt>length</tt> was 12, then you
+ * should expect an <tt>actual_length</tt> of 4 to indicate that the data was
+ * transferred in entirety.
+ *
+ * To simplify parsing of setup packets and obtaining the data from the
+ * correct offset, you may wish to use the libusb_control_transfer_get_data()
+ * and libusb_control_transfer_get_setup() functions within your transfer
+ * callback.
+ *
+ * Even though control endpoints do not halt, a completed control transfer
+ * may have a LIBUSB_TRANSFER_STALL status code. This indicates the control
+ * request was not supported.
+ *
+ * \section asyncintr Considerations for interrupt transfers
+ *
+ * All interrupt transfers are performed using the polling interval presented
+ * by the bInterval value of the endpoint descriptor.
+ *
+ * \section asynciso Considerations for isochronous transfers
+ *
+ * Isochronous transfers are more complicated than transfers to
+ * non-isochronous endpoints.
+ *
+ * To perform I/O to an isochronous endpoint, allocate the transfer by calling
+ * libusb_alloc_transfer() with an appropriate number of isochronous packets.
+ *
+ * During filling, set \ref libusb_transfer::type "type" to
+ * \ref libusb_transfer_type::LIBUSB_TRANSFER_TYPE_ISOCHRONOUS
+ * "LIBUSB_TRANSFER_TYPE_ISOCHRONOUS", and set
+ * \ref libusb_transfer::num_iso_packets "num_iso_packets" to a value less than
+ * or equal to the number of packets you requested during allocation.
+ * libusb_alloc_transfer() does not set either of these fields for you, given
+ * that you might not even use the transfer on an isochronous endpoint.
+ *
+ * Next, populate the length field for the first num_iso_packets entries in
+ * the \ref libusb_transfer::iso_packet_desc "iso_packet_desc" array. Section
+ * 5.6.3 of the USB2 specifications describe how the maximum isochronous
+ * packet length is determined by the wMaxPacketSize field in the endpoint
+ * descriptor.
+ * Two functions can help you here:
+ *
+ * - libusb_get_max_iso_packet_size() is an easy way to determine the max
+ *   packet size for an isochronous endpoint. Note that the maximum packet
+ *   size is actually the maximum number of bytes that can be transmitted in
+ *   a single microframe, therefore this function multiplies the maximum number
+ *   of bytes per transaction by the number of transaction opportunities per
+ *   microframe.
+ * - libusb_set_iso_packet_lengths() assigns the same length to all packets
+ *   within a transfer, which is usually what you want.
+ *
+ * For outgoing transfers, you'll obviously fill the buffer and populate the
+ * packet descriptors in hope that all the data gets transferred. For incoming
+ * transfers, you must ensure the buffer has sufficient capacity for
+ * the situation where all packets transfer the full amount of requested data.
+ *
+ * Completion handling requires some extra consideration. The
+ * \ref libusb_transfer::actual_length "actual_length" field of the transfer
+ * is meaningless and should not be examined; instead you must refer to the
+ * \ref libusb_iso_packet_descriptor::actual_length "actual_length" field of
+ * each individual packet.
+ *
+ * The \ref libusb_transfer::status "status" field of the transfer is also a
+ * little misleading:
+ *  - If the packets were submitted and the isochronous data microframes
+ *    completed normally, status will have value
+ *    \ref libusb_transfer_status::LIBUSB_TRANSFER_COMPLETED
+ *    "LIBUSB_TRANSFER_COMPLETED". Note that bus errors and software-incurred
+ *    delays are not counted as transfer errors; the transfer.status field may
+ *    indicate COMPLETED even if some or all of the packets failed. Refer to
+ *    the \ref libusb_iso_packet_descriptor::status "status" field of each
+ *    individual packet to determine packet failures.
+ *  - The status field will have value
+ *    \ref libusb_transfer_status::LIBUSB_TRANSFER_ERROR
+ *    "LIBUSB_TRANSFER_ERROR" only when serious errors were encountered.
+ *  - Other transfer status codes occur with normal behaviour.
+ *
+ * The data for each packet will be found at an offset into the buffer that
+ * can be calculated as if each prior packet completed in full. The
+ * libusb_get_iso_packet_buffer() and libusb_get_iso_packet_buffer_simple()
+ * functions may help you here.
+ *
+ * \section asynclimits Transfer length limitations
+ *
+ * Some operating systems may impose limits on the length of the transfer data
+ * buffer or, in the case of isochronous transfers, the length of individual
+ * isochronous packets. Such limits can be difficult for libusb to detect, so
+ * in most cases the library will simply try and submit the transfer as set up
+ * by you. If the transfer fails to submit because it is too large,
+ * libusb_submit_transfer() will return
+ * \ref libusb_error::LIBUSB_ERROR_INVALID_PARAM "LIBUSB_ERROR_INVALID_PARAM".
+ *
+ * The following are known limits for control transfer lengths. Note that this
+ * length includes the 8-byte setup packet.
+ * - Linux (4,096 bytes)
+ * - Windows (4,096 bytes)
+ *
+ * \section asyncmem Memory caveats
+ *
+ * In most circumstances, it is not safe to use stack memory for transfer
+ * buffers. This is because the function that fired off the asynchronous
+ * transfer may return before libusb has finished using the buffer, and when
+ * the function returns it's stack gets destroyed. This is true for both
+ * host-to-device and device-to-host transfers.
+ *
+ * The only case in which it is safe to use stack memory is where you can
+ * guarantee that the function owning the stack space for the buffer does not
+ * return until after the transfer's callback function has completed. In every
+ * other case, you need to use heap memory instead.
+ *
+ * \section asyncflags Fine control
+ *
+ * Through using this asynchronous interface, you may find yourself repeating
+ * a few simple operations many times. You can apply a bitwise OR of certain
+ * flags to a transfer to simplify certain things:
+ * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_SHORT_NOT_OK
+ *   "LIBUSB_TRANSFER_SHORT_NOT_OK" results in transfers which transferred
+ *   less than the requested amount of data being marked with status
+ *   \ref libusb_transfer_status::LIBUSB_TRANSFER_ERROR "LIBUSB_TRANSFER_ERROR"
+ *   (they would normally be regarded as COMPLETED)
+ * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_BUFFER
+ *   "LIBUSB_TRANSFER_FREE_BUFFER" allows you to ask libusb to free the transfer
+ *   buffer when freeing the transfer.
+ * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_TRANSFER
+ *   "LIBUSB_TRANSFER_FREE_TRANSFER" causes libusb to automatically free the
+ *   transfer after the transfer callback returns.
+ *
+ * \section asyncevent Event handling
+ *
+ * An asynchronous model requires that libusb perform work at various
+ * points in time - namely processing the results of previously-submitted
+ * transfers and invoking the user-supplied callback function.
+ *
+ * This gives rise to the libusb_handle_events() function which your
+ * application must call into when libusb has work do to. This gives libusb
+ * the opportunity to reap pending transfers, invoke callbacks, etc.
+ *
+ * \note
+ * All event handling is performed by whichever thread calls the
+ * libusb_handle_events() function. libusb does not invoke any callbacks
+ * outside of this context. Consequently, any callbacks will be run on the
+ * thread that calls the libusb_handle_events() function.
+ *
+ * When to call the libusb_handle_events() function depends on which model
+ * your application decides to use. The 2 different approaches:
+ *
+ * -# Repeatedly call libusb_handle_events() in blocking mode from a dedicated
+ *    thread.
+ * -# Integrate libusb with your application's main event loop. libusb
+ *    exposes a set of file descriptors which allow you to do this.
+ *
+ * The first approach has the big advantage that it will also work on Windows
+ * were libusb' poll API for select / poll integration is not available. So
+ * if you want to support Windows and use the async API, you must use this
+ * approach, see the \ref eventthread "Using an event handling thread" section
+ * below for details.
+ *
+ * If you prefer a single threaded approach with a single central event loop,
+ * see the \ref libusb_poll "polling and timing" section for how to integrate libusb
+ * into your application's main event loop.
+ *
+ * \section eventthread Using an event handling thread
+ *
+ * Lets begin with stating the obvious: If you're going to use a separate
+ * thread for libusb event handling, your callback functions MUST be
+ * thread-safe.
+ *
+ * Other then that doing event handling from a separate thread, is mostly
+ * simple. You can use an event thread function as follows:
+\code
+void *event_thread_func(void *ctx)
+{
+    while (event_thread_run)
+        libusb_handle_events(ctx);
+
+    return NULL;
+}
+\endcode
+ *
+ * There is one caveat though, stopping this thread requires setting the
+ * event_thread_run variable to 0, and after that libusb_handle_events() needs
+ * to return control to event_thread_func. But unless some event happens,
+ * libusb_handle_events() will not return.
+ *
+ * There are 2 different ways of dealing with this, depending on if your
+ * application uses libusb' \ref libusb_hotplug "hotplug" support or not.
+ *
+ * Applications which do not use hotplug support, should not start the event
+ * thread until after their first call to libusb_open(), and should stop the
+ * thread when closing the last open device as follows:
+\code
+void my_close_handle(libusb_device_handle *dev_handle)
+{
+    if (open_devs == 1)
+        event_thread_run = 0;
+
+    libusb_close(dev_handle); // This wakes up libusb_handle_events()
+
+    if (open_devs == 1)
+        pthread_join(event_thread);
+
+    open_devs--;
+}
+\endcode
+ *
+ * Applications using hotplug support should start the thread at program init,
+ * after having successfully called libusb_hotplug_register_callback(), and
+ * should stop the thread at program exit as follows:
+\code
+void my_libusb_exit(void)
+{
+    event_thread_run = 0;
+    libusb_hotplug_deregister_callback(ctx, hotplug_cb_handle); // This wakes up libusb_handle_events()
+    pthread_join(event_thread);
+    libusb_exit(ctx);
+}
+\endcode
+ */
+
+/**
+ * @defgroup libusb_poll Polling and timing
+ *
+ * This page documents libusb's functions for polling events and timing.
+ * These functions are only necessary for users of the
+ * \ref libusb_asyncio "asynchronous API". If you are only using the simpler
+ * \ref libusb_syncio "synchronous API" then you do not need to ever call these
+ * functions.
+ *
+ * The justification for the functionality described here has already been
+ * discussed in the \ref asyncevent "event handling" section of the
+ * asynchronous API documentation. In summary, libusb does not create internal
+ * threads for event processing and hence relies on your application calling
+ * into libusb at certain points in time so that pending events can be handled.
+ *
+ * Your main loop is probably already calling poll() or select() or a
+ * variant on a set of file descriptors for other event sources (e.g. keyboard
+ * button presses, mouse movements, network sockets, etc). You then add
+ * libusb's file descriptors to your poll()/select() calls, and when activity
+ * is detected on such descriptors you know it is time to call
+ * libusb_handle_events().
+ *
+ * There is one final event handling complication. libusb supports
+ * asynchronous transfers which time out after a specified time period.
+ *
+ * On some platforms a timerfd is used, so the timeout handling is just another
+ * fd, on other platforms this requires that libusb is called into at or after
+ * the timeout to handle it. So, in addition to considering libusb's file
+ * descriptors in your main event loop, you must also consider that libusb
+ * sometimes needs to be called into at fixed points in time even when there
+ * is no file descriptor activity, see \ref polltime details.
+ *
+ * In order to know precisely when libusb needs to be called into, libusb
+ * offers you a set of pollable file descriptors and information about when
+ * the next timeout expires.
+ *
+ * If you are using the asynchronous I/O API, you must take one of the two
+ * following options, otherwise your I/O will not complete.
+ *
+ * \section pollsimple The simple option
+ *
+ * If your application revolves solely around libusb and does not need to
+ * handle other event sources, you can have a program structure as follows:
+\code
+// initialize libusb
+// find and open device
+// maybe fire off some initial async I/O
+
+while (user_has_not_requested_exit)
+	libusb_handle_events(ctx);
+
+// clean up and exit
+\endcode
+ *
+ * With such a simple main loop, you do not have to worry about managing
+ * sets of file descriptors or handling timeouts. libusb_handle_events() will
+ * handle those details internally.
+ *
+ * \section libusb_pollmain The more advanced option
+ *
+ * \note This functionality is currently only available on Unix-like platforms.
+ * On Windows, libusb_get_pollfds() simply returns NULL. Applications which
+ * want to support Windows are advised to use an \ref eventthread
+ * "event handling thread" instead.
+ *
+ * In more advanced applications, you will already have a main loop which
+ * is monitoring other event sources: network sockets, X11 events, mouse
+ * movements, etc. Through exposing a set of file descriptors, libusb is
+ * designed to cleanly integrate into such main loops.
+ *
+ * In addition to polling file descriptors for the other event sources, you
+ * take a set of file descriptors from libusb and monitor those too. When you
+ * detect activity on libusb's file descriptors, you call
+ * libusb_handle_events_timeout() in non-blocking mode.
+ *
+ * What's more, libusb may also need to handle events at specific moments in
+ * time. No file descriptor activity is generated at these times, so your
+ * own application needs to be continually aware of when the next one of these
+ * moments occurs (through calling libusb_get_next_timeout()), and then it
+ * needs to call libusb_handle_events_timeout() in non-blocking mode when
+ * these moments occur. This means that you need to adjust your
+ * poll()/select() timeout accordingly.
+ *
+ * libusb provides you with a set of file descriptors to poll and expects you
+ * to poll all of them, treating them as a single entity. The meaning of each
+ * file descriptor in the set is an internal implementation detail,
+ * platform-dependent and may vary from release to release. Don't try and
+ * interpret the meaning of the file descriptors, just do as libusb indicates,
+ * polling all of them at once.
+ *
+ * In pseudo-code, you want something that looks like:
+\code
+// initialise libusb
+
+libusb_get_pollfds(ctx)
+while (user has not requested application exit) {
+	libusb_get_next_timeout(ctx);
+	poll(on libusb file descriptors plus any other event sources of interest,
+		using a timeout no larger than the value libusb just suggested)
+	if (poll() indicated activity on libusb file descriptors)
+		libusb_handle_events_timeout(ctx, &zero_tv);
+	if (time has elapsed to or beyond the libusb timeout)
+		libusb_handle_events_timeout(ctx, &zero_tv);
+	// handle events from other sources here
+}
+
+// clean up and exit
+\endcode
+ *
+ * \subsection polltime Notes on time-based events
+ *
+ * The above complication with having to track time and call into libusb at
+ * specific moments is a bit of a headache. For maximum compatibility, you do
+ * need to write your main loop as above, but you may decide that you can
+ * restrict the supported platforms of your application and get away with
+ * a more simplistic scheme.
+ *
+ * These time-based event complications are \b not required on the following
+ * platforms:
+ *  - Darwin
+ *  - Linux, provided that the following version requirements are satisfied:
+ *   - Linux v2.6.27 or newer, compiled with timerfd support
+ *   - glibc v2.9 or newer
+ *   - libusb v1.0.5 or newer
+ *
+ * Under these configurations, libusb_get_next_timeout() will \em always return
+ * 0, so your main loop can be simplified to:
+\code
+// initialise libusb
+
+libusb_get_pollfds(ctx)
+while (user has not requested application exit) {
+	poll(on libusb file descriptors plus any other event sources of interest,
+		using any timeout that you like)
+	if (poll() indicated activity on libusb file descriptors)
+		libusb_handle_events_timeout(ctx, &zero_tv);
+	// handle events from other sources here
+}
+
+// clean up and exit
+\endcode
+ *
+ * Do remember that if you simplify your main loop to the above, you will
+ * lose compatibility with some platforms (including legacy Linux platforms,
+ * and <em>any future platforms supported by libusb which may have time-based
+ * event requirements</em>). The resultant problems will likely appear as
+ * strange bugs in your application.
+ *
+ * You can use the libusb_pollfds_handle_timeouts() function to do a runtime
+ * check to see if it is safe to ignore the time-based event complications.
+ * If your application has taken the shortcut of ignoring libusb's next timeout
+ * in your main loop, then you are advised to check the return value of
+ * libusb_pollfds_handle_timeouts() during application startup, and to abort
+ * if the platform does suffer from these timing complications.
+ *
+ * \subsection fdsetchange Changes in the file descriptor set
+ *
+ * The set of file descriptors that libusb uses as event sources may change
+ * during the life of your application. Rather than having to repeatedly
+ * call libusb_get_pollfds(), you can set up notification functions for when
+ * the file descriptor set changes using libusb_set_pollfd_notifiers().
+ *
+ * \subsection mtissues Multi-threaded considerations
+ *
+ * Unfortunately, the situation is complicated further when multiple threads
+ * come into play. If two threads are monitoring the same file descriptors,
+ * the fact that only one thread will be woken up when an event occurs causes
+ * some headaches.
+ *
+ * The events lock, event waiters lock, and libusb_handle_events_locked()
+ * entities are added to solve these problems. You do not need to be concerned
+ * with these entities otherwise.
+ *
+ * See the extra documentation: \ref libusb_mtasync
+ */
+
+/** \page libusb_mtasync Multi-threaded applications and asynchronous I/O
+ *
+ * libusb is a thread-safe library, but extra considerations must be applied
+ * to applications which interact with libusb from multiple threads.
+ *
+ * The underlying issue that must be addressed is that all libusb I/O
+ * revolves around monitoring file descriptors through the poll()/select()
+ * system calls. This is directly exposed at the
+ * \ref libusb_asyncio "asynchronous interface" but it is important to note that the
+ * \ref libusb_syncio "synchronous interface" is implemented on top of the
+ * asynchronous interface, therefore the same considerations apply.
+ *
+ * The issue is that if two or more threads are concurrently calling poll()
+ * or select() on libusb's file descriptors then only one of those threads
+ * will be woken up when an event arrives. The others will be completely
+ * oblivious that anything has happened.
+ *
+ * Consider the following pseudo-code, which submits an asynchronous transfer
+ * then waits for its completion. This style is one way you could implement a
+ * synchronous interface on top of the asynchronous interface (and libusb
+ * does something similar, albeit more advanced due to the complications
+ * explained on this page).
+ *
+\code
+void cb(struct libusb_transfer *transfer)
+{
+	int *completed = transfer->user_data;
+	*completed = 1;
+}
+
+void myfunc() {
+	struct libusb_transfer *transfer;
+	unsigned char buffer[LIBUSB_CONTROL_SETUP_SIZE] __attribute__ ((aligned (2)));
+	int completed = 0;
+
+	transfer = libusb_alloc_transfer(0);
+	libusb_fill_control_setup(buffer,
+		LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_ENDPOINT_OUT, 0x04, 0x01, 0, 0);
+	libusb_fill_control_transfer(transfer, dev, buffer, cb, &completed, 1000);
+	libusb_submit_transfer(transfer);
+
+	while (!completed) {
+		poll(libusb file descriptors, 120*1000);
+		if (poll indicates activity)
+			libusb_handle_events_timeout(ctx, &zero_tv);
+	}
+	printf("completed!");
+	// other code here
+}
+\endcode
+ *
+ * Here we are <em>serializing</em> completion of an asynchronous event
+ * against a condition - the condition being completion of a specific transfer.
+ * The poll() loop has a long timeout to minimize CPU usage during situations
+ * when nothing is happening (it could reasonably be unlimited).
+ *
+ * If this is the only thread that is polling libusb's file descriptors, there
+ * is no problem: there is no danger that another thread will swallow up the
+ * event that we are interested in. On the other hand, if there is another
+ * thread polling the same descriptors, there is a chance that it will receive
+ * the event that we were interested in. In this situation, <tt>myfunc()</tt>
+ * will only realise that the transfer has completed on the next iteration of
+ * the loop, <em>up to 120 seconds later.</em> Clearly a two-minute delay is
+ * undesirable, and don't even think about using short timeouts to circumvent
+ * this issue!
+ *
+ * The solution here is to ensure that no two threads are ever polling the
+ * file descriptors at the same time. A naive implementation of this would
+ * impact the capabilities of the library, so libusb offers the scheme
+ * documented below to ensure no loss of functionality.
+ *
+ * Before we go any further, it is worth mentioning that all libusb-wrapped
+ * event handling procedures fully adhere to the scheme documented below.
+ * This includes libusb_handle_events() and its variants, and all the
+ * synchronous I/O functions - libusb hides this headache from you.
+ *
+ * \section Using libusb_handle_events() from multiple threads
+ *
+ * Even when only using libusb_handle_events() and synchronous I/O functions,
+ * you can still have a race condition. You might be tempted to solve the
+ * above with libusb_handle_events() like so:
+ *
+\code
+	libusb_submit_transfer(transfer);
+
+	while (!completed) {
+		libusb_handle_events(ctx);
+	}
+	printf("completed!");
+\endcode
+ *
+ * This however has a race between the checking of completed and
+ * libusb_handle_events() acquiring the events lock, so another thread
+ * could have completed the transfer, resulting in this thread hanging
+ * until either a timeout or another event occurs. See also commit
+ * 6696512aade99bb15d6792af90ae329af270eba6 which fixes this in the
+ * synchronous API implementation of libusb.
+ *
+ * Fixing this race requires checking the variable completed only after
+ * taking the event lock, which defeats the concept of just calling
+ * libusb_handle_events() without worrying about locking. This is why
+ * libusb-1.0.9 introduces the new libusb_handle_events_timeout_completed()
+ * and libusb_handle_events_completed() functions, which handles doing the
+ * completion check for you after they have acquired the lock:
+ *
+\code
+	libusb_submit_transfer(transfer);
+
+	while (!completed) {
+		libusb_handle_events_completed(ctx, &completed);
+	}
+	printf("completed!");
+\endcode
+ *
+ * This nicely fixes the race in our example. Note that if all you want to
+ * do is submit a single transfer and wait for its completion, then using
+ * one of the synchronous I/O functions is much easier.
+ *
+ * \note
+ * The `completed` variable must be modified while holding the event lock,
+ * otherwise a race condition can still exist. It is simplest to do so from
+ * within the transfer callback as shown above.
+ *
+ * \section eventlock The events lock
+ *
+ * The problem is when we consider the fact that libusb exposes file
+ * descriptors to allow for you to integrate asynchronous USB I/O into
+ * existing main loops, effectively allowing you to do some work behind
+ * libusb's back. If you do take libusb's file descriptors and pass them to
+ * poll()/select() yourself, you need to be aware of the associated issues.
+ *
+ * The first concept to be introduced is the events lock. The events lock
+ * is used to serialize threads that want to handle events, such that only
+ * one thread is handling events at any one time.
+ *
+ * You must take the events lock before polling libusb file descriptors,
+ * using libusb_lock_events(). You must release the lock as soon as you have
+ * aborted your poll()/select() loop, using libusb_unlock_events().
+ *
+ * \section threadwait Letting other threads do the work for you
+ *
+ * Although the events lock is a critical part of the solution, it is not
+ * enough on it's own. You might wonder if the following is sufficient...
+\code
+	libusb_lock_events(ctx);
+	while (!completed) {
+		poll(libusb file descriptors, 120*1000);
+		if (poll indicates activity)
+			libusb_handle_events_timeout(ctx, &zero_tv);
+	}
+	libusb_unlock_events(ctx);
+\endcode
+ * ...and the answer is that it is not. This is because the transfer in the
+ * code shown above may take a long time (say 30 seconds) to complete, and
+ * the lock is not released until the transfer is completed.
+ *
+ * Another thread with similar code that wants to do event handling may be
+ * working with a transfer that completes after a few milliseconds. Despite
+ * having such a quick completion time, the other thread cannot check that
+ * status of its transfer until the code above has finished (30 seconds later)
+ * due to contention on the lock.
+ *
+ * To solve this, libusb offers you a mechanism to determine when another
+ * thread is handling events. It also offers a mechanism to block your thread
+ * until the event handling thread has completed an event (and this mechanism
+ * does not involve polling of file descriptors).
+ *
+ * After determining that another thread is currently handling events, you
+ * obtain the <em>event waiters</em> lock using libusb_lock_event_waiters().
+ * You then re-check that some other thread is still handling events, and if
+ * so, you call libusb_wait_for_event().
+ *
+ * libusb_wait_for_event() puts your application to sleep until an event
+ * occurs, or until a thread releases the events lock. When either of these
+ * things happen, your thread is woken up, and should re-check the condition
+ * it was waiting on. It should also re-check that another thread is handling
+ * events, and if not, it should start handling events itself.
+ *
+ * This looks like the following, as pseudo-code:
+\code
+retry:
+if (libusb_try_lock_events(ctx) == 0) {
+	// we obtained the event lock: do our own event handling
+	while (!completed) {
+		if (!libusb_event_handling_ok(ctx)) {
+			libusb_unlock_events(ctx);
+			goto retry;
+		}
+		poll(libusb file descriptors, 120*1000);
+		if (poll indicates activity)
+			libusb_handle_events_locked(ctx, 0);
+	}
+	libusb_unlock_events(ctx);
+} else {
+	// another thread is doing event handling. wait for it to signal us that
+	// an event has completed
+	libusb_lock_event_waiters(ctx);
+
+	while (!completed) {
+		// now that we have the event waiters lock, double check that another
+		// thread is still handling events for us. (it may have ceased handling
+		// events in the time it took us to reach this point)
+		if (!libusb_event_handler_active(ctx)) {
+			// whoever was handling events is no longer doing so, try again
+			libusb_unlock_event_waiters(ctx);
+			goto retry;
+		}
+
+		libusb_wait_for_event(ctx, NULL);
+	}
+	libusb_unlock_event_waiters(ctx);
+}
+printf("completed!\n");
+\endcode
+ *
+ * A naive look at the above code may suggest that this can only support
+ * one event waiter (hence a total of 2 competing threads, the other doing
+ * event handling), because the event waiter seems to have taken the event
+ * waiters lock while waiting for an event. However, the system does support
+ * multiple event waiters, because libusb_wait_for_event() actually drops
+ * the lock while waiting, and reacquires it before continuing.
+ *
+ * We have now implemented code which can dynamically handle situations where
+ * nobody is handling events (so we should do it ourselves), and it can also
+ * handle situations where another thread is doing event handling (so we can
+ * piggyback onto them). It is also equipped to handle a combination of
+ * the two, for example, another thread is doing event handling, but for
+ * whatever reason it stops doing so before our condition is met, so we take
+ * over the event handling.
+ *
+ * Four functions were introduced in the above pseudo-code. Their importance
+ * should be apparent from the code shown above.
+ * -# libusb_try_lock_events() is a non-blocking function which attempts
+ *    to acquire the events lock but returns a failure code if it is contended.
+ * -# libusb_event_handling_ok() checks that libusb is still happy for your
+ *    thread to be performing event handling. Sometimes, libusb needs to
+ *    interrupt the event handler, and this is how you can check if you have
+ *    been interrupted. If this function returns 0, the correct behaviour is
+ *    for you to give up the event handling lock, and then to repeat the cycle.
+ *    The following libusb_try_lock_events() will fail, so you will become an
+ *    events waiter. For more information on this, read \ref fullstory below.
+ * -# libusb_handle_events_locked() is a variant of
+ *    libusb_handle_events_timeout() that you can call while holding the
+ *    events lock. libusb_handle_events_timeout() itself implements similar
+ *    logic to the above, so be sure not to call it when you are
+ *    "working behind libusb's back", as is the case here.
+ * -# libusb_event_handler_active() determines if someone is currently
+ *    holding the events lock
+ *
+ * You might be wondering why there is no function to wake up all threads
+ * blocked on libusb_wait_for_event(). This is because libusb can do this
+ * internally: it will wake up all such threads when someone calls
+ * libusb_unlock_events() or when a transfer completes (at the point after its
+ * callback has returned).
+ *
+ * \subsection fullstory The full story
+ *
+ * The above explanation should be enough to get you going, but if you're
+ * really thinking through the issues then you may be left with some more
+ * questions regarding libusb's internals. If you're curious, read on, and if
+ * not, skip to the next section to avoid confusing yourself!
+ *
+ * The immediate question that may spring to mind is: what if one thread
+ * modifies the set of file descriptors that need to be polled while another
+ * thread is doing event handling?
+ *
+ * There are 2 situations in which this may happen.
+ * -# libusb_open() will add another file descriptor to the poll set,
+ *    therefore it is desirable to interrupt the event handler so that it
+ *    restarts, picking up the new descriptor.
+ * -# libusb_close() will remove a file descriptor from the poll set. There
+ *    are all kinds of race conditions that could arise here, so it is
+ *    important that nobody is doing event handling at this time.
+ *
+ * libusb handles these issues internally, so application developers do not
+ * have to stop their event handlers while opening/closing devices. Here's how
+ * it works, focusing on the libusb_close() situation first:
+ *
+ * -# During initialization, libusb opens an internal pipe, and it adds the read
+ *    end of this pipe to the set of file descriptors to be polled.
+ * -# During libusb_close(), libusb writes some dummy data on this event pipe.
+ *    This immediately interrupts the event handler. libusb also records
+ *    internally that it is trying to interrupt event handlers for this
+ *    high-priority event.
+ * -# At this point, some of the functions described above start behaving
+ *    differently:
+ *   - libusb_event_handling_ok() starts returning 1, indicating that it is NOT
+ *     OK for event handling to continue.
+ *   - libusb_try_lock_events() starts returning 1, indicating that another
+ *     thread holds the event handling lock, even if the lock is uncontended.
+ *   - libusb_event_handler_active() starts returning 1, indicating that
+ *     another thread is doing event handling, even if that is not true.
+ * -# The above changes in behaviour result in the event handler stopping and
+ *    giving up the events lock very quickly, giving the high-priority
+ *    libusb_close() operation a "free ride" to acquire the events lock. All
+ *    threads that are competing to do event handling become event waiters.
+ * -# With the events lock held inside libusb_close(), libusb can safely remove
+ *    a file descriptor from the poll set, in the safety of knowledge that
+ *    nobody is polling those descriptors or trying to access the poll set.
+ * -# After obtaining the events lock, the close operation completes very
+ *    quickly (usually a matter of milliseconds) and then immediately releases
+ *    the events lock.
+ * -# At the same time, the behaviour of libusb_event_handling_ok() and friends
+ *    reverts to the original, documented behaviour.
+ * -# The release of the events lock causes the threads that are waiting for
+ *    events to be woken up and to start competing to become event handlers
+ *    again. One of them will succeed; it will then re-obtain the list of poll
+ *    descriptors, and USB I/O will then continue as normal.
+ *
+ * libusb_open() is similar, and is actually a more simplistic case. Upon a
+ * call to libusb_open():
+ *
+ * -# The device is opened and a file descriptor is added to the poll set.
+ * -# libusb sends some dummy data on the event pipe, and records that it
+ *    is trying to modify the poll descriptor set.
+ * -# The event handler is interrupted, and the same behaviour change as for
+ *    libusb_close() takes effect, causing all event handling threads to become
+ *    event waiters.
+ * -# The libusb_open() implementation takes its free ride to the events lock.
+ * -# Happy that it has successfully paused the events handler, libusb_open()
+ *    releases the events lock.
+ * -# The event waiter threads are all woken up and compete to become event
+ *    handlers again. The one that succeeds will obtain the list of poll
+ *    descriptors again, which will include the addition of the new device.
+ *
+ * \subsection concl Closing remarks
+ *
+ * The above may seem a little complicated, but hopefully I have made it clear
+ * why such complications are necessary. Also, do not forget that this only
+ * applies to applications that take libusb's file descriptors and integrate
+ * them into their own polling loops.
+ *
+ * You may decide that it is OK for your multi-threaded application to ignore
+ * some of the rules and locks detailed above, because you don't think that
+ * two threads can ever be polling the descriptors at the same time. If that
+ * is the case, then that's good news for you because you don't have to worry.
+ * But be careful here; remember that the synchronous I/O functions do event
+ * handling internally. If you have one thread doing event handling in a loop
+ * (without implementing the rules and locking semantics documented above)
+ * and another trying to send a synchronous USB transfer, you will end up with
+ * two threads monitoring the same descriptors, and the above-described
+ * undesirable behaviour occurring. The solution is for your polling thread to
+ * play by the rules; the synchronous I/O functions do so, and this will result
+ * in them getting along in perfect harmony.
+ *
+ * If you do have a dedicated thread doing event handling, it is perfectly
+ * legal for it to take the event handling lock for long periods of time. Any
+ * synchronous I/O functions you call from other threads will transparently
+ * fall back to the "event waiters" mechanism detailed above. The only
+ * consideration that your event handling thread must apply is the one related
+ * to libusb_event_handling_ok(): you must call this before every poll(), and
+ * give up the events lock if instructed.
+ */
+
+int usbi_io_init(struct libusb_context *ctx)
+{
+	int r;
+
+	usbi_mutex_init(&ctx->flying_transfers_lock);
+	usbi_mutex_init(&ctx->events_lock);
+	usbi_mutex_init(&ctx->event_waiters_lock);
+	usbi_cond_init(&ctx->event_waiters_cond);
+	usbi_mutex_init(&ctx->event_data_lock);
+	usbi_tls_key_create(&ctx->event_handling_key);
+	list_init(&ctx->flying_transfers);
+	list_init(&ctx->event_sources);
+	list_init(&ctx->removed_event_sources);
+	list_init(&ctx->hotplug_msgs);
+	list_init(&ctx->completed_transfers);
+
+	r = usbi_create_event(&ctx->event);
+	if (r < 0)
+		goto err;
+
+	r = usbi_add_event_source(ctx, USBI_EVENT_OS_HANDLE(&ctx->event), USBI_EVENT_POLL_EVENTS);
+	if (r < 0)
+		goto err_destroy_event;
+
+#ifdef HAVE_OS_TIMER
+	r = usbi_create_timer(&ctx->timer);
+	if (r == 0) {
+		usbi_dbg(ctx, "using timer for timeouts");
+		r = usbi_add_event_source(ctx, USBI_TIMER_OS_HANDLE(&ctx->timer), USBI_TIMER_POLL_EVENTS);
+		if (r < 0)
+			goto err_destroy_timer;
+	} else {
+		usbi_dbg(ctx, "timer not available for timeouts");
+	}
+#endif
+
+	return 0;
+
+#ifdef HAVE_OS_TIMER
+err_destroy_timer:
+	usbi_destroy_timer(&ctx->timer);
+	usbi_remove_event_source(ctx, USBI_EVENT_OS_HANDLE(&ctx->event));
+#endif
+err_destroy_event:
+	usbi_destroy_event(&ctx->event);
+err:
+	usbi_mutex_destroy(&ctx->flying_transfers_lock);
+	usbi_mutex_destroy(&ctx->events_lock);
+	usbi_mutex_destroy(&ctx->event_waiters_lock);
+	usbi_cond_destroy(&ctx->event_waiters_cond);
+	usbi_mutex_destroy(&ctx->event_data_lock);
+	usbi_tls_key_delete(ctx->event_handling_key);
+	return r;
+}
+
+static void cleanup_removed_event_sources(struct libusb_context *ctx)
+{
+	struct usbi_event_source *ievent_source, *tmp;
+
+	for_each_removed_event_source_safe(ctx, ievent_source, tmp) {
+		list_del(&ievent_source->list);
+		free(ievent_source);
+	}
+}
+
+void usbi_io_exit(struct libusb_context *ctx)
+{
+#ifdef HAVE_OS_TIMER
+	if (usbi_using_timer(ctx)) {
+		usbi_remove_event_source(ctx, USBI_TIMER_OS_HANDLE(&ctx->timer));
+		usbi_destroy_timer(&ctx->timer);
+	}
+#endif
+	usbi_remove_event_source(ctx, USBI_EVENT_OS_HANDLE(&ctx->event));
+	usbi_destroy_event(&ctx->event);
+	usbi_mutex_destroy(&ctx->flying_transfers_lock);
+	usbi_mutex_destroy(&ctx->events_lock);
+	usbi_mutex_destroy(&ctx->event_waiters_lock);
+	usbi_cond_destroy(&ctx->event_waiters_cond);
+	usbi_mutex_destroy(&ctx->event_data_lock);
+	usbi_tls_key_delete(ctx->event_handling_key);
+	cleanup_removed_event_sources(ctx);
+	free(ctx->event_data);
+}
+
+static void calculate_timeout(struct usbi_transfer *itransfer)
+{
+	unsigned int timeout =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer)->timeout;
+
+	if (!timeout) {
+		TIMESPEC_CLEAR(&itransfer->timeout);
+		return;
+	}
+
+	usbi_get_monotonic_time(&itransfer->timeout);
+
+	itransfer->timeout.tv_sec += timeout / 1000U;
+	itransfer->timeout.tv_nsec += (timeout % 1000U) * 1000000L;
+	if (itransfer->timeout.tv_nsec >= NSEC_PER_SEC) {
+		++itransfer->timeout.tv_sec;
+		itransfer->timeout.tv_nsec -= NSEC_PER_SEC;
+	}
+}
+
+/** \ingroup libusb_asyncio
+ * Allocate a libusb transfer with a specified number of isochronous packet
+ * descriptors. The returned transfer is pre-initialized for you. When the new
+ * transfer is no longer needed, it should be freed with
+ * libusb_free_transfer().
+ *
+ * Transfers intended for non-isochronous endpoints (e.g. control, bulk,
+ * interrupt) should specify an iso_packets count of zero.
+ *
+ * For transfers intended for isochronous endpoints, specify an appropriate
+ * number of packet descriptors to be allocated as part of the transfer.
+ * The returned transfer is not specially initialized for isochronous I/O;
+ * you are still required to set the
+ * \ref libusb_transfer::num_iso_packets "num_iso_packets" and
+ * \ref libusb_transfer::type "type" fields accordingly.
+ *
+ * It is safe to allocate a transfer with some isochronous packets and then
+ * use it on a non-isochronous endpoint. If you do this, ensure that at time
+ * of submission, num_iso_packets is 0 and that type is set appropriately.
+ *
+ * \param iso_packets number of isochronous packet descriptors to allocate. Must be non-negative.
+ * \returns a newly allocated transfer, or NULL on error
+ */
+DEFAULT_VISIBILITY
+struct libusb_transfer * LIBUSB_CALL libusb_alloc_transfer(
+	int iso_packets)
+{
+	size_t priv_size;
+	size_t alloc_size;
+	unsigned char *ptr;
+	struct usbi_transfer *itransfer;
+	struct libusb_transfer *transfer;
+
+	assert(iso_packets >= 0);
+	if (iso_packets < 0)
+		return NULL;
+
+	priv_size = PTR_ALIGN(usbi_backend.transfer_priv_size);
+	alloc_size = priv_size
+		+ sizeof(struct usbi_transfer)
+		+ sizeof(struct libusb_transfer)
+		+ (sizeof(struct libusb_iso_packet_descriptor) * (size_t)iso_packets);
+	ptr = calloc(1, alloc_size);
+	if (!ptr)
+		return NULL;
+
+	itransfer = (struct usbi_transfer *)(ptr + priv_size);
+	itransfer->num_iso_packets = iso_packets;
+	itransfer->priv = ptr;
+	usbi_mutex_init(&itransfer->lock);
+	transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	return transfer;
+}
+
+/** \ingroup libusb_asyncio
+ * Free a transfer structure. This should be called for all transfers
+ * allocated with libusb_alloc_transfer().
+ *
+ * If the \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_BUFFER
+ * "LIBUSB_TRANSFER_FREE_BUFFER" flag is set and the transfer buffer is
+ * non-NULL, this function will also free the transfer buffer using the
+ * standard system memory allocator (e.g. free()).
+ *
+ * It is legal to call this function with a NULL transfer. In this case,
+ * the function will simply return safely.
+ *
+ * It is not legal to free an active transfer (one which has been submitted
+ * and has not yet completed).
+ *
+ * \param transfer the transfer to free
+ */
+void API_EXPORTED libusb_free_transfer(struct libusb_transfer *transfer)
+{
+	struct usbi_transfer *itransfer;
+	size_t priv_size;
+	unsigned char *ptr;
+
+	if (!transfer)
+		return;
+
+	usbi_dbg(TRANSFER_CTX(transfer), "transfer %p", transfer);
+	if (transfer->flags & LIBUSB_TRANSFER_FREE_BUFFER)
+		free(transfer->buffer);
+
+	itransfer = LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+	usbi_mutex_destroy(&itransfer->lock);
+
+	priv_size = PTR_ALIGN(usbi_backend.transfer_priv_size);
+	ptr = (unsigned char *)itransfer - priv_size;
+	assert(ptr == itransfer->priv);
+	free(ptr);
+}
+
+/* iterates through the flying transfers, and rearms the timer based on the
+ * next upcoming timeout.
+ * must be called with flying_list locked.
+ * returns 0 on success or a LIBUSB_ERROR code on failure.
+ */
+#ifdef HAVE_OS_TIMER
+static int arm_timer_for_next_timeout(struct libusb_context *ctx)
+{
+	struct usbi_transfer *itransfer;
+
+	if (!usbi_using_timer(ctx))
+		return 0;
+
+	for_each_transfer(ctx, itransfer) {
+		struct timespec *cur_ts = &itransfer->timeout;
+
+		/* if we've reached transfers of infinite timeout, then we have no
+		 * arming to do */
+		if (!TIMESPEC_IS_SET(cur_ts))
+			break;
+
+		/* act on first transfer that has not already been handled */
+		if (!(itransfer->timeout_flags & (USBI_TRANSFER_TIMEOUT_HANDLED | USBI_TRANSFER_OS_HANDLES_TIMEOUT))) {
+			usbi_dbg(ctx, "next timeout originally %ums", USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer)->timeout);
+			return usbi_arm_timer(&ctx->timer, cur_ts);
+		}
+	}
+
+	usbi_dbg(ctx, "no timeouts, disarming timer");
+	return usbi_disarm_timer(&ctx->timer);
+}
+#else
+static inline int arm_timer_for_next_timeout(struct libusb_context *ctx)
+{
+	UNUSED(ctx);
+	return 0;
+}
+#endif
+
+/* add a transfer to the (timeout-sorted) active transfers list.
+ * This function will return non 0 if fails to update the timer,
+ * in which case the transfer is *not* on the flying_transfers list. */
+static int add_to_flying_list(struct usbi_transfer *itransfer)
+{
+	struct usbi_transfer *cur;
+	struct timespec *timeout = &itransfer->timeout;
+	struct libusb_context *ctx = ITRANSFER_CTX(itransfer);
+	int r = 0;
+	int first = 1;
+
+	calculate_timeout(itransfer);
+
+	/* if we have no other flying transfers, start the list with this one */
+	if (list_empty(&ctx->flying_transfers)) {
+		list_add(&itransfer->list, &ctx->flying_transfers);
+		goto out;
+	}
+
+	/* if we have infinite timeout, append to end of list */
+	if (!TIMESPEC_IS_SET(timeout)) {
+		list_add_tail(&itransfer->list, &ctx->flying_transfers);
+		/* first is irrelevant in this case */
+		goto out;
+	}
+
+	/* otherwise, find appropriate place in list */
+	for_each_transfer(ctx, cur) {
+		/* find first timeout that occurs after the transfer in question */
+		struct timespec *cur_ts = &cur->timeout;
+
+		if (!TIMESPEC_IS_SET(cur_ts) || TIMESPEC_CMP(cur_ts, timeout, >)) {
+			list_add_tail(&itransfer->list, &cur->list);
+			goto out;
+		}
+		first = 0;
+	}
+	/* first is 0 at this stage (list not empty) */
+
+	/* otherwise we need to be inserted at the end */
+	list_add_tail(&itransfer->list, &ctx->flying_transfers);
+out:
+#ifdef HAVE_OS_TIMER
+	if (first && usbi_using_timer(ctx) && TIMESPEC_IS_SET(timeout)) {
+		/* if this transfer has the lowest timeout of all active transfers,
+		 * rearm the timer with this transfer's timeout */
+		usbi_dbg(ctx, "arm timer for timeout in %ums (first in line)",
+			USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer)->timeout);
+		r = usbi_arm_timer(&ctx->timer, timeout);
+	}
+#else
+	UNUSED(first);
+#endif
+
+	if (r)
+		list_del(&itransfer->list);
+
+	return r;
+}
+
+/* remove a transfer from the active transfers list.
+ * This function will *always* remove the transfer from the
+ * flying_transfers list. It will return a LIBUSB_ERROR code
+ * if it fails to update the timer for the next timeout. */
+static int remove_from_flying_list(struct usbi_transfer *itransfer)
+{
+	struct libusb_context *ctx = ITRANSFER_CTX(itransfer);
+	int rearm_timer;
+	int r = 0;
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	rearm_timer = (TIMESPEC_IS_SET(&itransfer->timeout) &&
+		list_first_entry(&ctx->flying_transfers, struct usbi_transfer, list) == itransfer);
+	list_del(&itransfer->list);
+	if (rearm_timer)
+		r = arm_timer_for_next_timeout(ctx);
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	return r;
+}
+
+/** \ingroup libusb_asyncio
+ * Submit a transfer. This function will fire off the USB transfer and then
+ * return immediately.
+ *
+ * \param transfer the transfer to submit
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_BUSY if the transfer has already been submitted.
+ * \returns LIBUSB_ERROR_NOT_SUPPORTED if the transfer flags are not supported
+ * by the operating system.
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the transfer size is larger than
+ * the operating system and/or hardware can support (see \ref asynclimits)
+ * \returns another LIBUSB_ERROR code on other failure
+ */
+int API_EXPORTED libusb_submit_transfer(struct libusb_transfer *transfer)
+{
+	struct usbi_transfer *itransfer =
+		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+	struct libusb_context *ctx = TRANSFER_CTX(transfer);
+	int r;
+
+	usbi_dbg(ctx, "transfer %p", transfer);
+
+	/*
+	 * Important note on locking, this function takes / releases locks
+	 * in the following order:
+	 *  take flying_transfers_lock
+	 *  take itransfer->lock
+	 *  clear transfer
+	 *  add to flying_transfers list
+	 *  release flying_transfers_lock
+	 *  submit transfer
+	 *  release itransfer->lock
+	 *  if submit failed:
+	 *   take flying_transfers_lock
+	 *   remove from flying_transfers list
+	 *   release flying_transfers_lock
+	 *
+	 * Note that it takes locks in the order a-b and then releases them
+	 * in the same order a-b. This is somewhat unusual but not wrong,
+	 * release order is not important as long as *all* locks are released
+	 * before re-acquiring any locks.
+	 *
+	 * This means that the ordering of first releasing itransfer->lock
+	 * and then re-acquiring the flying_transfers_list on error is
+	 * important and must not be changed!
+	 *
+	 * This is done this way because when we take both locks we must always
+	 * take flying_transfers_lock first to avoid ab-ba style deadlocks with
+	 * the timeout handling and usbi_handle_disconnect paths.
+	 *
+	 * And we cannot release itransfer->lock before the submission is
+	 * complete otherwise timeout handling for transfers with short
+	 * timeouts may run before submission.
+	 */
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	usbi_mutex_lock(&itransfer->lock);
+	if (itransfer->state_flags & USBI_TRANSFER_IN_FLIGHT) {
+		usbi_mutex_unlock(&ctx->flying_transfers_lock);
+		usbi_mutex_unlock(&itransfer->lock);
+		return LIBUSB_ERROR_BUSY;
+	}
+	itransfer->transferred = 0;
+	itransfer->state_flags = 0;
+	itransfer->timeout_flags = 0;
+	r = add_to_flying_list(itransfer);
+	if (r) {
+		usbi_mutex_unlock(&ctx->flying_transfers_lock);
+		usbi_mutex_unlock(&itransfer->lock);
+		return r;
+	}
+	/*
+	 * We must release the flying transfers lock here, because with
+	 * some backends the submit_transfer method is synchronous.
+	 */
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	r = usbi_backend.submit_transfer(itransfer);
+	if (r == LIBUSB_SUCCESS) {
+		itransfer->state_flags |= USBI_TRANSFER_IN_FLIGHT;
+		/* keep a reference to this device */
+		libusb_ref_device(transfer->dev_handle->dev);
+	}
+	usbi_mutex_unlock(&itransfer->lock);
+
+	if (r != LIBUSB_SUCCESS)
+		remove_from_flying_list(itransfer);
+
+	return r;
+}
+
+/** \ingroup libusb_asyncio
+ * Asynchronously cancel a previously submitted transfer.
+ * This function returns immediately, but this does not indicate cancellation
+ * is complete. Your callback function will be invoked at some later time
+ * with a transfer status of
+ * \ref libusb_transfer_status::LIBUSB_TRANSFER_CANCELLED
+ * "LIBUSB_TRANSFER_CANCELLED."
+ *
+ * \param transfer the transfer to cancel
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_NOT_FOUND if the transfer is not in progress,
+ * already complete, or already cancelled.
+ * \returns a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_cancel_transfer(struct libusb_transfer *transfer)
+{
+	struct usbi_transfer *itransfer =
+		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+	struct libusb_context *ctx = ITRANSFER_CTX(itransfer);
+	int r;
+
+	usbi_dbg(ctx, "transfer %p", transfer );
+	usbi_mutex_lock(&itransfer->lock);
+	if (!(itransfer->state_flags & USBI_TRANSFER_IN_FLIGHT)
+			|| (itransfer->state_flags & USBI_TRANSFER_CANCELLING)) {
+		r = LIBUSB_ERROR_NOT_FOUND;
+		goto out;
+	}
+	r = usbi_backend.cancel_transfer(itransfer);
+	if (r < 0) {
+		if (r != LIBUSB_ERROR_NOT_FOUND &&
+		    r != LIBUSB_ERROR_NO_DEVICE)
+			usbi_err(ctx, "cancel transfer failed error %d", r);
+		else
+			usbi_dbg(ctx, "cancel transfer failed error %d", r);
+
+		if (r == LIBUSB_ERROR_NO_DEVICE)
+			itransfer->state_flags |= USBI_TRANSFER_DEVICE_DISAPPEARED;
+	}
+
+	itransfer->state_flags |= USBI_TRANSFER_CANCELLING;
+
+out:
+	usbi_mutex_unlock(&itransfer->lock);
+	return r;
+}
+
+/** \ingroup libusb_asyncio
+ * Set a transfers bulk stream id. Note users are advised to use
+ * libusb_fill_bulk_stream_transfer() instead of calling this function
+ * directly.
+ *
+ * Since version 1.0.19, \ref LIBUSB_API_VERSION >= 0x01000103
+ *
+ * \param transfer the transfer to set the stream id for
+ * \param stream_id the stream id to set
+ * \see libusb_alloc_streams()
+ */
+void API_EXPORTED libusb_transfer_set_stream_id(
+	struct libusb_transfer *transfer, uint32_t stream_id)
+{
+	struct usbi_transfer *itransfer =
+		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+
+	itransfer->stream_id = stream_id;
+}
+
+/** \ingroup libusb_asyncio
+ * Get a transfers bulk stream id.
+ *
+ * Since version 1.0.19, \ref LIBUSB_API_VERSION >= 0x01000103
+ *
+ * \param transfer the transfer to get the stream id for
+ * \returns the stream id for the transfer
+ */
+uint32_t API_EXPORTED libusb_transfer_get_stream_id(
+	struct libusb_transfer *transfer)
+{
+	struct usbi_transfer *itransfer =
+		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
+
+	return itransfer->stream_id;
+}
+
+/* Handle completion of a transfer (completion might be an error condition).
+ * This will invoke the user-supplied callback function, which may end up
+ * freeing the transfer. Therefore you cannot use the transfer structure
+ * after calling this function, and you should free all backend-specific
+ * data before calling it.
+ * Do not call this function with the usbi_transfer lock held. User-specified
+ * callback functions may attempt to directly resubmit the transfer, which
+ * will attempt to take the lock. */
+int usbi_handle_transfer_completion(struct usbi_transfer *itransfer,
+	enum libusb_transfer_status status)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct libusb_device_handle *dev_handle = transfer->dev_handle;
+	struct libusb_context *ctx = ITRANSFER_CTX(itransfer);
+	uint8_t flags;
+	int r;
+
+	r = remove_from_flying_list(itransfer);
+	if (r < 0)
+		usbi_err(ctx, "failed to set timer for next timeout");
+
+	usbi_mutex_lock(&itransfer->lock);
+	itransfer->state_flags &= ~USBI_TRANSFER_IN_FLIGHT;
+	usbi_mutex_unlock(&itransfer->lock);
+
+	if (status == LIBUSB_TRANSFER_COMPLETED
+			&& transfer->flags & LIBUSB_TRANSFER_SHORT_NOT_OK) {
+		int rqlen = transfer->length;
+		if (transfer->type == LIBUSB_TRANSFER_TYPE_CONTROL)
+			rqlen -= LIBUSB_CONTROL_SETUP_SIZE;
+		if (rqlen != itransfer->transferred) {
+			usbi_dbg(ctx, "interpreting short transfer as error");
+			status = LIBUSB_TRANSFER_ERROR;
+		}
+	}
+
+	flags = transfer->flags;
+	transfer->status = status;
+	transfer->actual_length = itransfer->transferred;
+	usbi_dbg(ctx, "transfer %p has callback %p", transfer, transfer->callback);
+	if (transfer->callback)
+		transfer->callback(transfer);
+	/* transfer might have been freed by the above call, do not use from
+	 * this point. */
+	if (flags & LIBUSB_TRANSFER_FREE_TRANSFER)
+		libusb_free_transfer(transfer);
+	libusb_unref_device(dev_handle->dev);
+	return r;
+}
+
+/* Similar to usbi_handle_transfer_completion() but exclusively for transfers
+ * that were asynchronously cancelled. The same concerns w.r.t. freeing of
+ * transfers exist here.
+ * Do not call this function with the usbi_transfer lock held. User-specified
+ * callback functions may attempt to directly resubmit the transfer, which
+ * will attempt to take the lock. */
+int usbi_handle_transfer_cancellation(struct usbi_transfer *itransfer)
+{
+	struct libusb_context *ctx = ITRANSFER_CTX(itransfer);
+	uint8_t timed_out;
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	timed_out = itransfer->timeout_flags & USBI_TRANSFER_TIMED_OUT;
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	/* if the URB was cancelled due to timeout, report timeout to the user */
+	if (timed_out) {
+		usbi_dbg(ctx, "detected timeout cancellation");
+		return usbi_handle_transfer_completion(itransfer, LIBUSB_TRANSFER_TIMED_OUT);
+	}
+
+	/* otherwise its a normal async cancel */
+	return usbi_handle_transfer_completion(itransfer, LIBUSB_TRANSFER_CANCELLED);
+}
+
+/* Add a completed transfer to the completed_transfers list of the
+ * context and signal the event. The backend's handle_transfer_completion()
+ * function will be called the next time an event handler runs. */
+void usbi_signal_transfer_completion(struct usbi_transfer *itransfer)
+{
+	libusb_device_handle *dev_handle = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer)->dev_handle;
+
+	if (dev_handle) {
+		struct libusb_context *ctx = HANDLE_CTX(dev_handle);
+		unsigned int event_flags;
+
+		usbi_mutex_lock(&ctx->event_data_lock);
+		event_flags = ctx->event_flags;
+		ctx->event_flags |= USBI_EVENT_TRANSFER_COMPLETED;
+		list_add_tail(&itransfer->completed_list, &ctx->completed_transfers);
+		if (!event_flags)
+			usbi_signal_event(&ctx->event);
+		usbi_mutex_unlock(&ctx->event_data_lock);
+	}
+}
+
+/** \ingroup libusb_poll
+ * Attempt to acquire the event handling lock. This lock is used to ensure that
+ * only one thread is monitoring libusb event sources at any one time.
+ *
+ * You only need to use this lock if you are developing an application
+ * which calls poll() or select() on libusb's file descriptors directly.
+ * If you stick to libusb's event handling loop functions (e.g.
+ * libusb_handle_events()) then you do not need to be concerned with this
+ * locking.
+ *
+ * While holding this lock, you are trusted to actually be handling events.
+ * If you are no longer handling events, you must call libusb_unlock_events()
+ * as soon as possible.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 0 if the lock was obtained successfully
+ * \returns 1 if the lock was not obtained (i.e. another thread holds the lock)
+ * \ref libusb_mtasync
+ */
+int API_EXPORTED libusb_try_lock_events(libusb_context *ctx)
+{
+	int r;
+	unsigned int ru;
+
+	ctx = usbi_get_context(ctx);
+
+	/* is someone else waiting to close a device? if so, don't let this thread
+	 * start event handling */
+	usbi_mutex_lock(&ctx->event_data_lock);
+	ru = ctx->device_close;
+	usbi_mutex_unlock(&ctx->event_data_lock);
+	if (ru) {
+		usbi_dbg(ctx, "someone else is closing a device");
+		return 1;
+	}
+
+	r = usbi_mutex_trylock(&ctx->events_lock);
+	if (!r)
+		return 1;
+
+	ctx->event_handler_active = 1;
+	return 0;
+}
+
+/** \ingroup libusb_poll
+ * Acquire the event handling lock, blocking until successful acquisition if
+ * it is contended. This lock is used to ensure that only one thread is
+ * monitoring libusb event sources at any one time.
+ *
+ * You only need to use this lock if you are developing an application
+ * which calls poll() or select() on libusb's file descriptors directly.
+ * If you stick to libusb's event handling loop functions (e.g.
+ * libusb_handle_events()) then you do not need to be concerned with this
+ * locking.
+ *
+ * While holding this lock, you are trusted to actually be handling events.
+ * If you are no longer handling events, you must call libusb_unlock_events()
+ * as soon as possible.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \ref libusb_mtasync
+ */
+void API_EXPORTED libusb_lock_events(libusb_context *ctx)
+{
+	ctx = usbi_get_context(ctx);
+	usbi_mutex_lock(&ctx->events_lock);
+	ctx->event_handler_active = 1;
+}
+
+/** \ingroup libusb_poll
+ * Release the lock previously acquired with libusb_try_lock_events() or
+ * libusb_lock_events(). Releasing this lock will wake up any threads blocked
+ * on libusb_wait_for_event().
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \ref libusb_mtasync
+ */
+void API_EXPORTED libusb_unlock_events(libusb_context *ctx)
+{
+	ctx = usbi_get_context(ctx);
+	ctx->event_handler_active = 0;
+	usbi_mutex_unlock(&ctx->events_lock);
+
+	/* FIXME: perhaps we should be a bit more efficient by not broadcasting
+	 * the availability of the events lock when we are modifying pollfds
+	 * (check ctx->device_close)? */
+	usbi_mutex_lock(&ctx->event_waiters_lock);
+	usbi_cond_broadcast(&ctx->event_waiters_cond);
+	usbi_mutex_unlock(&ctx->event_waiters_lock);
+}
+
+/** \ingroup libusb_poll
+ * Determine if it is still OK for this thread to be doing event handling.
+ *
+ * Sometimes, libusb needs to temporarily pause all event handlers, and this
+ * is the function you should use before polling file descriptors to see if
+ * this is the case.
+ *
+ * If this function instructs your thread to give up the events lock, you
+ * should just continue the usual logic that is documented in \ref libusb_mtasync.
+ * On the next iteration, your thread will fail to obtain the events lock,
+ * and will hence become an event waiter.
+ *
+ * This function should be called while the events lock is held: you don't
+ * need to worry about the results of this function if your thread is not
+ * the current event handler.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 1 if event handling can start or continue
+ * \returns 0 if this thread must give up the events lock
+ * \ref fullstory "Multi-threaded I/O: the full story"
+ */
+int API_EXPORTED libusb_event_handling_ok(libusb_context *ctx)
+{
+	unsigned int r;
+
+	ctx = usbi_get_context(ctx);
+
+	/* is someone else waiting to close a device? if so, don't let this thread
+	 * continue event handling */
+	usbi_mutex_lock(&ctx->event_data_lock);
+	r = ctx->device_close;
+	usbi_mutex_unlock(&ctx->event_data_lock);
+	if (r) {
+		usbi_dbg(ctx, "someone else is closing a device");
+		return 0;
+	}
+
+	return 1;
+}
+
+
+/** \ingroup libusb_poll
+ * Determine if an active thread is handling events (i.e. if anyone is holding
+ * the event handling lock).
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 1 if a thread is handling events
+ * \returns 0 if there are no threads currently handling events
+ * \ref libusb_mtasync
+ */
+int API_EXPORTED libusb_event_handler_active(libusb_context *ctx)
+{
+	unsigned int r;
+
+	ctx = usbi_get_context(ctx);
+
+	/* is someone else waiting to close a device? if so, don't let this thread
+	 * start event handling -- indicate that event handling is happening */
+	usbi_mutex_lock(&ctx->event_data_lock);
+	r = ctx->device_close;
+	usbi_mutex_unlock(&ctx->event_data_lock);
+	if (r) {
+		usbi_dbg(ctx, "someone else is closing a device");
+		return 1;
+	}
+
+	return ctx->event_handler_active;
+}
+
+/** \ingroup libusb_poll
+ * Interrupt any active thread that is handling events. This is mainly useful
+ * for interrupting a dedicated event handling thread when an application
+ * wishes to call libusb_exit().
+ *
+ * Since version 1.0.21, \ref LIBUSB_API_VERSION >= 0x01000105
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \ref libusb_mtasync
+ */
+void API_EXPORTED libusb_interrupt_event_handler(libusb_context *ctx)
+{
+	unsigned int event_flags;
+
+	usbi_dbg(ctx, " ");
+
+	ctx = usbi_get_context(ctx);
+	usbi_mutex_lock(&ctx->event_data_lock);
+
+	event_flags = ctx->event_flags;
+	ctx->event_flags |= USBI_EVENT_USER_INTERRUPT;
+	if (!event_flags)
+		usbi_signal_event(&ctx->event);
+
+	usbi_mutex_unlock(&ctx->event_data_lock);
+}
+
+/** \ingroup libusb_poll
+ * Acquire the event waiters lock. This lock is designed to be obtained under
+ * the situation where you want to be aware when events are completed, but
+ * some other thread is event handling so calling libusb_handle_events() is not
+ * allowed.
+ *
+ * You then obtain this lock, re-check that another thread is still handling
+ * events, then call libusb_wait_for_event().
+ *
+ * You only need to use this lock if you are developing an application
+ * which calls poll() or select() on libusb's file descriptors directly,
+ * <b>and</b> may potentially be handling events from 2 threads simultaneously.
+ * If you stick to libusb's event handling loop functions (e.g.
+ * libusb_handle_events()) then you do not need to be concerned with this
+ * locking.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \ref libusb_mtasync
+ */
+void API_EXPORTED libusb_lock_event_waiters(libusb_context *ctx)
+{
+	ctx = usbi_get_context(ctx);
+	usbi_mutex_lock(&ctx->event_waiters_lock);
+}
+
+/** \ingroup libusb_poll
+ * Release the event waiters lock.
+ * \param ctx the context to operate on, or NULL for the default context
+ * \ref libusb_mtasync
+ */
+void API_EXPORTED libusb_unlock_event_waiters(libusb_context *ctx)
+{
+	ctx = usbi_get_context(ctx);
+	usbi_mutex_unlock(&ctx->event_waiters_lock);
+}
+
+/** \ingroup libusb_poll
+ * Wait for another thread to signal completion of an event. Must be called
+ * with the event waiters lock held, see libusb_lock_event_waiters().
+ *
+ * This function will block until any of the following conditions are met:
+ * -# The timeout expires
+ * -# A transfer completes
+ * -# A thread releases the event handling lock through libusb_unlock_events()
+ *
+ * Condition 1 is obvious. Condition 2 unblocks your thread <em>after</em>
+ * the callback for the transfer has completed. Condition 3 is important
+ * because it means that the thread that was previously handling events is no
+ * longer doing so, so if any events are to complete, another thread needs to
+ * step up and start event handling.
+ *
+ * This function releases the event waiters lock before putting your thread
+ * to sleep, and reacquires the lock as it is being woken up.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv maximum timeout for this blocking function. A NULL value
+ * indicates unlimited timeout.
+ * \returns 0 after a transfer completes or another thread stops event handling
+ * \returns 1 if the timeout expired
+ * \returns LIBUSB_ERROR_INVALID_PARAM if timeval is invalid
+ * \ref libusb_mtasync
+ */
+int API_EXPORTED libusb_wait_for_event(libusb_context *ctx, struct timeval *tv)
+{
+	int r;
+
+	ctx = usbi_get_context(ctx);
+	if (!tv) {
+		usbi_cond_wait(&ctx->event_waiters_cond, &ctx->event_waiters_lock);
+		return 0;
+	}
+
+	if (!TIMEVAL_IS_VALID(tv))
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	r = usbi_cond_timedwait(&ctx->event_waiters_cond,
+		&ctx->event_waiters_lock, tv);
+	if (r < 0)
+		return r == LIBUSB_ERROR_TIMEOUT;
+
+	return 0;
+}
+
+static void handle_timeout(struct usbi_transfer *itransfer)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	int r;
+
+	itransfer->timeout_flags |= USBI_TRANSFER_TIMEOUT_HANDLED;
+	r = libusb_cancel_transfer(transfer);
+	if (r == LIBUSB_SUCCESS)
+		itransfer->timeout_flags |= USBI_TRANSFER_TIMED_OUT;
+	else
+		usbi_warn(TRANSFER_CTX(transfer),
+			"async cancel failed %d", r);
+}
+
+static void handle_timeouts_locked(struct libusb_context *ctx)
+{
+	struct timespec systime;
+	struct usbi_transfer *itransfer;
+
+	if (list_empty(&ctx->flying_transfers))
+		return;
+
+	/* get current time */
+	usbi_get_monotonic_time(&systime);
+
+	/* iterate through flying transfers list, finding all transfers that
+	 * have expired timeouts */
+	for_each_transfer(ctx, itransfer) {
+		struct timespec *cur_ts = &itransfer->timeout;
+
+		/* if we've reached transfers of infinite timeout, we're all done */
+		if (!TIMESPEC_IS_SET(cur_ts))
+			return;
+
+		/* ignore timeouts we've already handled */
+		if (itransfer->timeout_flags & (USBI_TRANSFER_TIMEOUT_HANDLED | USBI_TRANSFER_OS_HANDLES_TIMEOUT))
+			continue;
+
+		/* if transfer has non-expired timeout, nothing more to do */
+		if (TIMESPEC_CMP(cur_ts, &systime, >))
+			return;
+
+		/* otherwise, we've got an expired timeout to handle */
+		handle_timeout(itransfer);
+	}
+}
+
+static void handle_timeouts(struct libusb_context *ctx)
+{
+	ctx = usbi_get_context(ctx);
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	handle_timeouts_locked(ctx);
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+}
+
+static int handle_event_trigger(struct libusb_context *ctx)
+{
+	struct list_head hotplug_msgs;
+	int hotplug_event = 0;
+	int r = 0;
+
+	usbi_dbg(ctx, "event triggered");
+
+	list_init(&hotplug_msgs);
+
+	/* take the the event data lock while processing events */
+	usbi_mutex_lock(&ctx->event_data_lock);
+
+	/* check if someone modified the event sources */
+	if (ctx->event_flags & USBI_EVENT_EVENT_SOURCES_MODIFIED)
+		usbi_dbg(ctx, "someone updated the event sources");
+
+	if (ctx->event_flags & USBI_EVENT_USER_INTERRUPT) {
+		usbi_dbg(ctx, "someone purposefully interrupted");
+		ctx->event_flags &= ~USBI_EVENT_USER_INTERRUPT;
+	}
+
+	if (ctx->event_flags & USBI_EVENT_HOTPLUG_CB_DEREGISTERED) {
+		usbi_dbg(ctx, "someone unregistered a hotplug cb");
+		ctx->event_flags &= ~USBI_EVENT_HOTPLUG_CB_DEREGISTERED;
+		hotplug_event = 1;
+	}
+
+	/* check if someone is closing a device */
+	if (ctx->event_flags & USBI_EVENT_DEVICE_CLOSE)
+		usbi_dbg(ctx, "someone is closing a device");
+
+	/* check for any pending hotplug messages */
+	if (ctx->event_flags & USBI_EVENT_HOTPLUG_MSG_PENDING) {
+		usbi_dbg(ctx, "hotplug message received");
+		ctx->event_flags &= ~USBI_EVENT_HOTPLUG_MSG_PENDING;
+		hotplug_event = 1;
+		assert(!list_empty(&ctx->hotplug_msgs));
+		list_cut(&hotplug_msgs, &ctx->hotplug_msgs);
+	}
+
+	/* complete any pending transfers */
+	if (ctx->event_flags & USBI_EVENT_TRANSFER_COMPLETED) {
+		struct usbi_transfer *itransfer, *tmp;
+		struct list_head completed_transfers;
+
+		assert(!list_empty(&ctx->completed_transfers));
+		list_cut(&completed_transfers, &ctx->completed_transfers);
+		usbi_mutex_unlock(&ctx->event_data_lock);
+
+		__for_each_completed_transfer_safe(&completed_transfers, itransfer, tmp) {
+			list_del(&itransfer->completed_list);
+			r = usbi_backend.handle_transfer_completion(itransfer);
+			if (r) {
+				usbi_err(ctx, "backend handle_transfer_completion failed with error %d", r);
+				break;
+			}
+		}
+
+		usbi_mutex_lock(&ctx->event_data_lock);
+		if (!list_empty(&completed_transfers)) {
+			/* an error occurred, put the remaining transfers back on the list */
+			list_splice_front(&completed_transfers, &ctx->completed_transfers);
+		} else if (list_empty(&ctx->completed_transfers)) {
+			ctx->event_flags &= ~USBI_EVENT_TRANSFER_COMPLETED;
+		}
+	}
+
+	/* if no further pending events, clear the event */
+	if (!ctx->event_flags)
+		usbi_clear_event(&ctx->event);
+
+	usbi_mutex_unlock(&ctx->event_data_lock);
+
+	/* process the hotplug events, if any */
+	if (hotplug_event)
+		usbi_hotplug_process(ctx, &hotplug_msgs);
+
+	return r;
+}
+
+#ifdef HAVE_OS_TIMER
+static int handle_timer_trigger(struct libusb_context *ctx)
+{
+	int r;
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+
+	/* process the timeout that just happened */
+	handle_timeouts_locked(ctx);
+
+	/* arm for next timeout */
+	r = arm_timer_for_next_timeout(ctx);
+
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	return r;
+}
+#endif
+
+/* do the actual event handling. assumes that no other thread is concurrently
+ * doing the same thing. */
+static int handle_events(struct libusb_context *ctx, struct timeval *tv)
+{
+	struct usbi_reported_events reported_events;
+	int r, timeout_ms;
+
+	/* prevent attempts to recursively handle events (e.g. calling into
+	 * libusb_handle_events() from within a hotplug or transfer callback) */
+	if (usbi_handling_events(ctx))
+		return LIBUSB_ERROR_BUSY;
+
+	/* only reallocate the event source data when the list of event sources has
+	 * been modified since the last handle_events(), otherwise reuse them to
+	 * save the additional overhead */
+	usbi_mutex_lock(&ctx->event_data_lock);
+	if (ctx->event_flags & USBI_EVENT_EVENT_SOURCES_MODIFIED) {
+		usbi_dbg(ctx, "event sources modified, reallocating event data");
+
+		/* free anything removed since we last ran */
+		cleanup_removed_event_sources(ctx);
+
+		r = usbi_alloc_event_data(ctx);
+		if (r) {
+			usbi_mutex_unlock(&ctx->event_data_lock);
+			return r;
+		}
+
+		/* reset the flag now that we have the updated list */
+		ctx->event_flags &= ~USBI_EVENT_EVENT_SOURCES_MODIFIED;
+
+		/* if no further pending events, clear the event so that we do
+		 * not immediately return from the wait function */
+		if (!ctx->event_flags)
+			usbi_clear_event(&ctx->event);
+	}
+	usbi_mutex_unlock(&ctx->event_data_lock);
+
+	timeout_ms = (int)(tv->tv_sec * 1000) + (tv->tv_usec / 1000);
+
+	/* round up to next millisecond */
+	if (tv->tv_usec % 1000)
+		timeout_ms++;
+
+	reported_events.event_bits = 0;
+
+	usbi_start_event_handling(ctx);
+
+	r = usbi_wait_for_events(ctx, &reported_events, timeout_ms);
+	if (r != LIBUSB_SUCCESS) {
+		if (r == LIBUSB_ERROR_TIMEOUT) {
+			handle_timeouts(ctx);
+			r = LIBUSB_SUCCESS;
+		}
+		goto done;
+	}
+
+	if (reported_events.event_triggered) {
+		r = handle_event_trigger(ctx);
+		if (r) {
+			/* return error code */
+			goto done;
+		}
+	}
+
+#ifdef HAVE_OS_TIMER
+	if (reported_events.timer_triggered) {
+		r = handle_timer_trigger(ctx);
+		if (r) {
+			/* return error code */
+			goto done;
+		}
+	}
+#endif
+
+	if (!reported_events.num_ready)
+		goto done;
+
+	r = usbi_backend.handle_events(ctx, reported_events.event_data,
+		reported_events.event_data_count, reported_events.num_ready);
+	if (r)
+		usbi_err(ctx, "backend handle_events failed with error %d", r);
+
+done:
+	usbi_end_event_handling(ctx);
+	return r;
+}
+
+/* returns the smallest of:
+ *  1. timeout of next URB
+ *  2. user-supplied timeout
+ * returns 1 if there is an already-expired timeout, otherwise returns 0
+ * and populates out
+ */
+static int get_next_timeout(libusb_context *ctx, struct timeval *tv,
+	struct timeval *out)
+{
+	struct timeval timeout;
+	int r = libusb_get_next_timeout(ctx, &timeout);
+	if (r) {
+		/* timeout already expired? */
+		if (!timerisset(&timeout))
+			return 1;
+
+		/* choose the smallest of next URB timeout or user specified timeout */
+		if (timercmp(&timeout, tv, <))
+			*out = timeout;
+		else
+			*out = *tv;
+	} else {
+		*out = *tv;
+	}
+	return 0;
+}
+
+/** \ingroup libusb_poll
+ * Handle any pending events.
+ *
+ * libusb determines "pending events" by checking if any timeouts have expired
+ * and by checking the set of file descriptors for activity.
+ *
+ * If a zero timeval is passed, this function will handle any already-pending
+ * events and then immediately return in non-blocking style.
+ *
+ * If a non-zero timeval is passed and no events are currently pending, this
+ * function will block waiting for events to handle up until the specified
+ * timeout. If an event arrives or a signal is raised, this function will
+ * return early.
+ *
+ * If the parameter completed is not NULL then <em>after obtaining the event
+ * handling lock</em> this function will return immediately if the integer
+ * pointed to is not 0. This allows for race free waiting for the completion
+ * of a specific transfer.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv the maximum time to block waiting for events, or an all zero
+ * timeval struct for non-blocking mode
+ * \param completed pointer to completion integer to check, or NULL
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_INVALID_PARAM if timeval is invalid
+ * \returns another LIBUSB_ERROR code on other failure
+ * \ref libusb_mtasync
+ */
+int API_EXPORTED libusb_handle_events_timeout_completed(libusb_context *ctx,
+	struct timeval *tv, int *completed)
+{
+	int r;
+	struct timeval poll_timeout;
+
+	if (!TIMEVAL_IS_VALID(tv))
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	ctx = usbi_get_context(ctx);
+	r = get_next_timeout(ctx, tv, &poll_timeout);
+	if (r) {
+		/* timeout already expired */
+		handle_timeouts(ctx);
+		return 0;
+	}
+
+retry:
+	if (libusb_try_lock_events(ctx) == 0) {
+		if (completed == NULL || !*completed) {
+			/* we obtained the event lock: do our own event handling */
+			usbi_dbg(ctx, "doing our own event handling");
+			r = handle_events(ctx, &poll_timeout);
+		}
+		libusb_unlock_events(ctx);
+		return r;
+	}
+
+	/* another thread is doing event handling. wait for thread events that
+	 * notify event completion. */
+	libusb_lock_event_waiters(ctx);
+
+	if (completed && *completed)
+		goto already_done;
+
+	if (!libusb_event_handler_active(ctx)) {
+		/* we hit a race: whoever was event handling earlier finished in the
+		 * time it took us to reach this point. try the cycle again. */
+		libusb_unlock_event_waiters(ctx);
+		usbi_dbg(ctx, "event handler was active but went away, retrying");
+		goto retry;
+	}
+
+	usbi_dbg(ctx, "another thread is doing event handling");
+	r = libusb_wait_for_event(ctx, &poll_timeout);
+
+already_done:
+	libusb_unlock_event_waiters(ctx);
+
+	if (r < 0)
+		return r;
+	else if (r == 1)
+		handle_timeouts(ctx);
+	return 0;
+}
+
+/** \ingroup libusb_poll
+ * Handle any pending events
+ *
+ * Like libusb_handle_events_timeout_completed(), but without the completed
+ * parameter, calling this function is equivalent to calling
+ * libusb_handle_events_timeout_completed() with a NULL completed parameter.
+ *
+ * This function is kept primarily for backwards compatibility.
+ * All new code should call libusb_handle_events_completed() or
+ * libusb_handle_events_timeout_completed() to avoid race conditions.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv the maximum time to block waiting for events, or an all zero
+ * timeval struct for non-blocking mode
+ * \returns 0 on success, or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_handle_events_timeout(libusb_context *ctx,
+	struct timeval *tv)
+{
+	return libusb_handle_events_timeout_completed(ctx, tv, NULL);
+}
+
+/** \ingroup libusb_poll
+ * Handle any pending events in blocking mode. There is currently a timeout
+ * hard-coded at 60 seconds but we plan to make it unlimited in future. For
+ * finer control over whether this function is blocking or non-blocking, or
+ * for control over the timeout, use libusb_handle_events_timeout_completed()
+ * instead.
+ *
+ * This function is kept primarily for backwards compatibility.
+ * All new code should call libusb_handle_events_completed() or
+ * libusb_handle_events_timeout_completed() to avoid race conditions.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 0 on success, or a LIBUSB_ERROR code on failure
+ */
+int API_EXPORTED libusb_handle_events(libusb_context *ctx)
+{
+	struct timeval tv;
+	tv.tv_sec = 60;
+	tv.tv_usec = 0;
+	return libusb_handle_events_timeout_completed(ctx, &tv, NULL);
+}
+
+/** \ingroup libusb_poll
+ * Handle any pending events in blocking mode.
+ *
+ * Like libusb_handle_events(), with the addition of a completed parameter
+ * to allow for race free waiting for the completion of a specific transfer.
+ *
+ * See libusb_handle_events_timeout_completed() for details on the completed
+ * parameter.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param completed pointer to completion integer to check, or NULL
+ * \returns 0 on success, or a LIBUSB_ERROR code on failure
+ * \ref libusb_mtasync
+ */
+int API_EXPORTED libusb_handle_events_completed(libusb_context *ctx,
+	int *completed)
+{
+	struct timeval tv;
+	tv.tv_sec = 60;
+	tv.tv_usec = 0;
+	return libusb_handle_events_timeout_completed(ctx, &tv, completed);
+}
+
+/** \ingroup libusb_poll
+ * Handle any pending events by polling file descriptors, without checking if
+ * any other threads are already doing so. Must be called with the event lock
+ * held, see libusb_lock_events().
+ *
+ * This function is designed to be called under the situation where you have
+ * taken the event lock and are calling poll()/select() directly on libusb's
+ * file descriptors (as opposed to using libusb_handle_events() or similar).
+ * You detect events on libusb's descriptors, so you then call this function
+ * with a zero timeout value (while still holding the event lock).
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv the maximum time to block waiting for events, or zero for
+ * non-blocking mode
+ * \returns 0 on success
+ * \returns LIBUSB_ERROR_INVALID_PARAM if timeval is invalid
+ * \returns another LIBUSB_ERROR code on other failure
+ * \ref libusb_mtasync
+ */
+int API_EXPORTED libusb_handle_events_locked(libusb_context *ctx,
+	struct timeval *tv)
+{
+	int r;
+	struct timeval poll_timeout;
+
+	if (!TIMEVAL_IS_VALID(tv))
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	ctx = usbi_get_context(ctx);
+	r = get_next_timeout(ctx, tv, &poll_timeout);
+	if (r) {
+		/* timeout already expired */
+		handle_timeouts(ctx);
+		return 0;
+	}
+
+	return handle_events(ctx, &poll_timeout);
+}
+
+/** \ingroup libusb_poll
+ * Determines whether your application must apply special timing considerations
+ * when monitoring libusb's file descriptors.
+ *
+ * This function is only useful for applications which retrieve and poll
+ * libusb's file descriptors in their own main loop (\ref libusb_pollmain).
+ *
+ * Ordinarily, libusb's event handler needs to be called into at specific
+ * moments in time (in addition to times when there is activity on the file
+ * descriptor set). The usual approach is to use libusb_get_next_timeout()
+ * to learn about when the next timeout occurs, and to adjust your
+ * poll()/select() timeout accordingly so that you can make a call into the
+ * library at that time.
+ *
+ * Some platforms supported by libusb do not come with this baggage - any
+ * events relevant to timing will be represented by activity on the file
+ * descriptor set, and libusb_get_next_timeout() will always return 0.
+ * This function allows you to detect whether you are running on such a
+ * platform.
+ *
+ * Since v1.0.5.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns 0 if you must call into libusb at times determined by
+ * libusb_get_next_timeout(), or 1 if all timeout events are handled internally
+ * or through regular activity on the file descriptors.
+ * \ref libusb_pollmain "Polling libusb file descriptors for event handling"
+ */
+int API_EXPORTED libusb_pollfds_handle_timeouts(libusb_context *ctx)
+{
+	ctx = usbi_get_context(ctx);
+	return usbi_using_timer(ctx);
+}
+
+/** \ingroup libusb_poll
+ * Determine the next internal timeout that libusb needs to handle. You only
+ * need to use this function if you are calling poll() or select() or similar
+ * on libusb's file descriptors yourself - you do not need to use it if you
+ * are calling libusb_handle_events() or a variant directly.
+ *
+ * You should call this function in your main loop in order to determine how
+ * long to wait for select() or poll() to return results. libusb needs to be
+ * called into at this timeout, so you should use it as an upper bound on
+ * your select() or poll() call.
+ *
+ * When the timeout has expired, call into libusb_handle_events_timeout()
+ * (perhaps in non-blocking mode) so that libusb can handle the timeout.
+ *
+ * This function may return 1 (success) and an all-zero timeval. If this is
+ * the case, it indicates that libusb has a timeout that has already expired
+ * so you should call libusb_handle_events_timeout() or similar immediately.
+ * A return code of 0 indicates that there are no pending timeouts.
+ *
+ * On some platforms, this function will always returns 0 (no pending
+ * timeouts). See \ref polltime.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param tv output location for a relative time against the current
+ * clock in which libusb must be called into in order to process timeout events
+ * \returns 0 if there are no pending timeouts, 1 if a timeout was returned,
+ * or LIBUSB_ERROR_OTHER on failure
+ */
+int API_EXPORTED libusb_get_next_timeout(libusb_context *ctx,
+	struct timeval *tv)
+{
+	struct usbi_transfer *itransfer;
+	struct timespec systime;
+	struct timespec next_timeout = { 0, 0 };
+
+	ctx = usbi_get_context(ctx);
+	if (usbi_using_timer(ctx))
+		return 0;
+
+	usbi_mutex_lock(&ctx->flying_transfers_lock);
+	if (list_empty(&ctx->flying_transfers)) {
+		usbi_mutex_unlock(&ctx->flying_transfers_lock);
+		usbi_dbg(ctx, "no URBs, no timeout!");
+		return 0;
+	}
+
+	/* find next transfer which hasn't already been processed as timed out */
+	for_each_transfer(ctx, itransfer) {
+		if (itransfer->timeout_flags & (USBI_TRANSFER_TIMEOUT_HANDLED | USBI_TRANSFER_OS_HANDLES_TIMEOUT))
+			continue;
+
+		/* if we've reached transfers of infinite timeout, we're done looking */
+		if (!TIMESPEC_IS_SET(&itransfer->timeout))
+			break;
+
+		next_timeout = itransfer->timeout;
+		break;
+	}
+	usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+	if (!TIMESPEC_IS_SET(&next_timeout)) {
+		usbi_dbg(ctx, "no URB with timeout or all handled by OS; no timeout!");
+		return 0;
+	}
+
+	usbi_get_monotonic_time(&systime);
+
+	if (!TIMESPEC_CMP(&systime, &next_timeout, <)) {
+		usbi_dbg(ctx, "first timeout already expired");
+		timerclear(tv);
+	} else {
+		TIMESPEC_SUB(&next_timeout, &systime, &next_timeout);
+		TIMESPEC_TO_TIMEVAL(tv, &next_timeout);
+		usbi_dbg(ctx, "next timeout in %ld.%06lds", (long)tv->tv_sec, (long)tv->tv_usec);
+	}
+
+	return 1;
+}
+
+/** \ingroup libusb_poll
+ * Register notification functions for file descriptor additions/removals.
+ * These functions will be invoked for every new or removed file descriptor
+ * that libusb uses as an event source.
+ *
+ * To remove notifiers, pass NULL values for the function pointers.
+ *
+ * Note that file descriptors may have been added even before you register
+ * these notifiers (e.g. at libusb_init() time).
+ *
+ * Additionally, note that the removal notifier may be called during
+ * libusb_exit() (e.g. when it is closing file descriptors that were opened
+ * and added to the poll set at libusb_init() time). If you don't want this,
+ * remove the notifiers immediately before calling libusb_exit().
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \param added_cb pointer to function for addition notifications
+ * \param removed_cb pointer to function for removal notifications
+ * \param user_data User data to be passed back to callbacks (useful for
+ * passing context information)
+ */
+void API_EXPORTED libusb_set_pollfd_notifiers(libusb_context *ctx,
+	libusb_pollfd_added_cb added_cb, libusb_pollfd_removed_cb removed_cb,
+	void *user_data)
+{
+#if !defined(PLATFORM_WINDOWS)
+	ctx = usbi_get_context(ctx);
+	ctx->fd_added_cb = added_cb;
+	ctx->fd_removed_cb = removed_cb;
+	ctx->fd_cb_user_data = user_data;
+#else
+	usbi_err(ctx, "external polling of libusb's internal event sources " \
+		"is not yet supported on Windows");
+	UNUSED(added_cb);
+	UNUSED(removed_cb);
+	UNUSED(user_data);
+#endif
+}
+
+/*
+ * Interrupt the iteration of the event handling thread, so that it picks
+ * up the event source change. Callers of this function must hold the event_data_lock.
+ */
+static void usbi_event_source_notification(struct libusb_context *ctx)
+{
+	unsigned int event_flags;
+
+	/* Record that there is a new poll fd.
+	 * Only signal an event if there are no prior pending events. */
+	event_flags = ctx->event_flags;
+	ctx->event_flags |= USBI_EVENT_EVENT_SOURCES_MODIFIED;
+	if (!event_flags)
+		usbi_signal_event(&ctx->event);
+}
+
+/* Add an event source to the list of event sources to be monitored.
+ * poll_events should be specified as a bitmask of events passed to poll(), e.g.
+ * POLLIN and/or POLLOUT. */
+int usbi_add_event_source(struct libusb_context *ctx, usbi_os_handle_t os_handle, short poll_events)
+{
+	struct usbi_event_source *ievent_source = malloc(sizeof(*ievent_source));
+
+	if (!ievent_source)
+		return LIBUSB_ERROR_NO_MEM;
+
+	usbi_dbg(ctx, "add " USBI_OS_HANDLE_FORMAT_STRING " events %d", os_handle, poll_events);
+	ievent_source->data.os_handle = os_handle;
+	ievent_source->data.poll_events = poll_events;
+	usbi_mutex_lock(&ctx->event_data_lock);
+	list_add_tail(&ievent_source->list, &ctx->event_sources);
+	usbi_event_source_notification(ctx);
+	usbi_mutex_unlock(&ctx->event_data_lock);
+
+#if !defined(PLATFORM_WINDOWS)
+	if (ctx->fd_added_cb)
+		ctx->fd_added_cb(os_handle, poll_events, ctx->fd_cb_user_data);
+#endif
+
+	return 0;
+}
+
+/* Remove an event source from the list of event sources to be monitored. */
+void usbi_remove_event_source(struct libusb_context *ctx, usbi_os_handle_t os_handle)
+{
+	struct usbi_event_source *ievent_source;
+	int found = 0;
+
+	usbi_dbg(ctx, "remove " USBI_OS_HANDLE_FORMAT_STRING, os_handle);
+	usbi_mutex_lock(&ctx->event_data_lock);
+	for_each_event_source(ctx, ievent_source) {
+		if (ievent_source->data.os_handle == os_handle) {
+			found = 1;
+			break;
+		}
+	}
+
+	if (!found) {
+		usbi_dbg(ctx, "couldn't find " USBI_OS_HANDLE_FORMAT_STRING " to remove", os_handle);
+		usbi_mutex_unlock(&ctx->event_data_lock);
+		return;
+	}
+
+	list_del(&ievent_source->list);
+	list_add_tail(&ievent_source->list, &ctx->removed_event_sources);
+	usbi_event_source_notification(ctx);
+	usbi_mutex_unlock(&ctx->event_data_lock);
+
+#if !defined(PLATFORM_WINDOWS)
+	if (ctx->fd_removed_cb)
+		ctx->fd_removed_cb(os_handle, ctx->fd_cb_user_data);
+#endif
+}
+
+/** \ingroup libusb_poll
+ * Retrieve a list of file descriptors that should be polled by your main loop
+ * as libusb event sources.
+ *
+ * The returned list is NULL-terminated and should be freed with libusb_free_pollfds()
+ * when done. The actual list contents must not be touched.
+ *
+ * As file descriptors are a Unix-specific concept, this function is not
+ * available on Windows and will always return NULL.
+ *
+ * \param ctx the context to operate on, or NULL for the default context
+ * \returns a NULL-terminated list of libusb_pollfd structures
+ * \returns NULL on error
+ * \returns NULL on platforms where the functionality is not available
+ */
+DEFAULT_VISIBILITY
+const struct libusb_pollfd ** LIBUSB_CALL libusb_get_pollfds(
+	libusb_context *ctx)
+{
+#if !defined(PLATFORM_WINDOWS)
+	struct libusb_pollfd **ret = NULL;
+	struct usbi_event_source *ievent_source;
+	size_t i;
+
+	static_assert(sizeof(struct usbi_event_source_data) == sizeof(struct libusb_pollfd),
+		      "mismatch between usbi_event_source_data and libusb_pollfd sizes");
+
+	ctx = usbi_get_context(ctx);
+
+	usbi_mutex_lock(&ctx->event_data_lock);
+
+	i = 0;
+	for_each_event_source(ctx, ievent_source)
+		i++;
+
+	ret = calloc(i + 1, sizeof(struct libusb_pollfd *));
+	if (!ret)
+		goto out;
+
+	i = 0;
+	for_each_event_source(ctx, ievent_source)
+		ret[i++] = (struct libusb_pollfd *)ievent_source;
+
+out:
+	usbi_mutex_unlock(&ctx->event_data_lock);
+	return (const struct libusb_pollfd **)ret;
+#else
+	usbi_err(ctx, "external polling of libusb's internal event sources " \
+		"is not yet supported on Windows");
+	return NULL;
+#endif
+}
+
+/** \ingroup libusb_poll
+ * Free a list of libusb_pollfd structures. This should be called for all
+ * pollfd lists allocated with libusb_get_pollfds().
+ *
+ * Since version 1.0.20, \ref LIBUSB_API_VERSION >= 0x01000104
+ *
+ * It is legal to call this function with a NULL pollfd list. In this case,
+ * the function will simply do nothing.
+ *
+ * \param pollfds the list of libusb_pollfd structures to free
+ */
+void API_EXPORTED libusb_free_pollfds(const struct libusb_pollfd **pollfds)
+{
+#if !defined(PLATFORM_WINDOWS)
+	free((void *)pollfds);
+#else
+	UNUSED(pollfds);
+#endif
+}
+
+/* Backends may call this from handle_events to report disconnection of a
+ * device. This function ensures transfers get cancelled appropriately.
+ * Callers of this function must hold the events_lock.
+ */
+void usbi_handle_disconnect(struct libusb_device_handle *dev_handle)
+{
+	struct libusb_context *ctx = HANDLE_CTX(dev_handle);
+	struct usbi_transfer *cur;
+	struct usbi_transfer *to_cancel;
+
+	usbi_dbg(ctx, "device %d.%d",
+		dev_handle->dev->bus_number, dev_handle->dev->device_address);
+
+	/* terminate all pending transfers with the LIBUSB_TRANSFER_NO_DEVICE
+	 * status code.
+	 *
+	 * when we find a transfer for this device on the list, there are two
+	 * possible scenarios:
+	 * 1. the transfer is currently in-flight, in which case we terminate the
+	 *    transfer here
+	 * 2. the transfer has been added to the flying transfer list by
+	 *    libusb_submit_transfer, has failed to submit and
+	 *    libusb_submit_transfer is waiting for us to release the
+	 *    flying_transfers_lock to remove it, so we ignore it
+	 */
+
+	while (1) {
+		to_cancel = NULL;
+		usbi_mutex_lock(&ctx->flying_transfers_lock);
+		for_each_transfer(ctx, cur) {
+			if (USBI_TRANSFER_TO_LIBUSB_TRANSFER(cur)->dev_handle == dev_handle) {
+				usbi_mutex_lock(&cur->lock);
+				if (cur->state_flags & USBI_TRANSFER_IN_FLIGHT)
+					to_cancel = cur;
+				usbi_mutex_unlock(&cur->lock);
+
+				if (to_cancel)
+					break;
+			}
+		}
+		usbi_mutex_unlock(&ctx->flying_transfers_lock);
+
+		if (!to_cancel)
+			break;
+
+		usbi_dbg(ctx, "cancelling transfer %p from disconnect",
+			 USBI_TRANSFER_TO_LIBUSB_TRANSFER(to_cancel));
+
+		usbi_mutex_lock(&to_cancel->lock);
+		usbi_backend.clear_transfer_priv(to_cancel);
+		usbi_mutex_unlock(&to_cancel->lock);
+		usbi_handle_transfer_completion(to_cancel, LIBUSB_TRANSFER_NO_DEVICE);
+	}
+}
diff --git a/hardware/src/libusb/libusb.h b/hardware/src/libusb/libusb.h
new file mode 100644
index 0000000..61cacc9
--- /dev/null
+++ b/hardware/src/libusb/libusb.h
@@ -0,0 +1,2135 @@
+/*
+ * Public libusb header file
+ * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ * Copyright © 2007-2008 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2012 Pete Batard <pete@akeo.ie>
+ * Copyright © 2012-2018 Nathan Hjelm <hjelmn@cs.unm.edu>
+ * Copyright © 2014-2020 Chris Dickens <christopher.a.dickens@gmail.com>
+ * For more information, please visit: http://libusb.info
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef LIBUSB_H
+#define LIBUSB_H
+
+#if defined(_MSC_VER)
+/* on MS environments, the inline keyword is available in C++ only */
+#if !defined(__cplusplus)
+#define inline __inline
+#endif
+/* ssize_t is also not available */
+#ifndef _SSIZE_T_DEFINED
+#define _SSIZE_T_DEFINED
+#include <basetsd.h>
+typedef SSIZE_T ssize_t;
+#endif /* _SSIZE_T_DEFINED */
+#endif /* _MSC_VER */
+
+#include <limits.h>
+#include <stdint.h>
+#include <sys/types.h>
+#if !defined(_MSC_VER)
+#include <sys/time.h>
+#endif
+#include <time.h>
+
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
+#define ZERO_SIZED_ARRAY		/* [] - valid C99 code */
+#else
+#define ZERO_SIZED_ARRAY	0	/* [0] - non-standard, but usually working code */
+#endif /* __STDC_VERSION__ */
+
+/* 'interface' might be defined as a macro on Windows, so we need to
+ * undefine it so as not to break the current libusb API, because
+ * libusb_config_descriptor has an 'interface' member
+ * As this can be problematic if you include windows.h after libusb.h
+ * in your sources, we force windows.h to be included first. */
+#if defined(_WIN32) || defined(__CYGWIN__)
+#include <windows.h>
+#if defined(interface)
+#undef interface
+#endif
+#if !defined(__CYGWIN__)
+#include <winsock.h>
+#endif
+#endif /* _WIN32 || __CYGWIN__ */
+
+#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5))
+#define LIBUSB_DEPRECATED_FOR(f) __attribute__ ((deprecated ("Use " #f " instead")))
+#elif defined(__GNUC__) && (__GNUC__ >= 3)
+#define LIBUSB_DEPRECATED_FOR(f) __attribute__ ((deprecated))
+#else
+#define LIBUSB_DEPRECATED_FOR(f)
+#endif /* __GNUC__ */
+
+#if defined(__GNUC__)
+#define LIBUSB_PACKED __attribute__ ((packed))
+#else
+#define LIBUSB_PACKED
+#endif /* __GNUC__ */
+
+/** \def LIBUSB_CALL
+ * \ingroup libusb_misc
+ * libusb's Windows calling convention.
+ *
+ * Under Windows, the selection of available compilers and configurations
+ * means that, unlike other platforms, there is not <em>one true calling
+ * convention</em> (calling convention: the manner in which parameters are
+ * passed to functions in the generated assembly code).
+ *
+ * Matching the Windows API itself, libusb uses the WINAPI convention (which
+ * translates to the <tt>stdcall</tt> convention) and guarantees that the
+ * library is compiled in this way. The public header file also includes
+ * appropriate annotations so that your own software will use the right
+ * convention, even if another convention is being used by default within
+ * your codebase.
+ *
+ * The one consideration that you must apply in your software is to mark
+ * all functions which you use as libusb callbacks with this LIBUSB_CALL
+ * annotation, so that they too get compiled for the correct calling
+ * convention.
+ *
+ * On non-Windows operating systems, this macro is defined as nothing. This
+ * means that you can apply it to your code without worrying about
+ * cross-platform compatibility.
+ */
+/* LIBUSB_CALL must be defined on both definition and declaration of libusb
+ * functions. You'd think that declaration would be enough, but cygwin will
+ * complain about conflicting types unless both are marked this way.
+ * The placement of this macro is important too; it must appear after the
+ * return type, before the function name. See internal documentation for
+ * API_EXPORTED.
+ */
+#if defined(_WIN32) || defined(__CYGWIN__)
+#define LIBUSB_CALL WINAPI
+#else
+#define LIBUSB_CALL
+#endif /* _WIN32 || __CYGWIN__ */
+
+/** \def LIBUSB_API_VERSION
+ * \ingroup libusb_misc
+ * libusb's API version.
+ *
+ * Since version 1.0.13, to help with feature detection, libusb defines
+ * a LIBUSB_API_VERSION macro that gets increased every time there is a
+ * significant change to the API, such as the introduction of a new call,
+ * the definition of a new macro/enum member, or any other element that
+ * libusb applications may want to detect at compilation time.
+ *
+ * The macro is typically used in an application as follows:
+ * \code
+ * #if defined(LIBUSB_API_VERSION) && (LIBUSB_API_VERSION >= 0x01001234)
+ * // Use one of the newer features from the libusb API
+ * #endif
+ * \endcode
+ *
+ * Internally, LIBUSB_API_VERSION is defined as follows:
+ * (libusb major << 24) | (libusb minor << 16) | (16 bit incremental)
+ */
+#define LIBUSB_API_VERSION 0x01000109
+
+/* The following is kept for compatibility, but will be deprecated in the future */
+#define LIBUSBX_API_VERSION LIBUSB_API_VERSION
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+/**
+ * \ingroup libusb_misc
+ * Convert a 16-bit value from host-endian to little-endian format. On
+ * little endian systems, this function does nothing. On big endian systems,
+ * the bytes are swapped.
+ * \param x the host-endian value to convert
+ * \returns the value in little-endian byte order
+ */
+static inline uint16_t libusb_cpu_to_le16(const uint16_t x)
+{
+	union {
+		uint8_t  b8[2];
+		uint16_t b16;
+	} _tmp;
+	_tmp.b8[1] = (uint8_t) (x >> 8);
+	_tmp.b8[0] = (uint8_t) (x & 0xff);
+	return _tmp.b16;
+}
+
+/** \def libusb_le16_to_cpu
+ * \ingroup libusb_misc
+ * Convert a 16-bit value from little-endian to host-endian format. On
+ * little endian systems, this function does nothing. On big endian systems,
+ * the bytes are swapped.
+ * \param x the little-endian value to convert
+ * \returns the value in host-endian byte order
+ */
+#define libusb_le16_to_cpu libusb_cpu_to_le16
+
+/* standard USB stuff */
+
+/** \ingroup libusb_desc
+ * Device and/or Interface Class codes */
+enum libusb_class_code {
+	/** In the context of a \ref libusb_device_descriptor "device descriptor",
+	 * this bDeviceClass value indicates that each interface specifies its
+	 * own class information and all interfaces operate independently.
+	 */
+	LIBUSB_CLASS_PER_INTERFACE = 0x00,
+
+	/** Audio class */
+	LIBUSB_CLASS_AUDIO = 0x01,
+
+	/** Communications class */
+	LIBUSB_CLASS_COMM = 0x02,
+
+	/** Human Interface Device class */
+	LIBUSB_CLASS_HID = 0x03,
+
+	/** Physical */
+	LIBUSB_CLASS_PHYSICAL = 0x05,
+
+	/** Image class */
+	LIBUSB_CLASS_IMAGE = 0x06,
+	LIBUSB_CLASS_PTP = 0x06, /* legacy name from libusb-0.1 usb.h */
+
+	/** Printer class */
+	LIBUSB_CLASS_PRINTER = 0x07,
+
+	/** Mass storage class */
+	LIBUSB_CLASS_MASS_STORAGE = 0x08,
+
+	/** Hub class */
+	LIBUSB_CLASS_HUB = 0x09,
+
+	/** Data class */
+	LIBUSB_CLASS_DATA = 0x0a,
+
+	/** Smart Card */
+	LIBUSB_CLASS_SMART_CARD = 0x0b,
+
+	/** Content Security */
+	LIBUSB_CLASS_CONTENT_SECURITY = 0x0d,
+
+	/** Video */
+	LIBUSB_CLASS_VIDEO = 0x0e,
+
+	/** Personal Healthcare */
+	LIBUSB_CLASS_PERSONAL_HEALTHCARE = 0x0f,
+
+	/** Diagnostic Device */
+	LIBUSB_CLASS_DIAGNOSTIC_DEVICE = 0xdc,
+
+	/** Wireless class */
+	LIBUSB_CLASS_WIRELESS = 0xe0,
+
+	/** Miscellaneous class */
+	LIBUSB_CLASS_MISCELLANEOUS = 0xef,
+
+	/** Application class */
+	LIBUSB_CLASS_APPLICATION = 0xfe,
+
+	/** Class is vendor-specific */
+	LIBUSB_CLASS_VENDOR_SPEC = 0xff
+};
+
+/** \ingroup libusb_desc
+ * Descriptor types as defined by the USB specification. */
+enum libusb_descriptor_type {
+	/** Device descriptor. See libusb_device_descriptor. */
+	LIBUSB_DT_DEVICE = 0x01,
+
+	/** Configuration descriptor. See libusb_config_descriptor. */
+	LIBUSB_DT_CONFIG = 0x02,
+
+	/** String descriptor */
+	LIBUSB_DT_STRING = 0x03,
+
+	/** Interface descriptor. See libusb_interface_descriptor. */
+	LIBUSB_DT_INTERFACE = 0x04,
+
+	/** Endpoint descriptor. See libusb_endpoint_descriptor. */
+	LIBUSB_DT_ENDPOINT = 0x05,
+
+	/** BOS descriptor */
+	LIBUSB_DT_BOS = 0x0f,
+
+	/** Device Capability descriptor */
+	LIBUSB_DT_DEVICE_CAPABILITY = 0x10,
+
+	/** HID descriptor */
+	LIBUSB_DT_HID = 0x21,
+
+	/** HID report descriptor */
+	LIBUSB_DT_REPORT = 0x22,
+
+	/** Physical descriptor */
+	LIBUSB_DT_PHYSICAL = 0x23,
+
+	/** Hub descriptor */
+	LIBUSB_DT_HUB = 0x29,
+
+	/** SuperSpeed Hub descriptor */
+	LIBUSB_DT_SUPERSPEED_HUB = 0x2a,
+
+	/** SuperSpeed Endpoint Companion descriptor */
+	LIBUSB_DT_SS_ENDPOINT_COMPANION = 0x30
+};
+
+/* Descriptor sizes per descriptor type */
+#define LIBUSB_DT_DEVICE_SIZE			18
+#define LIBUSB_DT_CONFIG_SIZE			9
+#define LIBUSB_DT_INTERFACE_SIZE		9
+#define LIBUSB_DT_ENDPOINT_SIZE			7
+#define LIBUSB_DT_ENDPOINT_AUDIO_SIZE		9	/* Audio extension */
+#define LIBUSB_DT_HUB_NONVAR_SIZE		7
+#define LIBUSB_DT_SS_ENDPOINT_COMPANION_SIZE	6
+#define LIBUSB_DT_BOS_SIZE			5
+#define LIBUSB_DT_DEVICE_CAPABILITY_SIZE	3
+
+/* BOS descriptor sizes */
+#define LIBUSB_BT_USB_2_0_EXTENSION_SIZE	7
+#define LIBUSB_BT_SS_USB_DEVICE_CAPABILITY_SIZE	10
+#define LIBUSB_BT_CONTAINER_ID_SIZE		20
+
+/* We unwrap the BOS => define its max size */
+#define LIBUSB_DT_BOS_MAX_SIZE				\
+	(LIBUSB_DT_BOS_SIZE +				\
+	 LIBUSB_BT_USB_2_0_EXTENSION_SIZE +		\
+	 LIBUSB_BT_SS_USB_DEVICE_CAPABILITY_SIZE +	\
+	 LIBUSB_BT_CONTAINER_ID_SIZE)
+
+#define LIBUSB_ENDPOINT_ADDRESS_MASK		0x0f	/* in bEndpointAddress */
+#define LIBUSB_ENDPOINT_DIR_MASK		0x80
+
+/** \ingroup libusb_desc
+ * Endpoint direction. Values for bit 7 of the
+ * \ref libusb_endpoint_descriptor::bEndpointAddress "endpoint address" scheme.
+ */
+enum libusb_endpoint_direction {
+	/** Out: host-to-device */
+	LIBUSB_ENDPOINT_OUT = 0x00,
+
+	/** In: device-to-host */
+	LIBUSB_ENDPOINT_IN = 0x80
+};
+
+#define LIBUSB_TRANSFER_TYPE_MASK		0x03	/* in bmAttributes */
+
+/** \ingroup libusb_desc
+ * Endpoint transfer type. Values for bits 0:1 of the
+ * \ref libusb_endpoint_descriptor::bmAttributes "endpoint attributes" field.
+ */
+enum libusb_endpoint_transfer_type {
+	/** Control endpoint */
+	LIBUSB_ENDPOINT_TRANSFER_TYPE_CONTROL = 0x0,
+
+	/** Isochronous endpoint */
+	LIBUSB_ENDPOINT_TRANSFER_TYPE_ISOCHRONOUS = 0x1,
+
+	/** Bulk endpoint */
+	LIBUSB_ENDPOINT_TRANSFER_TYPE_BULK = 0x2,
+
+	/** Interrupt endpoint */
+	LIBUSB_ENDPOINT_TRANSFER_TYPE_INTERRUPT = 0x3
+};
+
+/** \ingroup libusb_misc
+ * Standard requests, as defined in table 9-5 of the USB 3.0 specifications */
+enum libusb_standard_request {
+	/** Request status of the specific recipient */
+	LIBUSB_REQUEST_GET_STATUS = 0x00,
+
+	/** Clear or disable a specific feature */
+	LIBUSB_REQUEST_CLEAR_FEATURE = 0x01,
+
+	/* 0x02 is reserved */
+
+	/** Set or enable a specific feature */
+	LIBUSB_REQUEST_SET_FEATURE = 0x03,
+
+	/* 0x04 is reserved */
+
+	/** Set device address for all future accesses */
+	LIBUSB_REQUEST_SET_ADDRESS = 0x05,
+
+	/** Get the specified descriptor */
+	LIBUSB_REQUEST_GET_DESCRIPTOR = 0x06,
+
+	/** Used to update existing descriptors or add new descriptors */
+	LIBUSB_REQUEST_SET_DESCRIPTOR = 0x07,
+
+	/** Get the current device configuration value */
+	LIBUSB_REQUEST_GET_CONFIGURATION = 0x08,
+
+	/** Set device configuration */
+	LIBUSB_REQUEST_SET_CONFIGURATION = 0x09,
+
+	/** Return the selected alternate setting for the specified interface */
+	LIBUSB_REQUEST_GET_INTERFACE = 0x0a,
+
+	/** Select an alternate interface for the specified interface */
+	LIBUSB_REQUEST_SET_INTERFACE = 0x0b,
+
+	/** Set then report an endpoint's synchronization frame */
+	LIBUSB_REQUEST_SYNCH_FRAME = 0x0c,
+
+	/** Sets both the U1 and U2 Exit Latency */
+	LIBUSB_REQUEST_SET_SEL = 0x30,
+
+	/** Delay from the time a host transmits a packet to the time it is
+	  * received by the device. */
+	LIBUSB_SET_ISOCH_DELAY = 0x31
+};
+
+/** \ingroup libusb_misc
+ * Request type bits of the
+ * \ref libusb_control_setup::bmRequestType "bmRequestType" field in control
+ * transfers. */
+enum libusb_request_type {
+	/** Standard */
+	LIBUSB_REQUEST_TYPE_STANDARD = (0x00 << 5),
+
+	/** Class */
+	LIBUSB_REQUEST_TYPE_CLASS = (0x01 << 5),
+
+	/** Vendor */
+	LIBUSB_REQUEST_TYPE_VENDOR = (0x02 << 5),
+
+	/** Reserved */
+	LIBUSB_REQUEST_TYPE_RESERVED = (0x03 << 5)
+};
+
+/** \ingroup libusb_misc
+ * Recipient bits of the
+ * \ref libusb_control_setup::bmRequestType "bmRequestType" field in control
+ * transfers. Values 4 through 31 are reserved. */
+enum libusb_request_recipient {
+	/** Device */
+	LIBUSB_RECIPIENT_DEVICE = 0x00,
+
+	/** Interface */
+	LIBUSB_RECIPIENT_INTERFACE = 0x01,
+
+	/** Endpoint */
+	LIBUSB_RECIPIENT_ENDPOINT = 0x02,
+
+	/** Other */
+	LIBUSB_RECIPIENT_OTHER = 0x03
+};
+
+#define LIBUSB_ISO_SYNC_TYPE_MASK	0x0c
+
+/** \ingroup libusb_desc
+ * Synchronization type for isochronous endpoints. Values for bits 2:3 of the
+ * \ref libusb_endpoint_descriptor::bmAttributes "bmAttributes" field in
+ * libusb_endpoint_descriptor.
+ */
+enum libusb_iso_sync_type {
+	/** No synchronization */
+	LIBUSB_ISO_SYNC_TYPE_NONE = 0x0,
+
+	/** Asynchronous */
+	LIBUSB_ISO_SYNC_TYPE_ASYNC = 0x1,
+
+	/** Adaptive */
+	LIBUSB_ISO_SYNC_TYPE_ADAPTIVE = 0x2,
+
+	/** Synchronous */
+	LIBUSB_ISO_SYNC_TYPE_SYNC = 0x3
+};
+
+#define LIBUSB_ISO_USAGE_TYPE_MASK	0x30
+
+/** \ingroup libusb_desc
+ * Usage type for isochronous endpoints. Values for bits 4:5 of the
+ * \ref libusb_endpoint_descriptor::bmAttributes "bmAttributes" field in
+ * libusb_endpoint_descriptor.
+ */
+enum libusb_iso_usage_type {
+	/** Data endpoint */
+	LIBUSB_ISO_USAGE_TYPE_DATA = 0x0,
+
+	/** Feedback endpoint */
+	LIBUSB_ISO_USAGE_TYPE_FEEDBACK = 0x1,
+
+	/** Implicit feedback Data endpoint */
+	LIBUSB_ISO_USAGE_TYPE_IMPLICIT = 0x2
+};
+
+/** \ingroup libusb_desc
+ * Supported speeds (wSpeedSupported) bitfield. Indicates what
+ * speeds the device supports.
+ */
+enum libusb_supported_speed {
+	/** Low speed operation supported (1.5MBit/s). */
+	LIBUSB_LOW_SPEED_OPERATION = (1 << 0),
+
+	/** Full speed operation supported (12MBit/s). */
+	LIBUSB_FULL_SPEED_OPERATION = (1 << 1),
+
+	/** High speed operation supported (480MBit/s). */
+	LIBUSB_HIGH_SPEED_OPERATION = (1 << 2),
+
+	/** Superspeed operation supported (5000MBit/s). */
+	LIBUSB_SUPER_SPEED_OPERATION = (1 << 3)
+};
+
+/** \ingroup libusb_desc
+ * Masks for the bits of the
+ * \ref libusb_usb_2_0_extension_descriptor::bmAttributes "bmAttributes" field
+ * of the USB 2.0 Extension descriptor.
+ */
+enum libusb_usb_2_0_extension_attributes {
+	/** Supports Link Power Management (LPM) */
+	LIBUSB_BM_LPM_SUPPORT = (1 << 1)
+};
+
+/** \ingroup libusb_desc
+ * Masks for the bits of the
+ * \ref libusb_ss_usb_device_capability_descriptor::bmAttributes "bmAttributes" field
+ * field of the SuperSpeed USB Device Capability descriptor.
+ */
+enum libusb_ss_usb_device_capability_attributes {
+	/** Supports Latency Tolerance Messages (LTM) */
+	LIBUSB_BM_LTM_SUPPORT = (1 << 1)
+};
+
+/** \ingroup libusb_desc
+ * USB capability types
+ */
+enum libusb_bos_type {
+	/** Wireless USB device capability */
+	LIBUSB_BT_WIRELESS_USB_DEVICE_CAPABILITY = 0x01,
+
+	/** USB 2.0 extensions */
+	LIBUSB_BT_USB_2_0_EXTENSION = 0x02,
+
+	/** SuperSpeed USB device capability */
+	LIBUSB_BT_SS_USB_DEVICE_CAPABILITY = 0x03,
+
+	/** Container ID type */
+	LIBUSB_BT_CONTAINER_ID = 0x04
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the standard USB device descriptor. This
+ * descriptor is documented in section 9.6.1 of the USB 3.0 specification.
+ * All multiple-byte fields are represented in host-endian format.
+ */
+struct libusb_device_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_DEVICE LIBUSB_DT_DEVICE in this
+	 * context. */
+	uint8_t  bDescriptorType;
+
+	/** USB specification release number in binary-coded decimal. A value of
+	 * 0x0200 indicates USB 2.0, 0x0110 indicates USB 1.1, etc. */
+	uint16_t bcdUSB;
+
+	/** USB-IF class code for the device. See \ref libusb_class_code. */
+	uint8_t  bDeviceClass;
+
+	/** USB-IF subclass code for the device, qualified by the bDeviceClass
+	 * value */
+	uint8_t  bDeviceSubClass;
+
+	/** USB-IF protocol code for the device, qualified by the bDeviceClass and
+	 * bDeviceSubClass values */
+	uint8_t  bDeviceProtocol;
+
+	/** Maximum packet size for endpoint 0 */
+	uint8_t  bMaxPacketSize0;
+
+	/** USB-IF vendor ID */
+	uint16_t idVendor;
+
+	/** USB-IF product ID */
+	uint16_t idProduct;
+
+	/** Device release number in binary-coded decimal */
+	uint16_t bcdDevice;
+
+	/** Index of string descriptor describing manufacturer */
+	uint8_t  iManufacturer;
+
+	/** Index of string descriptor describing product */
+	uint8_t  iProduct;
+
+	/** Index of string descriptor containing device serial number */
+	uint8_t  iSerialNumber;
+
+	/** Number of possible configurations */
+	uint8_t  bNumConfigurations;
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the standard USB endpoint descriptor. This
+ * descriptor is documented in section 9.6.6 of the USB 3.0 specification.
+ * All multiple-byte fields are represented in host-endian format.
+ */
+struct libusb_endpoint_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_ENDPOINT LIBUSB_DT_ENDPOINT in
+	 * this context. */
+	uint8_t  bDescriptorType;
+
+	/** The address of the endpoint described by this descriptor. Bits 0:3 are
+	 * the endpoint number. Bits 4:6 are reserved. Bit 7 indicates direction,
+	 * see \ref libusb_endpoint_direction. */
+	uint8_t  bEndpointAddress;
+
+	/** Attributes which apply to the endpoint when it is configured using
+	 * the bConfigurationValue. Bits 0:1 determine the transfer type and
+	 * correspond to \ref libusb_endpoint_transfer_type. Bits 2:3 are only used
+	 * for isochronous endpoints and correspond to \ref libusb_iso_sync_type.
+	 * Bits 4:5 are also only used for isochronous endpoints and correspond to
+	 * \ref libusb_iso_usage_type. Bits 6:7 are reserved. */
+	uint8_t  bmAttributes;
+
+	/** Maximum packet size this endpoint is capable of sending/receiving. */
+	uint16_t wMaxPacketSize;
+
+	/** Interval for polling endpoint for data transfers. */
+	uint8_t  bInterval;
+
+	/** For audio devices only: the rate at which synchronization feedback
+	 * is provided. */
+	uint8_t  bRefresh;
+
+	/** For audio devices only: the address if the synch endpoint */
+	uint8_t  bSynchAddress;
+
+	/** Extra descriptors. If libusb encounters unknown endpoint descriptors,
+	 * it will store them here, should you wish to parse them. */
+	const unsigned char *extra;
+
+	/** Length of the extra descriptors, in bytes. Must be non-negative. */
+	int extra_length;
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the standard USB interface descriptor. This
+ * descriptor is documented in section 9.6.5 of the USB 3.0 specification.
+ * All multiple-byte fields are represented in host-endian format.
+ */
+struct libusb_interface_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_INTERFACE LIBUSB_DT_INTERFACE
+	 * in this context. */
+	uint8_t  bDescriptorType;
+
+	/** Number of this interface */
+	uint8_t  bInterfaceNumber;
+
+	/** Value used to select this alternate setting for this interface */
+	uint8_t  bAlternateSetting;
+
+	/** Number of endpoints used by this interface (excluding the control
+	 * endpoint). */
+	uint8_t  bNumEndpoints;
+
+	/** USB-IF class code for this interface. See \ref libusb_class_code. */
+	uint8_t  bInterfaceClass;
+
+	/** USB-IF subclass code for this interface, qualified by the
+	 * bInterfaceClass value */
+	uint8_t  bInterfaceSubClass;
+
+	/** USB-IF protocol code for this interface, qualified by the
+	 * bInterfaceClass and bInterfaceSubClass values */
+	uint8_t  bInterfaceProtocol;
+
+	/** Index of string descriptor describing this interface */
+	uint8_t  iInterface;
+
+	/** Array of endpoint descriptors. This length of this array is determined
+	 * by the bNumEndpoints field. */
+	const struct libusb_endpoint_descriptor *endpoint;
+
+	/** Extra descriptors. If libusb encounters unknown interface descriptors,
+	 * it will store them here, should you wish to parse them. */
+	const unsigned char *extra;
+
+	/** Length of the extra descriptors, in bytes. Must be non-negative. */
+	int extra_length;
+};
+
+/** \ingroup libusb_desc
+ * A collection of alternate settings for a particular USB interface.
+ */
+struct libusb_interface {
+	/** Array of interface descriptors. The length of this array is determined
+	 * by the num_altsetting field. */
+	const struct libusb_interface_descriptor *altsetting;
+
+	/** The number of alternate settings that belong to this interface.
+	 * Must be non-negative. */
+	int num_altsetting;
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the standard USB configuration descriptor. This
+ * descriptor is documented in section 9.6.3 of the USB 3.0 specification.
+ * All multiple-byte fields are represented in host-endian format.
+ */
+struct libusb_config_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_CONFIG LIBUSB_DT_CONFIG
+	 * in this context. */
+	uint8_t  bDescriptorType;
+
+	/** Total length of data returned for this configuration */
+	uint16_t wTotalLength;
+
+	/** Number of interfaces supported by this configuration */
+	uint8_t  bNumInterfaces;
+
+	/** Identifier value for this configuration */
+	uint8_t  bConfigurationValue;
+
+	/** Index of string descriptor describing this configuration */
+	uint8_t  iConfiguration;
+
+	/** Configuration characteristics */
+	uint8_t  bmAttributes;
+
+	/** Maximum power consumption of the USB device from this bus in this
+	 * configuration when the device is fully operation. Expressed in units
+	 * of 2 mA when the device is operating in high-speed mode and in units
+	 * of 8 mA when the device is operating in super-speed mode. */
+	uint8_t  MaxPower;
+
+	/** Array of interfaces supported by this configuration. The length of
+	 * this array is determined by the bNumInterfaces field. */
+	const struct libusb_interface *interface;
+
+	/** Extra descriptors. If libusb encounters unknown configuration
+	 * descriptors, it will store them here, should you wish to parse them. */
+	const unsigned char *extra;
+
+	/** Length of the extra descriptors, in bytes. Must be non-negative. */
+	int extra_length;
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the superspeed endpoint companion
+ * descriptor. This descriptor is documented in section 9.6.7 of
+ * the USB 3.0 specification. All multiple-byte fields are represented in
+ * host-endian format.
+ */
+struct libusb_ss_endpoint_companion_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_SS_ENDPOINT_COMPANION in
+	 * this context. */
+	uint8_t  bDescriptorType;
+
+	/** The maximum number of packets the endpoint can send or
+	 *  receive as part of a burst. */
+	uint8_t  bMaxBurst;
+
+	/** In bulk EP: bits 4:0 represents the maximum number of
+	 *  streams the EP supports. In isochronous EP: bits 1:0
+	 *  represents the Mult - a zero based value that determines
+	 *  the maximum number of packets within a service interval  */
+	uint8_t  bmAttributes;
+
+	/** The total number of bytes this EP will transfer every
+	 *  service interval. Valid only for periodic EPs. */
+	uint16_t wBytesPerInterval;
+};
+
+/** \ingroup libusb_desc
+ * A generic representation of a BOS Device Capability descriptor. It is
+ * advised to check bDevCapabilityType and call the matching
+ * libusb_get_*_descriptor function to get a structure fully matching the type.
+ */
+struct libusb_bos_dev_capability_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_DEVICE_CAPABILITY
+	 * LIBUSB_DT_DEVICE_CAPABILITY in this context. */
+	uint8_t  bDescriptorType;
+
+	/** Device Capability type */
+	uint8_t  bDevCapabilityType;
+
+	/** Device Capability data (bLength - 3 bytes) */
+	uint8_t  dev_capability_data[ZERO_SIZED_ARRAY];
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the Binary Device Object Store (BOS) descriptor.
+ * This descriptor is documented in section 9.6.2 of the USB 3.0 specification.
+ * All multiple-byte fields are represented in host-endian format.
+ */
+struct libusb_bos_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_BOS LIBUSB_DT_BOS
+	 * in this context. */
+	uint8_t  bDescriptorType;
+
+	/** Length of this descriptor and all of its sub descriptors */
+	uint16_t wTotalLength;
+
+	/** The number of separate device capability descriptors in
+	 * the BOS */
+	uint8_t  bNumDeviceCaps;
+
+	/** bNumDeviceCap Device Capability Descriptors */
+	struct libusb_bos_dev_capability_descriptor *dev_capability[ZERO_SIZED_ARRAY];
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the USB 2.0 Extension descriptor
+ * This descriptor is documented in section 9.6.2.1 of the USB 3.0 specification.
+ * All multiple-byte fields are represented in host-endian format.
+ */
+struct libusb_usb_2_0_extension_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_DEVICE_CAPABILITY
+	 * LIBUSB_DT_DEVICE_CAPABILITY in this context. */
+	uint8_t  bDescriptorType;
+
+	/** Capability type. Will have value
+	 * \ref libusb_capability_type::LIBUSB_BT_USB_2_0_EXTENSION
+	 * LIBUSB_BT_USB_2_0_EXTENSION in this context. */
+	uint8_t  bDevCapabilityType;
+
+	/** Bitmap encoding of supported device level features.
+	 * A value of one in a bit location indicates a feature is
+	 * supported; a value of zero indicates it is not supported.
+	 * See \ref libusb_usb_2_0_extension_attributes. */
+	uint32_t bmAttributes;
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the SuperSpeed USB Device Capability descriptor
+ * This descriptor is documented in section 9.6.2.2 of the USB 3.0 specification.
+ * All multiple-byte fields are represented in host-endian format.
+ */
+struct libusb_ss_usb_device_capability_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_DEVICE_CAPABILITY
+	 * LIBUSB_DT_DEVICE_CAPABILITY in this context. */
+	uint8_t  bDescriptorType;
+
+	/** Capability type. Will have value
+	 * \ref libusb_capability_type::LIBUSB_BT_SS_USB_DEVICE_CAPABILITY
+	 * LIBUSB_BT_SS_USB_DEVICE_CAPABILITY in this context. */
+	uint8_t  bDevCapabilityType;
+
+	/** Bitmap encoding of supported device level features.
+	 * A value of one in a bit location indicates a feature is
+	 * supported; a value of zero indicates it is not supported.
+	 * See \ref libusb_ss_usb_device_capability_attributes. */
+	uint8_t  bmAttributes;
+
+	/** Bitmap encoding of the speed supported by this device when
+	 * operating in SuperSpeed mode. See \ref libusb_supported_speed. */
+	uint16_t wSpeedSupported;
+
+	/** The lowest speed at which all the functionality supported
+	 * by the device is available to the user. For example if the
+	 * device supports all its functionality when connected at
+	 * full speed and above then it sets this value to 1. */
+	uint8_t  bFunctionalitySupport;
+
+	/** U1 Device Exit Latency. */
+	uint8_t  bU1DevExitLat;
+
+	/** U2 Device Exit Latency. */
+	uint16_t bU2DevExitLat;
+};
+
+/** \ingroup libusb_desc
+ * A structure representing the Container ID descriptor.
+ * This descriptor is documented in section 9.6.2.3 of the USB 3.0 specification.
+ * All multiple-byte fields, except UUIDs, are represented in host-endian format.
+ */
+struct libusb_container_id_descriptor {
+	/** Size of this descriptor (in bytes) */
+	uint8_t  bLength;
+
+	/** Descriptor type. Will have value
+	 * \ref libusb_descriptor_type::LIBUSB_DT_DEVICE_CAPABILITY
+	 * LIBUSB_DT_DEVICE_CAPABILITY in this context. */
+	uint8_t  bDescriptorType;
+
+	/** Capability type. Will have value
+	 * \ref libusb_capability_type::LIBUSB_BT_CONTAINER_ID
+	 * LIBUSB_BT_CONTAINER_ID in this context. */
+	uint8_t  bDevCapabilityType;
+
+	/** Reserved field */
+	uint8_t  bReserved;
+
+	/** 128 bit UUID */
+	uint8_t  ContainerID[16];
+};
+
+/** \ingroup libusb_asyncio
+ * Setup packet for control transfers. */
+#if defined(_MSC_VER) || defined(__WATCOMC__)
+#pragma pack(push, 1)
+#endif
+struct libusb_control_setup {
+	/** Request type. Bits 0:4 determine recipient, see
+	 * \ref libusb_request_recipient. Bits 5:6 determine type, see
+	 * \ref libusb_request_type. Bit 7 determines data transfer direction, see
+	 * \ref libusb_endpoint_direction.
+	 */
+	uint8_t  bmRequestType;
+
+	/** Request. If the type bits of bmRequestType are equal to
+	 * \ref libusb_request_type::LIBUSB_REQUEST_TYPE_STANDARD
+	 * "LIBUSB_REQUEST_TYPE_STANDARD" then this field refers to
+	 * \ref libusb_standard_request. For other cases, use of this field is
+	 * application-specific. */
+	uint8_t  bRequest;
+
+	/** Value. Varies according to request */
+	uint16_t wValue;
+
+	/** Index. Varies according to request, typically used to pass an index
+	 * or offset */
+	uint16_t wIndex;
+
+	/** Number of bytes to transfer */
+	uint16_t wLength;
+} LIBUSB_PACKED;
+#if defined(_MSC_VER) || defined(__WATCOMC__)
+#pragma pack(pop)
+#endif
+
+#define LIBUSB_CONTROL_SETUP_SIZE (sizeof(struct libusb_control_setup))
+
+/* libusb */
+
+struct libusb_context;
+struct libusb_device;
+struct libusb_device_handle;
+
+/** \ingroup libusb_lib
+ * Structure providing the version of the libusb runtime
+ */
+struct libusb_version {
+	/** Library major version. */
+	const uint16_t major;
+
+	/** Library minor version. */
+	const uint16_t minor;
+
+	/** Library micro version. */
+	const uint16_t micro;
+
+	/** Library nano version. */
+	const uint16_t nano;
+
+	/** Library release candidate suffix string, e.g. "-rc4". */
+	const char *rc;
+
+	/** For ABI compatibility only. */
+	const char *describe;
+};
+
+/** \ingroup libusb_lib
+ * Structure representing a libusb session. The concept of individual libusb
+ * sessions allows for your program to use two libraries (or dynamically
+ * load two modules) which both independently use libusb. This will prevent
+ * interference between the individual libusb users - for example
+ * libusb_set_option() will not affect the other user of the library, and
+ * libusb_exit() will not destroy resources that the other user is still
+ * using.
+ *
+ * Sessions are created by libusb_init() and destroyed through libusb_exit().
+ * If your application is guaranteed to only ever include a single libusb
+ * user (i.e. you), you do not have to worry about contexts: pass NULL in
+ * every function call where a context is required, and the default context
+ * will be used. Note that libusb_set_option(NULL, ...) is special, and adds
+ * an option to a list of default options for new contexts.
+ *
+ * For more information, see \ref libusb_contexts.
+ */
+typedef struct libusb_context libusb_context;
+
+/** \ingroup libusb_dev
+ * Structure representing a USB device detected on the system. This is an
+ * opaque type for which you are only ever provided with a pointer, usually
+ * originating from libusb_get_device_list() or libusb_hotplug_register_callback().
+ *
+ * Certain operations can be performed on a device, but in order to do any
+ * I/O you will have to first obtain a device handle using libusb_open().
+ *
+ * Devices are reference counted with libusb_ref_device() and
+ * libusb_unref_device(), and are freed when the reference count reaches 0.
+ * New devices presented by libusb_get_device_list() have a reference count of
+ * 1, and libusb_free_device_list() can optionally decrease the reference count
+ * on all devices in the list. libusb_open() adds another reference which is
+ * later destroyed by libusb_close().
+ */
+typedef struct libusb_device libusb_device;
+
+
+/** \ingroup libusb_dev
+ * Structure representing a handle on a USB device. This is an opaque type for
+ * which you are only ever provided with a pointer, usually originating from
+ * libusb_open().
+ *
+ * A device handle is used to perform I/O and other operations. When finished
+ * with a device handle, you should call libusb_close().
+ */
+typedef struct libusb_device_handle libusb_device_handle;
+
+/** \ingroup libusb_dev
+ * Speed codes. Indicates the speed at which the device is operating.
+ */
+enum libusb_speed {
+	/** The OS doesn't report or know the device speed. */
+	LIBUSB_SPEED_UNKNOWN = 0,
+
+	/** The device is operating at low speed (1.5MBit/s). */
+	LIBUSB_SPEED_LOW = 1,
+
+	/** The device is operating at full speed (12MBit/s). */
+	LIBUSB_SPEED_FULL = 2,
+
+	/** The device is operating at high speed (480MBit/s). */
+	LIBUSB_SPEED_HIGH = 3,
+
+	/** The device is operating at super speed (5000MBit/s). */
+	LIBUSB_SPEED_SUPER = 4,
+
+	/** The device is operating at super speed plus (10000MBit/s). */
+	LIBUSB_SPEED_SUPER_PLUS = 5
+};
+
+/** \ingroup libusb_misc
+ * Error codes. Most libusb functions return 0 on success or one of these
+ * codes on failure.
+ * You can call libusb_error_name() to retrieve a string representation of an
+ * error code or libusb_strerror() to get an end-user suitable description of
+ * an error code.
+ */
+enum libusb_error {
+	/** Success (no error) */
+	LIBUSB_SUCCESS = 0,
+
+	/** Input/output error */
+	LIBUSB_ERROR_IO = -1,
+
+	/** Invalid parameter */
+	LIBUSB_ERROR_INVALID_PARAM = -2,
+
+	/** Access denied (insufficient permissions) */
+	LIBUSB_ERROR_ACCESS = -3,
+
+	/** No such device (it may have been disconnected) */
+	LIBUSB_ERROR_NO_DEVICE = -4,
+
+	/** Entity not found */
+	LIBUSB_ERROR_NOT_FOUND = -5,
+
+	/** Resource busy */
+	LIBUSB_ERROR_BUSY = -6,
+
+	/** Operation timed out */
+	LIBUSB_ERROR_TIMEOUT = -7,
+
+	/** Overflow */
+	LIBUSB_ERROR_OVERFLOW = -8,
+
+	/** Pipe error */
+	LIBUSB_ERROR_PIPE = -9,
+
+	/** System call interrupted (perhaps due to signal) */
+	LIBUSB_ERROR_INTERRUPTED = -10,
+
+	/** Insufficient memory */
+	LIBUSB_ERROR_NO_MEM = -11,
+
+	/** Operation not supported or unimplemented on this platform */
+	LIBUSB_ERROR_NOT_SUPPORTED = -12,
+
+	/* NB: Remember to update LIBUSB_ERROR_COUNT below as well as the
+	   message strings in strerror.c when adding new error codes here. */
+
+	/** Other error */
+	LIBUSB_ERROR_OTHER = -99
+};
+
+/* Total number of error codes in enum libusb_error */
+#define LIBUSB_ERROR_COUNT 14
+
+/** \ingroup libusb_asyncio
+ * Transfer type */
+enum libusb_transfer_type {
+	/** Control transfer */
+	LIBUSB_TRANSFER_TYPE_CONTROL = 0U,
+
+	/** Isochronous transfer */
+	LIBUSB_TRANSFER_TYPE_ISOCHRONOUS = 1U,
+
+	/** Bulk transfer */
+	LIBUSB_TRANSFER_TYPE_BULK = 2U,
+
+	/** Interrupt transfer */
+	LIBUSB_TRANSFER_TYPE_INTERRUPT = 3U,
+
+	/** Bulk stream transfer */
+	LIBUSB_TRANSFER_TYPE_BULK_STREAM = 4U
+};
+
+/** \ingroup libusb_asyncio
+ * Transfer status codes */
+enum libusb_transfer_status {
+	/** Transfer completed without error. Note that this does not indicate
+	 * that the entire amount of requested data was transferred. */
+	LIBUSB_TRANSFER_COMPLETED,
+
+	/** Transfer failed */
+	LIBUSB_TRANSFER_ERROR,
+
+	/** Transfer timed out */
+	LIBUSB_TRANSFER_TIMED_OUT,
+
+	/** Transfer was cancelled */
+	LIBUSB_TRANSFER_CANCELLED,
+
+	/** For bulk/interrupt endpoints: halt condition detected (endpoint
+	 * stalled). For control endpoints: control request not supported. */
+	LIBUSB_TRANSFER_STALL,
+
+	/** Device was disconnected */
+	LIBUSB_TRANSFER_NO_DEVICE,
+
+	/** Device sent more data than requested */
+	LIBUSB_TRANSFER_OVERFLOW
+
+	/* NB! Remember to update libusb_error_name()
+	   when adding new status codes here. */
+};
+
+/** \ingroup libusb_asyncio
+ * libusb_transfer.flags values */
+enum libusb_transfer_flags {
+	/** Report short frames as errors */
+	LIBUSB_TRANSFER_SHORT_NOT_OK = (1U << 0),
+
+	/** Automatically free() transfer buffer during libusb_free_transfer().
+	 * Note that buffers allocated with libusb_dev_mem_alloc() should not
+	 * be attempted freed in this way, since free() is not an appropriate
+	 * way to release such memory. */
+	LIBUSB_TRANSFER_FREE_BUFFER = (1U << 1),
+
+	/** Automatically call libusb_free_transfer() after callback returns.
+	 * If this flag is set, it is illegal to call libusb_free_transfer()
+	 * from your transfer callback, as this will result in a double-free
+	 * when this flag is acted upon. */
+	LIBUSB_TRANSFER_FREE_TRANSFER = (1U << 2),
+
+	/** Terminate transfers that are a multiple of the endpoint's
+	 * wMaxPacketSize with an extra zero length packet. This is useful
+	 * when a device protocol mandates that each logical request is
+	 * terminated by an incomplete packet (i.e. the logical requests are
+	 * not separated by other means).
+	 *
+	 * This flag only affects host-to-device transfers to bulk and interrupt
+	 * endpoints. In other situations, it is ignored.
+	 *
+	 * This flag only affects transfers with a length that is a multiple of
+	 * the endpoint's wMaxPacketSize. On transfers of other lengths, this
+	 * flag has no effect. Therefore, if you are working with a device that
+	 * needs a ZLP whenever the end of the logical request falls on a packet
+	 * boundary, then it is sensible to set this flag on <em>every</em>
+	 * transfer (you do not have to worry about only setting it on transfers
+	 * that end on the boundary).
+	 *
+	 * This flag is currently only supported on Linux.
+	 * On other systems, libusb_submit_transfer() will return
+	 * LIBUSB_ERROR_NOT_SUPPORTED for every transfer where this flag is set.
+	 *
+	 * Available since libusb-1.0.9.
+	 */
+	LIBUSB_TRANSFER_ADD_ZERO_PACKET = (1U << 3)
+};
+
+/** \ingroup libusb_asyncio
+ * Isochronous packet descriptor. */
+struct libusb_iso_packet_descriptor {
+	/** Length of data to request in this packet */
+	unsigned int length;
+
+	/** Amount of data that was actually transferred */
+	unsigned int actual_length;
+
+	/** Status code for this packet */
+	enum libusb_transfer_status status;
+};
+
+struct libusb_transfer;
+
+/** \ingroup libusb_asyncio
+ * Asynchronous transfer callback function type. When submitting asynchronous
+ * transfers, you pass a pointer to a callback function of this type via the
+ * \ref libusb_transfer::callback "callback" member of the libusb_transfer
+ * structure. libusb will call this function later, when the transfer has
+ * completed or failed. See \ref libusb_asyncio for more information.
+ * \param transfer The libusb_transfer struct the callback function is being
+ * notified about.
+ */
+typedef void (LIBUSB_CALL *libusb_transfer_cb_fn)(struct libusb_transfer *transfer);
+
+/** \ingroup libusb_asyncio
+ * The generic USB transfer structure. The user populates this structure and
+ * then submits it in order to request a transfer. After the transfer has
+ * completed, the library populates the transfer with the results and passes
+ * it back to the user.
+ */
+struct libusb_transfer {
+	/** Handle of the device that this transfer will be submitted to */
+	libusb_device_handle *dev_handle;
+
+	/** A bitwise OR combination of \ref libusb_transfer_flags. */
+	uint8_t flags;
+
+	/** Address of the endpoint where this transfer will be sent. */
+	unsigned char endpoint;
+
+	/** Type of the transfer from \ref libusb_transfer_type */
+	unsigned char type;
+
+	/** Timeout for this transfer in milliseconds. A value of 0 indicates no
+	 * timeout. */
+	unsigned int timeout;
+
+	/** The status of the transfer. Read-only, and only for use within
+	 * transfer callback function.
+	 *
+	 * If this is an isochronous transfer, this field may read COMPLETED even
+	 * if there were errors in the frames. Use the
+	 * \ref libusb_iso_packet_descriptor::status "status" field in each packet
+	 * to determine if errors occurred. */
+	enum libusb_transfer_status status;
+
+	/** Length of the data buffer. Must be non-negative. */
+	int length;
+
+	/** Actual length of data that was transferred. Read-only, and only for
+	 * use within transfer callback function. Not valid for isochronous
+	 * endpoint transfers. */
+	int actual_length;
+
+	/** Callback function. This will be invoked when the transfer completes,
+	 * fails, or is cancelled. */
+	libusb_transfer_cb_fn callback;
+
+	/** User context data. Useful for associating specific data to a transfer
+	 * that can be accessed from within the callback function.
+	 *
+	 * This field may be set manually or is taken as the `user_data` parameter
+	 * of the following functions:
+	 * - libusb_fill_bulk_transfer()
+	 * - libusb_fill_bulk_stream_transfer()
+	 * - libusb_fill_control_transfer()
+	 * - libusb_fill_interrupt_transfer()
+	 * - libusb_fill_iso_transfer() */
+	void *user_data;
+
+	/** Data buffer */
+	unsigned char *buffer;
+
+	/** Number of isochronous packets. Only used for I/O with isochronous
+	 * endpoints. Must be non-negative. */
+	int num_iso_packets;
+
+	/** Isochronous packet descriptors, for isochronous transfers only. */
+	struct libusb_iso_packet_descriptor iso_packet_desc[ZERO_SIZED_ARRAY];
+};
+
+/** \ingroup libusb_misc
+ * Capabilities supported by an instance of libusb on the current running
+ * platform. Test if the loaded library supports a given capability by calling
+ * \ref libusb_has_capability().
+ */
+enum libusb_capability {
+	/** The libusb_has_capability() API is available. */
+	LIBUSB_CAP_HAS_CAPABILITY = 0x0000U,
+
+	/** Hotplug support is available on this platform. */
+	LIBUSB_CAP_HAS_HOTPLUG = 0x0001U,
+
+	/** The library can access HID devices without requiring user intervention.
+	 * Note that before being able to actually access an HID device, you may
+	 * still have to call additional libusb functions such as
+	 * \ref libusb_detach_kernel_driver(). */
+	LIBUSB_CAP_HAS_HID_ACCESS = 0x0100U,
+
+	/** The library supports detaching of the default USB driver, using
+	 * \ref libusb_detach_kernel_driver(), if one is set by the OS kernel */
+	LIBUSB_CAP_SUPPORTS_DETACH_KERNEL_DRIVER = 0x0101U
+};
+
+/** \ingroup libusb_lib
+ *  Log message levels.
+ */
+enum libusb_log_level {
+	/** (0) : No messages ever emitted by the library (default) */
+	LIBUSB_LOG_LEVEL_NONE = 0,
+
+	/** (1) : Error messages are emitted */
+	LIBUSB_LOG_LEVEL_ERROR = 1,
+
+	/** (2) : Warning and error messages are emitted */
+	LIBUSB_LOG_LEVEL_WARNING = 2,
+
+	/** (3) : Informational, warning and error messages are emitted */
+	LIBUSB_LOG_LEVEL_INFO = 3,
+
+	/** (4) : All messages are emitted */
+	LIBUSB_LOG_LEVEL_DEBUG = 4
+};
+
+/** \ingroup libusb_lib
+ *  Log callback mode.
+ *
+ *  Since version 1.0.23, \ref LIBUSB_API_VERSION >= 0x01000107
+ *
+ * \see libusb_set_log_cb()
+ */
+enum libusb_log_cb_mode {
+	/** Callback function handling all log messages. */
+	LIBUSB_LOG_CB_GLOBAL = (1 << 0),
+
+	/** Callback function handling context related log messages. */
+	LIBUSB_LOG_CB_CONTEXT = (1 << 1)
+};
+
+/** \ingroup libusb_lib
+ * Callback function for handling log messages.
+ * \param ctx the context which is related to the log message, or NULL if it
+ * is a global log message
+ * \param level the log level, see \ref libusb_log_level for a description
+ * \param str the log message
+ *
+ * Since version 1.0.23, \ref LIBUSB_API_VERSION >= 0x01000107
+ *
+ * \see libusb_set_log_cb()
+ */
+typedef void (LIBUSB_CALL *libusb_log_cb)(libusb_context *ctx,
+	enum libusb_log_level level, const char *str);
+
+int LIBUSB_CALL libusb_init(libusb_context **ctx);
+void LIBUSB_CALL libusb_exit(libusb_context *ctx);
+LIBUSB_DEPRECATED_FOR(libusb_set_option)
+void LIBUSB_CALL libusb_set_debug(libusb_context *ctx, int level);
+void LIBUSB_CALL libusb_set_log_cb(libusb_context *ctx, libusb_log_cb cb, int mode);
+const struct libusb_version * LIBUSB_CALL libusb_get_version(void);
+int LIBUSB_CALL libusb_has_capability(uint32_t capability);
+const char * LIBUSB_CALL libusb_error_name(int errcode);
+int LIBUSB_CALL libusb_setlocale(const char *locale);
+const char * LIBUSB_CALL libusb_strerror(int errcode);
+
+ssize_t LIBUSB_CALL libusb_get_device_list(libusb_context *ctx,
+	libusb_device ***list);
+void LIBUSB_CALL libusb_free_device_list(libusb_device **list,
+	int unref_devices);
+libusb_device * LIBUSB_CALL libusb_ref_device(libusb_device *dev);
+void LIBUSB_CALL libusb_unref_device(libusb_device *dev);
+
+int LIBUSB_CALL libusb_get_configuration(libusb_device_handle *dev,
+	int *config);
+int LIBUSB_CALL libusb_get_device_descriptor(libusb_device *dev,
+	struct libusb_device_descriptor *desc);
+int LIBUSB_CALL libusb_get_active_config_descriptor(libusb_device *dev,
+	struct libusb_config_descriptor **config);
+int LIBUSB_CALL libusb_get_config_descriptor(libusb_device *dev,
+	uint8_t config_index, struct libusb_config_descriptor **config);
+int LIBUSB_CALL libusb_get_config_descriptor_by_value(libusb_device *dev,
+	uint8_t bConfigurationValue, struct libusb_config_descriptor **config);
+void LIBUSB_CALL libusb_free_config_descriptor(
+	struct libusb_config_descriptor *config);
+int LIBUSB_CALL libusb_get_ss_endpoint_companion_descriptor(
+	libusb_context *ctx,
+	const struct libusb_endpoint_descriptor *endpoint,
+	struct libusb_ss_endpoint_companion_descriptor **ep_comp);
+void LIBUSB_CALL libusb_free_ss_endpoint_companion_descriptor(
+	struct libusb_ss_endpoint_companion_descriptor *ep_comp);
+int LIBUSB_CALL libusb_get_bos_descriptor(libusb_device_handle *dev_handle,
+	struct libusb_bos_descriptor **bos);
+void LIBUSB_CALL libusb_free_bos_descriptor(struct libusb_bos_descriptor *bos);
+int LIBUSB_CALL libusb_get_usb_2_0_extension_descriptor(
+	libusb_context *ctx,
+	struct libusb_bos_dev_capability_descriptor *dev_cap,
+	struct libusb_usb_2_0_extension_descriptor **usb_2_0_extension);
+void LIBUSB_CALL libusb_free_usb_2_0_extension_descriptor(
+	struct libusb_usb_2_0_extension_descriptor *usb_2_0_extension);
+int LIBUSB_CALL libusb_get_ss_usb_device_capability_descriptor(
+	libusb_context *ctx,
+	struct libusb_bos_dev_capability_descriptor *dev_cap,
+	struct libusb_ss_usb_device_capability_descriptor **ss_usb_device_cap);
+void LIBUSB_CALL libusb_free_ss_usb_device_capability_descriptor(
+	struct libusb_ss_usb_device_capability_descriptor *ss_usb_device_cap);
+int LIBUSB_CALL libusb_get_container_id_descriptor(libusb_context *ctx,
+	struct libusb_bos_dev_capability_descriptor *dev_cap,
+	struct libusb_container_id_descriptor **container_id);
+void LIBUSB_CALL libusb_free_container_id_descriptor(
+	struct libusb_container_id_descriptor *container_id);
+uint8_t LIBUSB_CALL libusb_get_bus_number(libusb_device *dev);
+uint8_t LIBUSB_CALL libusb_get_port_number(libusb_device *dev);
+int LIBUSB_CALL libusb_get_port_numbers(libusb_device *dev, uint8_t *port_numbers, int port_numbers_len);
+LIBUSB_DEPRECATED_FOR(libusb_get_port_numbers)
+int LIBUSB_CALL libusb_get_port_path(libusb_context *ctx, libusb_device *dev, uint8_t *path, uint8_t path_length);
+libusb_device * LIBUSB_CALL libusb_get_parent(libusb_device *dev);
+uint8_t LIBUSB_CALL libusb_get_device_address(libusb_device *dev);
+int LIBUSB_CALL libusb_get_device_speed(libusb_device *dev);
+int LIBUSB_CALL libusb_get_max_packet_size(libusb_device *dev,
+	unsigned char endpoint);
+int LIBUSB_CALL libusb_get_max_iso_packet_size(libusb_device *dev,
+	unsigned char endpoint);
+
+int LIBUSB_CALL libusb_wrap_sys_device(libusb_context *ctx, intptr_t sys_dev, libusb_device_handle **dev_handle);
+int LIBUSB_CALL libusb_open(libusb_device *dev, libusb_device_handle **dev_handle);
+void LIBUSB_CALL libusb_close(libusb_device_handle *dev_handle);
+libusb_device * LIBUSB_CALL libusb_get_device(libusb_device_handle *dev_handle);
+
+int LIBUSB_CALL libusb_set_configuration(libusb_device_handle *dev_handle,
+	int configuration);
+int LIBUSB_CALL libusb_claim_interface(libusb_device_handle *dev_handle,
+	int interface_number);
+int LIBUSB_CALL libusb_release_interface(libusb_device_handle *dev_handle,
+	int interface_number);
+
+libusb_device_handle * LIBUSB_CALL libusb_open_device_with_vid_pid(
+	libusb_context *ctx, uint16_t vendor_id, uint16_t product_id);
+
+int LIBUSB_CALL libusb_set_interface_alt_setting(libusb_device_handle *dev_handle,
+	int interface_number, int alternate_setting);
+int LIBUSB_CALL libusb_clear_halt(libusb_device_handle *dev_handle,
+	unsigned char endpoint);
+int LIBUSB_CALL libusb_reset_device(libusb_device_handle *dev_handle);
+
+int LIBUSB_CALL libusb_alloc_streams(libusb_device_handle *dev_handle,
+	uint32_t num_streams, unsigned char *endpoints, int num_endpoints);
+int LIBUSB_CALL libusb_free_streams(libusb_device_handle *dev_handle,
+	unsigned char *endpoints, int num_endpoints);
+
+unsigned char * LIBUSB_CALL libusb_dev_mem_alloc(libusb_device_handle *dev_handle,
+	size_t length);
+int LIBUSB_CALL libusb_dev_mem_free(libusb_device_handle *dev_handle,
+	unsigned char *buffer, size_t length);
+
+int LIBUSB_CALL libusb_kernel_driver_active(libusb_device_handle *dev_handle,
+	int interface_number);
+int LIBUSB_CALL libusb_detach_kernel_driver(libusb_device_handle *dev_handle,
+	int interface_number);
+int LIBUSB_CALL libusb_attach_kernel_driver(libusb_device_handle *dev_handle,
+	int interface_number);
+int LIBUSB_CALL libusb_set_auto_detach_kernel_driver(
+	libusb_device_handle *dev_handle, int enable);
+
+/* async I/O */
+
+/** \ingroup libusb_asyncio
+ * Get the data section of a control transfer. This convenience function is here
+ * to remind you that the data does not start until 8 bytes into the actual
+ * buffer, as the setup packet comes first.
+ *
+ * Calling this function only makes sense from a transfer callback function,
+ * or situations where you have already allocated a suitably sized buffer at
+ * transfer->buffer.
+ *
+ * \param transfer a transfer
+ * \returns pointer to the first byte of the data section
+ */
+static inline unsigned char *libusb_control_transfer_get_data(
+	struct libusb_transfer *transfer)
+{
+	return transfer->buffer + LIBUSB_CONTROL_SETUP_SIZE;
+}
+
+/** \ingroup libusb_asyncio
+ * Get the control setup packet of a control transfer. This convenience
+ * function is here to remind you that the control setup occupies the first
+ * 8 bytes of the transfer data buffer.
+ *
+ * Calling this function only makes sense from a transfer callback function,
+ * or situations where you have already allocated a suitably sized buffer at
+ * transfer->buffer.
+ *
+ * \param transfer a transfer
+ * \returns a casted pointer to the start of the transfer data buffer
+ */
+static inline struct libusb_control_setup *libusb_control_transfer_get_setup(
+	struct libusb_transfer *transfer)
+{
+	return (struct libusb_control_setup *)(void *)transfer->buffer;
+}
+
+/** \ingroup libusb_asyncio
+ * Helper function to populate the setup packet (first 8 bytes of the data
+ * buffer) for a control transfer. The wIndex, wValue and wLength values should
+ * be given in host-endian byte order.
+ *
+ * \param buffer buffer to output the setup packet into
+ * This pointer must be aligned to at least 2 bytes boundary.
+ * \param bmRequestType see the
+ * \ref libusb_control_setup::bmRequestType "bmRequestType" field of
+ * \ref libusb_control_setup
+ * \param bRequest see the
+ * \ref libusb_control_setup::bRequest "bRequest" field of
+ * \ref libusb_control_setup
+ * \param wValue see the
+ * \ref libusb_control_setup::wValue "wValue" field of
+ * \ref libusb_control_setup
+ * \param wIndex see the
+ * \ref libusb_control_setup::wIndex "wIndex" field of
+ * \ref libusb_control_setup
+ * \param wLength see the
+ * \ref libusb_control_setup::wLength "wLength" field of
+ * \ref libusb_control_setup
+ */
+static inline void libusb_fill_control_setup(unsigned char *buffer,
+	uint8_t bmRequestType, uint8_t bRequest, uint16_t wValue, uint16_t wIndex,
+	uint16_t wLength)
+{
+	struct libusb_control_setup *setup = (struct libusb_control_setup *)(void *)buffer;
+	setup->bmRequestType = bmRequestType;
+	setup->bRequest = bRequest;
+	setup->wValue = libusb_cpu_to_le16(wValue);
+	setup->wIndex = libusb_cpu_to_le16(wIndex);
+	setup->wLength = libusb_cpu_to_le16(wLength);
+}
+
+struct libusb_transfer * LIBUSB_CALL libusb_alloc_transfer(int iso_packets);
+int LIBUSB_CALL libusb_submit_transfer(struct libusb_transfer *transfer);
+int LIBUSB_CALL libusb_cancel_transfer(struct libusb_transfer *transfer);
+void LIBUSB_CALL libusb_free_transfer(struct libusb_transfer *transfer);
+void LIBUSB_CALL libusb_transfer_set_stream_id(
+	struct libusb_transfer *transfer, uint32_t stream_id);
+uint32_t LIBUSB_CALL libusb_transfer_get_stream_id(
+	struct libusb_transfer *transfer);
+
+/** \ingroup libusb_asyncio
+ * Helper function to populate the required \ref libusb_transfer fields
+ * for a control transfer.
+ *
+ * If you pass a transfer buffer to this function, the first 8 bytes will
+ * be interpreted as a control setup packet, and the wLength field will be
+ * used to automatically populate the \ref libusb_transfer::length "length"
+ * field of the transfer. Therefore the recommended approach is:
+ * -# Allocate a suitably sized data buffer (including space for control setup)
+ * -# Call libusb_fill_control_setup()
+ * -# If this is a host-to-device transfer with a data stage, put the data
+ *    in place after the setup packet
+ * -# Call this function
+ * -# Call libusb_submit_transfer()
+ *
+ * It is also legal to pass a NULL buffer to this function, in which case this
+ * function will not attempt to populate the length field. Remember that you
+ * must then populate the buffer and length fields later.
+ *
+ * \param transfer the transfer to populate
+ * \param dev_handle handle of the device that will handle the transfer
+ * \param buffer data buffer. If provided, this function will interpret the
+ * first 8 bytes as a setup packet and infer the transfer length from that.
+ * This pointer must be aligned to at least 2 bytes boundary.
+ * \param callback callback function to be invoked on transfer completion
+ * \param user_data user data to pass to callback function
+ * \param timeout timeout for the transfer in milliseconds
+ */
+static inline void libusb_fill_control_transfer(
+	struct libusb_transfer *transfer, libusb_device_handle *dev_handle,
+	unsigned char *buffer, libusb_transfer_cb_fn callback, void *user_data,
+	unsigned int timeout)
+{
+	struct libusb_control_setup *setup = (struct libusb_control_setup *)(void *)buffer;
+	transfer->dev_handle = dev_handle;
+	transfer->endpoint = 0;
+	transfer->type = LIBUSB_TRANSFER_TYPE_CONTROL;
+	transfer->timeout = timeout;
+	transfer->buffer = buffer;
+	if (setup)
+		transfer->length = (int) (LIBUSB_CONTROL_SETUP_SIZE
+			+ libusb_le16_to_cpu(setup->wLength));
+	transfer->user_data = user_data;
+	transfer->callback = callback;
+}
+
+/** \ingroup libusb_asyncio
+ * Helper function to populate the required \ref libusb_transfer fields
+ * for a bulk transfer.
+ *
+ * \param transfer the transfer to populate
+ * \param dev_handle handle of the device that will handle the transfer
+ * \param endpoint address of the endpoint where this transfer will be sent
+ * \param buffer data buffer
+ * \param length length of data buffer
+ * \param callback callback function to be invoked on transfer completion
+ * \param user_data user data to pass to callback function
+ * \param timeout timeout for the transfer in milliseconds
+ */
+static inline void libusb_fill_bulk_transfer(struct libusb_transfer *transfer,
+	libusb_device_handle *dev_handle, unsigned char endpoint,
+	unsigned char *buffer, int length, libusb_transfer_cb_fn callback,
+	void *user_data, unsigned int timeout)
+{
+	transfer->dev_handle = dev_handle;
+	transfer->endpoint = endpoint;
+	transfer->type = LIBUSB_TRANSFER_TYPE_BULK;
+	transfer->timeout = timeout;
+	transfer->buffer = buffer;
+	transfer->length = length;
+	transfer->user_data = user_data;
+	transfer->callback = callback;
+}
+
+/** \ingroup libusb_asyncio
+ * Helper function to populate the required \ref libusb_transfer fields
+ * for a bulk transfer using bulk streams.
+ *
+ * Since version 1.0.19, \ref LIBUSB_API_VERSION >= 0x01000103
+ *
+ * \param transfer the transfer to populate
+ * \param dev_handle handle of the device that will handle the transfer
+ * \param endpoint address of the endpoint where this transfer will be sent
+ * \param stream_id bulk stream id for this transfer
+ * \param buffer data buffer
+ * \param length length of data buffer
+ * \param callback callback function to be invoked on transfer completion
+ * \param user_data user data to pass to callback function
+ * \param timeout timeout for the transfer in milliseconds
+ */
+static inline void libusb_fill_bulk_stream_transfer(
+	struct libusb_transfer *transfer, libusb_device_handle *dev_handle,
+	unsigned char endpoint, uint32_t stream_id,
+	unsigned char *buffer, int length, libusb_transfer_cb_fn callback,
+	void *user_data, unsigned int timeout)
+{
+	libusb_fill_bulk_transfer(transfer, dev_handle, endpoint, buffer,
+				  length, callback, user_data, timeout);
+	transfer->type = LIBUSB_TRANSFER_TYPE_BULK_STREAM;
+	libusb_transfer_set_stream_id(transfer, stream_id);
+}
+
+/** \ingroup libusb_asyncio
+ * Helper function to populate the required \ref libusb_transfer fields
+ * for an interrupt transfer.
+ *
+ * \param transfer the transfer to populate
+ * \param dev_handle handle of the device that will handle the transfer
+ * \param endpoint address of the endpoint where this transfer will be sent
+ * \param buffer data buffer
+ * \param length length of data buffer
+ * \param callback callback function to be invoked on transfer completion
+ * \param user_data user data to pass to callback function
+ * \param timeout timeout for the transfer in milliseconds
+ */
+static inline void libusb_fill_interrupt_transfer(
+	struct libusb_transfer *transfer, libusb_device_handle *dev_handle,
+	unsigned char endpoint, unsigned char *buffer, int length,
+	libusb_transfer_cb_fn callback, void *user_data, unsigned int timeout)
+{
+	transfer->dev_handle = dev_handle;
+	transfer->endpoint = endpoint;
+	transfer->type = LIBUSB_TRANSFER_TYPE_INTERRUPT;
+	transfer->timeout = timeout;
+	transfer->buffer = buffer;
+	transfer->length = length;
+	transfer->user_data = user_data;
+	transfer->callback = callback;
+}
+
+/** \ingroup libusb_asyncio
+ * Helper function to populate the required \ref libusb_transfer fields
+ * for an isochronous transfer.
+ *
+ * \param transfer the transfer to populate
+ * \param dev_handle handle of the device that will handle the transfer
+ * \param endpoint address of the endpoint where this transfer will be sent
+ * \param buffer data buffer
+ * \param length length of data buffer
+ * \param num_iso_packets the number of isochronous packets
+ * \param callback callback function to be invoked on transfer completion
+ * \param user_data user data to pass to callback function
+ * \param timeout timeout for the transfer in milliseconds
+ */
+static inline void libusb_fill_iso_transfer(struct libusb_transfer *transfer,
+	libusb_device_handle *dev_handle, unsigned char endpoint,
+	unsigned char *buffer, int length, int num_iso_packets,
+	libusb_transfer_cb_fn callback, void *user_data, unsigned int timeout)
+{
+	transfer->dev_handle = dev_handle;
+	transfer->endpoint = endpoint;
+	transfer->type = LIBUSB_TRANSFER_TYPE_ISOCHRONOUS;
+	transfer->timeout = timeout;
+	transfer->buffer = buffer;
+	transfer->length = length;
+	transfer->num_iso_packets = num_iso_packets;
+	transfer->user_data = user_data;
+	transfer->callback = callback;
+}
+
+/** \ingroup libusb_asyncio
+ * Convenience function to set the length of all packets in an isochronous
+ * transfer, based on the num_iso_packets field in the transfer structure.
+ *
+ * \param transfer a transfer
+ * \param length the length to set in each isochronous packet descriptor
+ * \see libusb_get_max_packet_size()
+ */
+static inline void libusb_set_iso_packet_lengths(
+	struct libusb_transfer *transfer, unsigned int length)
+{
+	int i;
+
+	for (i = 0; i < transfer->num_iso_packets; i++)
+		transfer->iso_packet_desc[i].length = length;
+}
+
+/** \ingroup libusb_asyncio
+ * Convenience function to locate the position of an isochronous packet
+ * within the buffer of an isochronous transfer.
+ *
+ * This is a thorough function which loops through all preceding packets,
+ * accumulating their lengths to find the position of the specified packet.
+ * Typically you will assign equal lengths to each packet in the transfer,
+ * and hence the above method is sub-optimal. You may wish to use
+ * libusb_get_iso_packet_buffer_simple() instead.
+ *
+ * \param transfer a transfer
+ * \param packet the packet to return the address of
+ * \returns the base address of the packet buffer inside the transfer buffer,
+ * or NULL if the packet does not exist.
+ * \see libusb_get_iso_packet_buffer_simple()
+ */
+static inline unsigned char *libusb_get_iso_packet_buffer(
+	struct libusb_transfer *transfer, unsigned int packet)
+{
+	int i;
+	size_t offset = 0;
+	int _packet;
+
+	/* oops..slight bug in the API. packet is an unsigned int, but we use
+	 * signed integers almost everywhere else. range-check and convert to
+	 * signed to avoid compiler warnings. FIXME for libusb-2. */
+	if (packet > INT_MAX)
+		return NULL;
+	_packet = (int) packet;
+
+	if (_packet >= transfer->num_iso_packets)
+		return NULL;
+
+	for (i = 0; i < _packet; i++)
+		offset += transfer->iso_packet_desc[i].length;
+
+	return transfer->buffer + offset;
+}
+
+/** \ingroup libusb_asyncio
+ * Convenience function to locate the position of an isochronous packet
+ * within the buffer of an isochronous transfer, for transfers where each
+ * packet is of identical size.
+ *
+ * This function relies on the assumption that every packet within the transfer
+ * is of identical size to the first packet. Calculating the location of
+ * the packet buffer is then just a simple calculation:
+ * <tt>buffer + (packet_size * packet)</tt>
+ *
+ * Do not use this function on transfers other than those that have identical
+ * packet lengths for each packet.
+ *
+ * \param transfer a transfer
+ * \param packet the packet to return the address of
+ * \returns the base address of the packet buffer inside the transfer buffer,
+ * or NULL if the packet does not exist.
+ * \see libusb_get_iso_packet_buffer()
+ */
+static inline unsigned char *libusb_get_iso_packet_buffer_simple(
+	struct libusb_transfer *transfer, unsigned int packet)
+{
+	int _packet;
+
+	/* oops..slight bug in the API. packet is an unsigned int, but we use
+	 * signed integers almost everywhere else. range-check and convert to
+	 * signed to avoid compiler warnings. FIXME for libusb-2. */
+	if (packet > INT_MAX)
+		return NULL;
+	_packet = (int) packet;
+
+	if (_packet >= transfer->num_iso_packets)
+		return NULL;
+
+	return transfer->buffer + ((int) transfer->iso_packet_desc[0].length * _packet);
+}
+
+/* sync I/O */
+
+int LIBUSB_CALL libusb_control_transfer(libusb_device_handle *dev_handle,
+	uint8_t request_type, uint8_t bRequest, uint16_t wValue, uint16_t wIndex,
+	unsigned char *data, uint16_t wLength, unsigned int timeout);
+
+int LIBUSB_CALL libusb_bulk_transfer(libusb_device_handle *dev_handle,
+	unsigned char endpoint, unsigned char *data, int length,
+	int *actual_length, unsigned int timeout);
+
+int LIBUSB_CALL libusb_interrupt_transfer(libusb_device_handle *dev_handle,
+	unsigned char endpoint, unsigned char *data, int length,
+	int *actual_length, unsigned int timeout);
+
+/** \ingroup libusb_desc
+ * Retrieve a descriptor from the default control pipe.
+ * This is a convenience function which formulates the appropriate control
+ * message to retrieve the descriptor.
+ *
+ * \param dev_handle a device handle
+ * \param desc_type the descriptor type, see \ref libusb_descriptor_type
+ * \param desc_index the index of the descriptor to retrieve
+ * \param data output buffer for descriptor
+ * \param length size of data buffer
+ * \returns number of bytes returned in data, or LIBUSB_ERROR code on failure
+ */
+static inline int libusb_get_descriptor(libusb_device_handle *dev_handle,
+	uint8_t desc_type, uint8_t desc_index, unsigned char *data, int length)
+{
+	return libusb_control_transfer(dev_handle, LIBUSB_ENDPOINT_IN,
+		LIBUSB_REQUEST_GET_DESCRIPTOR, (uint16_t) ((desc_type << 8) | desc_index),
+		0, data, (uint16_t) length, 1000);
+}
+
+/** \ingroup libusb_desc
+ * Retrieve a descriptor from a device.
+ * This is a convenience function which formulates the appropriate control
+ * message to retrieve the descriptor. The string returned is Unicode, as
+ * detailed in the USB specifications.
+ *
+ * \param dev_handle a device handle
+ * \param desc_index the index of the descriptor to retrieve
+ * \param langid the language ID for the string descriptor
+ * \param data output buffer for descriptor
+ * \param length size of data buffer
+ * \returns number of bytes returned in data, or LIBUSB_ERROR code on failure
+ * \see libusb_get_string_descriptor_ascii()
+ */
+static inline int libusb_get_string_descriptor(libusb_device_handle *dev_handle,
+	uint8_t desc_index, uint16_t langid, unsigned char *data, int length)
+{
+	return libusb_control_transfer(dev_handle, LIBUSB_ENDPOINT_IN,
+		LIBUSB_REQUEST_GET_DESCRIPTOR, (uint16_t)((LIBUSB_DT_STRING << 8) | desc_index),
+		langid, data, (uint16_t) length, 1000);
+}
+
+int LIBUSB_CALL libusb_get_string_descriptor_ascii(libusb_device_handle *dev_handle,
+	uint8_t desc_index, unsigned char *data, int length);
+
+/* polling and timeouts */
+
+int LIBUSB_CALL libusb_try_lock_events(libusb_context *ctx);
+void LIBUSB_CALL libusb_lock_events(libusb_context *ctx);
+void LIBUSB_CALL libusb_unlock_events(libusb_context *ctx);
+int LIBUSB_CALL libusb_event_handling_ok(libusb_context *ctx);
+int LIBUSB_CALL libusb_event_handler_active(libusb_context *ctx);
+void LIBUSB_CALL libusb_interrupt_event_handler(libusb_context *ctx);
+void LIBUSB_CALL libusb_lock_event_waiters(libusb_context *ctx);
+void LIBUSB_CALL libusb_unlock_event_waiters(libusb_context *ctx);
+int LIBUSB_CALL libusb_wait_for_event(libusb_context *ctx, struct timeval *tv);
+
+int LIBUSB_CALL libusb_handle_events_timeout(libusb_context *ctx,
+	struct timeval *tv);
+int LIBUSB_CALL libusb_handle_events_timeout_completed(libusb_context *ctx,
+	struct timeval *tv, int *completed);
+int LIBUSB_CALL libusb_handle_events(libusb_context *ctx);
+int LIBUSB_CALL libusb_handle_events_completed(libusb_context *ctx, int *completed);
+int LIBUSB_CALL libusb_handle_events_locked(libusb_context *ctx,
+	struct timeval *tv);
+int LIBUSB_CALL libusb_pollfds_handle_timeouts(libusb_context *ctx);
+int LIBUSB_CALL libusb_get_next_timeout(libusb_context *ctx,
+	struct timeval *tv);
+
+/** \ingroup libusb_poll
+ * File descriptor for polling
+ */
+struct libusb_pollfd {
+	/** Numeric file descriptor */
+	int fd;
+
+	/** Event flags to poll for from <poll.h>. POLLIN indicates that you
+	 * should monitor this file descriptor for becoming ready to read from,
+	 * and POLLOUT indicates that you should monitor this file descriptor for
+	 * nonblocking write readiness. */
+	short events;
+};
+
+/** \ingroup libusb_poll
+ * Callback function, invoked when a new file descriptor should be added
+ * to the set of file descriptors monitored for events.
+ * \param fd the new file descriptor
+ * \param events events to monitor for, see \ref libusb_pollfd for a
+ * description
+ * \param user_data User data pointer specified in
+ * libusb_set_pollfd_notifiers() call
+ * \see libusb_set_pollfd_notifiers()
+ */
+typedef void (LIBUSB_CALL *libusb_pollfd_added_cb)(int fd, short events,
+	void *user_data);
+
+/** \ingroup libusb_poll
+ * Callback function, invoked when a file descriptor should be removed from
+ * the set of file descriptors being monitored for events. After returning
+ * from this callback, do not use that file descriptor again.
+ * \param fd the file descriptor to stop monitoring
+ * \param user_data User data pointer specified in
+ * libusb_set_pollfd_notifiers() call
+ * \see libusb_set_pollfd_notifiers()
+ */
+typedef void (LIBUSB_CALL *libusb_pollfd_removed_cb)(int fd, void *user_data);
+
+const struct libusb_pollfd ** LIBUSB_CALL libusb_get_pollfds(
+	libusb_context *ctx);
+void LIBUSB_CALL libusb_free_pollfds(const struct libusb_pollfd **pollfds);
+void LIBUSB_CALL libusb_set_pollfd_notifiers(libusb_context *ctx,
+	libusb_pollfd_added_cb added_cb, libusb_pollfd_removed_cb removed_cb,
+	void *user_data);
+
+/** \ingroup libusb_hotplug
+ * Callback handle.
+ *
+ * Callbacks handles are generated by libusb_hotplug_register_callback()
+ * and can be used to deregister callbacks. Callback handles are unique
+ * per libusb_context and it is safe to call libusb_hotplug_deregister_callback()
+ * on an already deregistered callback.
+ *
+ * Since version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102
+ *
+ * For more information, see \ref libusb_hotplug.
+ */
+typedef int libusb_hotplug_callback_handle;
+
+/** \ingroup libusb_hotplug
+ *
+ * Since version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102
+ *
+ * Hotplug events */
+typedef enum {
+	/** A device has been plugged in and is ready to use */
+	LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED = (1 << 0),
+
+	/** A device has left and is no longer available.
+	 * It is the user's responsibility to call libusb_close on any handle associated with a disconnected device.
+	 * It is safe to call libusb_get_device_descriptor on a device that has left */
+	LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT = (1 << 1)
+} libusb_hotplug_event;
+
+/** \ingroup libusb_hotplug
+ *
+ * Since version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102
+ *
+ * Hotplug flags */
+typedef enum {
+	/** Arm the callback and fire it for all matching currently attached devices. */
+	LIBUSB_HOTPLUG_ENUMERATE = (1 << 0)
+} libusb_hotplug_flag;
+
+/** \ingroup libusb_hotplug
+ * Convenience macro when not using any flags */
+#define LIBUSB_HOTPLUG_NO_FLAGS 0
+
+/** \ingroup libusb_hotplug
+ * Wildcard matching for hotplug events */
+#define LIBUSB_HOTPLUG_MATCH_ANY -1
+
+/** \ingroup libusb_hotplug
+ * Hotplug callback function type. When requesting hotplug event notifications,
+ * you pass a pointer to a callback function of this type.
+ *
+ * This callback may be called by an internal event thread and as such it is
+ * recommended the callback do minimal processing before returning.
+ *
+ * libusb will call this function later, when a matching event had happened on
+ * a matching device. See \ref libusb_hotplug for more information.
+ *
+ * It is safe to call either libusb_hotplug_register_callback() or
+ * libusb_hotplug_deregister_callback() from within a callback function.
+ *
+ * Since version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102
+ *
+ * \param ctx            context of this notification
+ * \param device         libusb_device this event occurred on
+ * \param event          event that occurred
+ * \param user_data      user data provided when this callback was registered
+ * \returns bool whether this callback is finished processing events.
+ *                       returning 1 will cause this callback to be deregistered
+ */
+typedef int (LIBUSB_CALL *libusb_hotplug_callback_fn)(libusb_context *ctx,
+	libusb_device *device, libusb_hotplug_event event, void *user_data);
+
+/** \ingroup libusb_hotplug
+ * Register a hotplug callback function
+ *
+ * Register a callback with the libusb_context. The callback will fire
+ * when a matching event occurs on a matching device. The callback is
+ * armed until either it is deregistered with libusb_hotplug_deregister_callback()
+ * or the supplied callback returns 1 to indicate it is finished processing events.
+ *
+ * If the \ref LIBUSB_HOTPLUG_ENUMERATE is passed the callback will be
+ * called with a \ref LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED for all devices
+ * already plugged into the machine. Note that libusb modifies its internal
+ * device list from a separate thread, while calling hotplug callbacks from
+ * libusb_handle_events(), so it is possible for a device to already be present
+ * on, or removed from, its internal device list, while the hotplug callbacks
+ * still need to be dispatched. This means that when using \ref
+ * LIBUSB_HOTPLUG_ENUMERATE, your callback may be called twice for the arrival
+ * of the same device, once from libusb_hotplug_register_callback() and once
+ * from libusb_handle_events(); and/or your callback may be called for the
+ * removal of a device for which an arrived call was never made.
+ *
+ * Since version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102
+ *
+ * \param[in] ctx context to register this callback with
+ * \param[in] events bitwise or of hotplug events that will trigger this callback.
+ *            See \ref libusb_hotplug_event
+ * \param[in] flags bitwise or of hotplug flags that affect registration.
+ *            See \ref libusb_hotplug_flag
+ * \param[in] vendor_id the vendor id to match or \ref LIBUSB_HOTPLUG_MATCH_ANY
+ * \param[in] product_id the product id to match or \ref LIBUSB_HOTPLUG_MATCH_ANY
+ * \param[in] dev_class the device class to match or \ref LIBUSB_HOTPLUG_MATCH_ANY
+ * \param[in] cb_fn the function to be invoked on a matching event/device
+ * \param[in] user_data user data to pass to the callback function
+ * \param[out] callback_handle pointer to store the handle of the allocated callback (can be NULL)
+ * \returns LIBUSB_SUCCESS on success LIBUSB_ERROR code on failure
+ */
+int LIBUSB_CALL libusb_hotplug_register_callback(libusb_context *ctx,
+	int events, int flags,
+	int vendor_id, int product_id, int dev_class,
+	libusb_hotplug_callback_fn cb_fn, void *user_data,
+	libusb_hotplug_callback_handle *callback_handle);
+
+/** \ingroup libusb_hotplug
+ * Deregisters a hotplug callback.
+ *
+ * Deregister a callback from a libusb_context. This function is safe to call from within
+ * a hotplug callback.
+ *
+ * Since version 1.0.16, \ref LIBUSB_API_VERSION >= 0x01000102
+ *
+ * \param[in] ctx context this callback is registered with
+ * \param[in] callback_handle the handle of the callback to deregister
+ */
+void LIBUSB_CALL libusb_hotplug_deregister_callback(libusb_context *ctx,
+	libusb_hotplug_callback_handle callback_handle);
+
+/** \ingroup libusb_hotplug
+ * Gets the user_data associated with a hotplug callback.
+ *
+ * Since version v1.0.24 \ref LIBUSB_API_VERSION >= 0x01000108
+ *
+ * \param[in] ctx context this callback is registered with
+ * \param[in] callback_handle the handle of the callback to get the user_data of
+ */
+void * LIBUSB_CALL libusb_hotplug_get_user_data(libusb_context *ctx,
+	libusb_hotplug_callback_handle callback_handle);
+
+/** \ingroup libusb_lib
+ * Available option values for libusb_set_option().
+ */
+enum libusb_option {
+	/** Set the log message verbosity.
+	 *
+	 * The default level is LIBUSB_LOG_LEVEL_NONE, which means no messages are ever
+	 * printed. If you choose to increase the message verbosity level, ensure
+	 * that your application does not close the stderr file descriptor.
+	 *
+	 * You are advised to use level LIBUSB_LOG_LEVEL_WARNING. libusb is conservative
+	 * with its message logging and most of the time, will only log messages that
+	 * explain error conditions and other oddities. This will help you debug
+	 * your software.
+	 *
+	 * If the LIBUSB_DEBUG environment variable was set when libusb was
+	 * initialized, this function does nothing: the message verbosity is fixed
+	 * to the value in the environment variable.
+	 *
+	 * If libusb was compiled without any message logging, this function does
+	 * nothing: you'll never get any messages.
+	 *
+	 * If libusb was compiled with verbose debug message logging, this function
+	 * does nothing: you'll always get messages from all levels.
+	 */
+	LIBUSB_OPTION_LOG_LEVEL = 0,
+
+	/** Use the UsbDk backend for a specific context, if available.
+	 *
+	 * This option should be set immediately after calling libusb_init(), otherwise
+	 * unspecified behavior may occur.
+	 *
+	 * Only valid on Windows.
+	 */
+	LIBUSB_OPTION_USE_USBDK = 1,
+
+	/** Do not scan for devices
+	 *
+	 * With this option set, libusb will skip scanning devices in
+	 * libusb_init(). Must be set before calling libusb_init().
+	 *
+	 * Hotplug functionality will also be deactivated.
+	 *
+	 * The option is useful in combination with libusb_wrap_sys_device(),
+	 * which can access a device directly without prior device scanning.
+	 *
+	 * This is typically needed on Android, where access to USB devices
+	 * is limited.
+	 *
+	 * For LIBUSB_API_VERSION 0x01000108 it was called LIBUSB_OPTION_WEAK_AUTHORITY
+	 *
+	 * Only valid on Linux.
+	 */
+	LIBUSB_OPTION_NO_DEVICE_DISCOVERY = 2,
+
+#define LIBUSB_OPTION_WEAK_AUTHORITY LIBUSB_OPTION_NO_DEVICE_DISCOVERY
+
+	LIBUSB_OPTION_MAX = 3
+};
+
+int LIBUSB_CALL libusb_set_option(libusb_context *ctx, enum libusb_option option, ...);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
diff --git a/hardware/src/libusb/libusbi.h b/hardware/src/libusb/libusbi.h
new file mode 100644
index 0000000..6f38828
--- /dev/null
+++ b/hardware/src/libusb/libusbi.h
@@ -0,0 +1,1483 @@
+/*
+ * Internal header for libusb
+ * Copyright © 2007-2009 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ * Copyright © 2019 Nathan Hjelm <hjelmn@cs.umm.edu>
+ * Copyright © 2019-2020 Google LLC. All rights reserved.
+ * Copyright © 2020 Chris Dickens <christopher.a.dickens@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef LIBUSBI_H
+#define LIBUSBI_H
+
+#include "config.h"
+
+#include <assert.h>
+#include <inttypes.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdlib.h>
+#ifdef HAVE_SYS_TIME_H
+#include <sys/time.h>
+#endif
+
+#include "libusb.h"
+
+/* Not all C standard library headers define static_assert in assert.h
+ * Additionally, Visual Studio treats static_assert as a keyword.
+ */
+#if !defined(__cplusplus) && !defined(static_assert) && !defined(_MSC_VER)
+#define static_assert(cond, msg) _Static_assert(cond, msg)
+#endif
+
+#ifdef NDEBUG
+#define ASSERT_EQ(expression, value)	(void)expression
+#define ASSERT_NE(expression, value)	(void)expression
+#else
+#define ASSERT_EQ(expression, value)	assert(expression == value)
+#define ASSERT_NE(expression, value)	assert(expression != value)
+#endif
+
+#define container_of(ptr, type, member) \
+	((type *)((uintptr_t)(ptr) - (uintptr_t)offsetof(type, member)))
+
+#ifndef ARRAYSIZE
+#define ARRAYSIZE(array) (sizeof(array) / sizeof(array[0]))
+#endif
+
+#ifndef CLAMP
+#define CLAMP(val, min, max) \
+	((val) < (min) ? (min) : ((val) > (max) ? (max) : (val)))
+#endif
+
+#ifndef MIN
+#define MIN(a, b)	((a) < (b) ? (a) : (b))
+#endif
+
+#ifndef MAX
+#define MAX(a, b)	((a) > (b) ? (a) : (b))
+#endif
+
+/* The following is used to silence warnings for unused variables */
+#if defined(UNREFERENCED_PARAMETER)
+#define UNUSED(var)	UNREFERENCED_PARAMETER(var)
+#else
+#define UNUSED(var)	do { (void)(var); } while(0)
+#endif
+
+/* Macro to align a value up to the next multiple of the size of a pointer */
+#define PTR_ALIGN(v) \
+	(((v) + (sizeof(void *) - 1)) & ~(sizeof(void *) - 1))
+
+/* Atomic operations
+ *
+ * Useful for reference counting or when accessing a value without a lock
+ *
+ * The following atomic operations are defined:
+ *   usbi_atomic_load() - Atomically read a variable's value
+ *   usbi_atomic_store() - Atomically write a new value value to a variable
+ *   usbi_atomic_inc() - Atomically increment a variable's value and return the new value
+ *   usbi_atomic_dec() - Atomically decrement a variable's value and return the new value
+ *
+ * All of these operations are ordered with each other, thus the effects of
+ * any one operation is guaranteed to be seen by any other operation.
+ */
+#ifdef _MSC_VER
+typedef volatile LONG usbi_atomic_t;
+#define usbi_atomic_load(a)	(*(a))
+#define usbi_atomic_store(a, v)	(*(a)) = (v)
+#define usbi_atomic_inc(a)	InterlockedIncrement((a))
+#define usbi_atomic_dec(a)	InterlockedDecrement((a))
+#else
+#include <stdatomic.h>
+typedef atomic_long usbi_atomic_t;
+#define usbi_atomic_load(a)	atomic_load((a))
+#define usbi_atomic_store(a, v)	atomic_store((a), (v))
+#define usbi_atomic_inc(a)	(atomic_fetch_add((a), 1) + 1)
+#define usbi_atomic_dec(a)	(atomic_fetch_add((a), -1) - 1)
+#endif
+
+/* Internal abstractions for event handling and thread synchronization */
+#if defined(PLATFORM_POSIX)
+#include "os/events_posix.h"
+#include "os/threads_posix.h"
+#elif defined(PLATFORM_WINDOWS)
+#include "os/events_windows.h"
+#include "os/threads_windows.h"
+#endif
+
+/* Inside the libusb code, mark all public functions as follows:
+ *   return_type API_EXPORTED function_name(params) { ... }
+ * But if the function returns a pointer, mark it as follows:
+ *   DEFAULT_VISIBILITY return_type * LIBUSB_CALL function_name(params) { ... }
+ * In the libusb public header, mark all declarations as:
+ *   return_type LIBUSB_CALL function_name(params);
+ */
+#define API_EXPORTED LIBUSB_CALL DEFAULT_VISIBILITY
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define USB_MAXENDPOINTS	32
+#define USB_MAXINTERFACES	32
+#define USB_MAXCONFIG		8
+
+/* Backend specific capabilities */
+#define USBI_CAP_HAS_HID_ACCESS			0x00010000
+#define USBI_CAP_SUPPORTS_DETACH_KERNEL_DRIVER	0x00020000
+
+/* Maximum number of bytes in a log line */
+#define USBI_MAX_LOG_LEN	1024
+/* Terminator for log lines */
+#define USBI_LOG_LINE_END	"\n"
+
+struct list_head {
+	struct list_head *prev, *next;
+};
+
+/* Get an entry from the list
+ *  ptr - the address of this list_head element in "type"
+ *  type - the data type that contains "member"
+ *  member - the list_head element in "type"
+ */
+#define list_entry(ptr, type, member) \
+	container_of(ptr, type, member)
+
+#define list_first_entry(ptr, type, member) \
+	list_entry((ptr)->next, type, member)
+
+#define list_next_entry(ptr, type, member) \
+	list_entry((ptr)->member.next, type, member)
+
+/* Get each entry from a list
+ *  pos - A structure pointer has a "member" element
+ *  head - list head
+ *  member - the list_head element in "pos"
+ *  type - the type of the first parameter
+ */
+#define list_for_each_entry(pos, head, member, type)			\
+	for (pos = list_first_entry(head, type, member);		\
+		 &pos->member != (head);				\
+		 pos = list_next_entry(pos, type, member))
+
+#define list_for_each_entry_safe(pos, n, head, member, type)		\
+	for (pos = list_first_entry(head, type, member),		\
+		 n = list_next_entry(pos, type, member);		\
+		 &pos->member != (head);				\
+		 pos = n, n = list_next_entry(n, type, member))
+
+/* Helper macros to iterate over a list. The structure pointed
+ * to by "pos" must have a list_head member named "list".
+ */
+#define for_each_helper(pos, head, type) \
+	list_for_each_entry(pos, head, list, type)
+
+#define for_each_safe_helper(pos, n, head, type) \
+	list_for_each_entry_safe(pos, n, head, list, type)
+
+#define list_empty(entry) ((entry)->next == (entry))
+
+static inline void list_init(struct list_head *entry)
+{
+	entry->prev = entry->next = entry;
+}
+
+static inline void list_add(struct list_head *entry, struct list_head *head)
+{
+	entry->next = head->next;
+	entry->prev = head;
+
+	head->next->prev = entry;
+	head->next = entry;
+}
+
+static inline void list_add_tail(struct list_head *entry,
+	struct list_head *head)
+{
+	entry->next = head;
+	entry->prev = head->prev;
+
+	head->prev->next = entry;
+	head->prev = entry;
+}
+
+static inline void list_del(struct list_head *entry)
+{
+	entry->next->prev = entry->prev;
+	entry->prev->next = entry->next;
+	entry->next = entry->prev = NULL;
+}
+
+static inline void list_cut(struct list_head *list, struct list_head *head)
+{
+	if (list_empty(head)) {
+		list_init(list);
+		return;
+	}
+
+	list->next = head->next;
+	list->next->prev = list;
+	list->prev = head->prev;
+	list->prev->next = list;
+
+	list_init(head);
+}
+
+static inline void list_splice_front(struct list_head *list, struct list_head *head)
+{
+	list->next->prev = head;
+	list->prev->next = head->next;
+	head->next->prev = list->prev;
+	head->next = list->next;
+}
+
+static inline void *usbi_reallocf(void *ptr, size_t size)
+{
+	void *ret = realloc(ptr, size);
+
+	if (!ret)
+		free(ptr);
+	return ret;
+}
+
+#if !defined(USEC_PER_SEC)
+#define USEC_PER_SEC	1000000L
+#endif
+
+#if !defined(NSEC_PER_SEC)
+#define NSEC_PER_SEC	1000000000L
+#endif
+
+#define TIMEVAL_IS_VALID(tv)						\
+	((tv)->tv_sec >= 0 &&						\
+	 (tv)->tv_usec >= 0 && (tv)->tv_usec < USEC_PER_SEC)
+
+#define TIMESPEC_IS_SET(ts)	((ts)->tv_sec || (ts)->tv_nsec)
+#define TIMESPEC_CLEAR(ts)	(ts)->tv_sec = (ts)->tv_nsec = 0
+#define TIMESPEC_CMP(a, b, CMP)						\
+	(((a)->tv_sec == (b)->tv_sec)					\
+	 ? ((a)->tv_nsec CMP (b)->tv_nsec)				\
+	 : ((a)->tv_sec CMP (b)->tv_sec))
+#define TIMESPEC_SUB(a, b, result)					\
+	do {								\
+		(result)->tv_sec = (a)->tv_sec - (b)->tv_sec;		\
+		(result)->tv_nsec = (a)->tv_nsec - (b)->tv_nsec;	\
+		if ((result)->tv_nsec < 0L) {				\
+			--(result)->tv_sec;				\
+			(result)->tv_nsec += NSEC_PER_SEC;		\
+		}							\
+	} while (0)
+
+#if defined(PLATFORM_WINDOWS)
+#define TIMEVAL_TV_SEC_TYPE	long
+#else
+#define TIMEVAL_TV_SEC_TYPE	time_t
+#endif
+
+/* Some platforms don't have this define */
+#ifndef TIMESPEC_TO_TIMEVAL
+#define TIMESPEC_TO_TIMEVAL(tv, ts)					\
+	do {								\
+		(tv)->tv_sec = (TIMEVAL_TV_SEC_TYPE) (ts)->tv_sec;	\
+		(tv)->tv_usec = (ts)->tv_nsec / 1000L;			\
+	} while (0)
+#endif
+
+#ifdef ENABLE_LOGGING
+
+#if defined(_MSC_VER) && (_MSC_VER < 1900)
+#include <stdio.h>
+#define snprintf usbi_snprintf
+#define vsnprintf usbi_vsnprintf
+int usbi_snprintf(char *dst, size_t size, const char *format, ...);
+int usbi_vsnprintf(char *dst, size_t size, const char *format, va_list args);
+#define LIBUSB_PRINTF_WIN32
+#endif /* defined(_MSC_VER) && (_MSC_VER < 1900) */
+
+void usbi_log(struct libusb_context *ctx, enum libusb_log_level level,
+	const char *function, const char *format, ...) PRINTF_FORMAT(4, 5);
+
+#define _usbi_log(ctx, level, ...) usbi_log(ctx, level, __func__, __VA_ARGS__)
+
+#define usbi_err(ctx, ...)	_usbi_log(ctx, LIBUSB_LOG_LEVEL_ERROR, __VA_ARGS__)
+#define usbi_warn(ctx, ...)	_usbi_log(ctx, LIBUSB_LOG_LEVEL_WARNING, __VA_ARGS__)
+#define usbi_info(ctx, ...)	_usbi_log(ctx, LIBUSB_LOG_LEVEL_INFO, __VA_ARGS__)
+#define usbi_dbg(ctx ,...)      	_usbi_log(ctx, LIBUSB_LOG_LEVEL_DEBUG, __VA_ARGS__)
+
+#else /* ENABLE_LOGGING */
+
+#define usbi_err(ctx, ...)	UNUSED(ctx)
+#define usbi_warn(ctx, ...)	UNUSED(ctx)
+#define usbi_info(ctx, ...)	UNUSED(ctx)
+#define usbi_dbg(ctx, ...)	do {} while (0)
+
+#endif /* ENABLE_LOGGING */
+
+#define DEVICE_CTX(dev)		((dev)->ctx)
+#define HANDLE_CTX(handle)	(DEVICE_CTX((handle)->dev))
+#define TRANSFER_CTX(transfer)	(HANDLE_CTX((transfer)->dev_handle))
+#define ITRANSFER_CTX(itransfer) \
+	(TRANSFER_CTX(USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer)))
+
+#define IS_EPIN(ep)		(0 != ((ep) & LIBUSB_ENDPOINT_IN))
+#define IS_EPOUT(ep)		(!IS_EPIN(ep))
+#define IS_XFERIN(xfer)		(0 != ((xfer)->endpoint & LIBUSB_ENDPOINT_IN))
+#define IS_XFEROUT(xfer)	(!IS_XFERIN(xfer))
+
+struct libusb_context {
+#if defined(ENABLE_LOGGING) && !defined(ENABLE_DEBUG_LOGGING)
+	enum libusb_log_level debug;
+	int debug_fixed;
+	libusb_log_cb log_handler;
+#endif
+
+	/* used for signalling occurrence of an internal event. */
+	usbi_event_t event;
+
+#ifdef HAVE_OS_TIMER
+	/* used for timeout handling, if supported by OS.
+	 * this timer is maintained to trigger on the next pending timeout */
+	usbi_timer_t timer;
+#endif
+
+	struct list_head usb_devs;
+	usbi_mutex_t usb_devs_lock;
+
+	/* A list of open handles. Backends are free to traverse this if required.
+	 */
+	struct list_head open_devs;
+	usbi_mutex_t open_devs_lock;
+
+	/* A list of registered hotplug callbacks */
+	struct list_head hotplug_cbs;
+	libusb_hotplug_callback_handle next_hotplug_cb_handle;
+	usbi_mutex_t hotplug_cbs_lock;
+
+	/* A flag to indicate that the context is ready for hotplug notifications */
+	usbi_atomic_t hotplug_ready;
+
+	/* this is a list of in-flight transfer handles, sorted by timeout
+	 * expiration. URBs to timeout the soonest are placed at the beginning of
+	 * the list, URBs that will time out later are placed after, and urbs with
+	 * infinite timeout are always placed at the very end. */
+	struct list_head flying_transfers;
+	/* Note paths taking both this and usbi_transfer->lock must always
+	 * take this lock first */
+	usbi_mutex_t flying_transfers_lock;
+
+#if !defined(PLATFORM_WINDOWS)
+	/* user callbacks for pollfd changes */
+	libusb_pollfd_added_cb fd_added_cb;
+	libusb_pollfd_removed_cb fd_removed_cb;
+	void *fd_cb_user_data;
+#endif
+
+	/* ensures that only one thread is handling events at any one time */
+	usbi_mutex_t events_lock;
+
+	/* used to see if there is an active thread doing event handling */
+	int event_handler_active;
+
+	/* A thread-local storage key to track which thread is performing event
+	 * handling */
+	usbi_tls_key_t event_handling_key;
+
+	/* used to wait for event completion in threads other than the one that is
+	 * event handling */
+	usbi_mutex_t event_waiters_lock;
+	usbi_cond_t event_waiters_cond;
+
+	/* A lock to protect internal context event data. */
+	usbi_mutex_t event_data_lock;
+
+	/* A bitmask of flags that are set to indicate specific events that need to
+	 * be handled. Protected by event_data_lock. */
+	unsigned int event_flags;
+
+	/* A counter that is set when we want to interrupt and prevent event handling,
+	 * in order to safely close a device. Protected by event_data_lock. */
+	unsigned int device_close;
+
+	/* A list of currently active event sources. Protected by event_data_lock. */
+	struct list_head event_sources;
+
+	/* A list of event sources that have been removed since the last time
+	 * event sources were waited on. Protected by event_data_lock. */
+	struct list_head removed_event_sources;
+
+	/* A pointer and count to platform-specific data used for monitoring event
+	 * sources. Only accessed during event handling. */
+	void *event_data;
+	unsigned int event_data_cnt;
+
+	/* A list of pending hotplug messages. Protected by event_data_lock. */
+	struct list_head hotplug_msgs;
+
+	/* A list of pending completed transfers. Protected by event_data_lock. */
+	struct list_head completed_transfers;
+
+	struct list_head list;
+};
+
+extern struct libusb_context *usbi_default_context;
+
+extern struct list_head active_contexts_list;
+extern usbi_mutex_static_t active_contexts_lock;
+
+static inline struct libusb_context *usbi_get_context(struct libusb_context *ctx)
+{
+	return ctx ? ctx : usbi_default_context;
+}
+
+enum usbi_event_flags {
+	/* The list of event sources has been modified */
+	USBI_EVENT_EVENT_SOURCES_MODIFIED = 1U << 0,
+
+	/* The user has interrupted the event handler */
+	USBI_EVENT_USER_INTERRUPT = 1U << 1,
+
+	/* A hotplug callback deregistration is pending */
+	USBI_EVENT_HOTPLUG_CB_DEREGISTERED = 1U << 2,
+
+	/* One or more hotplug messages are pending */
+	USBI_EVENT_HOTPLUG_MSG_PENDING = 1U << 3,
+
+	/* One or more completed transfers are pending */
+	USBI_EVENT_TRANSFER_COMPLETED = 1U << 4,
+
+	/* A device is in the process of being closed */
+	USBI_EVENT_DEVICE_CLOSE = 1U << 5,
+};
+
+/* Macros for managing event handling state */
+static inline int usbi_handling_events(struct libusb_context *ctx)
+{
+	return usbi_tls_key_get(ctx->event_handling_key) != NULL;
+}
+
+static inline void usbi_start_event_handling(struct libusb_context *ctx)
+{
+	usbi_tls_key_set(ctx->event_handling_key, ctx);
+}
+
+static inline void usbi_end_event_handling(struct libusb_context *ctx)
+{
+	usbi_tls_key_set(ctx->event_handling_key, NULL);
+}
+
+struct libusb_device {
+	usbi_atomic_t refcnt;
+
+	struct libusb_context *ctx;
+	struct libusb_device *parent_dev;
+
+	uint8_t bus_number;
+	uint8_t port_number;
+	uint8_t device_address;
+	enum libusb_speed speed;
+
+	struct list_head list;
+	unsigned long session_data;
+
+	struct libusb_device_descriptor device_descriptor;
+	usbi_atomic_t attached;
+};
+
+struct libusb_device_handle {
+	/* lock protects claimed_interfaces */
+	usbi_mutex_t lock;
+	unsigned long claimed_interfaces;
+
+	struct list_head list;
+	struct libusb_device *dev;
+	int auto_detach_kernel_driver;
+};
+
+/* Function called by backend during device initialization to convert
+ * multi-byte fields in the device descriptor to host-endian format.
+ */
+static inline void usbi_localize_device_descriptor(struct libusb_device_descriptor *desc)
+{
+	desc->bcdUSB = libusb_le16_to_cpu(desc->bcdUSB);
+	desc->idVendor = libusb_le16_to_cpu(desc->idVendor);
+	desc->idProduct = libusb_le16_to_cpu(desc->idProduct);
+	desc->bcdDevice = libusb_le16_to_cpu(desc->bcdDevice);
+}
+
+#ifdef HAVE_CLOCK_GETTIME
+static inline void usbi_get_monotonic_time(struct timespec *tp)
+{
+	ASSERT_EQ(clock_gettime(CLOCK_MONOTONIC, tp), 0);
+}
+static inline void usbi_get_real_time(struct timespec *tp)
+{
+	ASSERT_EQ(clock_gettime(CLOCK_REALTIME, tp), 0);
+}
+#else
+/* If the platform doesn't provide the clock_gettime() function, the backend
+ * must provide its own clock implementations.  Two clock functions are
+ * required:
+ *
+ *   usbi_get_monotonic_time(): returns the time since an unspecified starting
+ *                              point (usually boot) that is monotonically
+ *                              increasing.
+ *   usbi_get_real_time(): returns the time since system epoch.
+ */
+void usbi_get_monotonic_time(struct timespec *tp);
+void usbi_get_real_time(struct timespec *tp);
+#endif
+
+/* in-memory transfer layout:
+ *
+ * 1. os private data
+ * 2. struct usbi_transfer
+ * 3. struct libusb_transfer (which includes iso packets) [variable size]
+ *
+ * from a libusb_transfer, you can get the usbi_transfer by rewinding the
+ * appropriate number of bytes.
+ */
+
+struct usbi_transfer {
+	int num_iso_packets;
+	struct list_head list;
+	struct list_head completed_list;
+	struct timespec timeout;
+	int transferred;
+	uint32_t stream_id;
+	uint32_t state_flags;   /* Protected by usbi_transfer->lock */
+	uint32_t timeout_flags; /* Protected by the flying_stransfers_lock */
+
+	/* this lock is held during libusb_submit_transfer() and
+	 * libusb_cancel_transfer() (allowing the OS backend to prevent duplicate
+	 * cancellation, submission-during-cancellation, etc). the OS backend
+	 * should also take this lock in the handle_events path, to prevent the user
+	 * cancelling the transfer from another thread while you are processing
+	 * its completion (presumably there would be races within your OS backend
+	 * if this were possible).
+	 * Note paths taking both this and the flying_transfers_lock must
+	 * always take the flying_transfers_lock first */
+	usbi_mutex_t lock;
+
+	void *priv;
+};
+
+enum usbi_transfer_state_flags {
+	/* Transfer successfully submitted by backend */
+	USBI_TRANSFER_IN_FLIGHT = 1U << 0,
+
+	/* Cancellation was requested via libusb_cancel_transfer() */
+	USBI_TRANSFER_CANCELLING = 1U << 1,
+
+	/* Operation on the transfer failed because the device disappeared */
+	USBI_TRANSFER_DEVICE_DISAPPEARED = 1U << 2,
+};
+
+enum usbi_transfer_timeout_flags {
+	/* Set by backend submit_transfer() if the OS handles timeout */
+	USBI_TRANSFER_OS_HANDLES_TIMEOUT = 1U << 0,
+
+	/* The transfer timeout has been handled */
+	USBI_TRANSFER_TIMEOUT_HANDLED = 1U << 1,
+
+	/* The transfer timeout was successfully processed */
+	USBI_TRANSFER_TIMED_OUT = 1U << 2,
+};
+
+#define USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer)	\
+	((struct libusb_transfer *)			\
+	 ((unsigned char *)(itransfer)			\
+	  + PTR_ALIGN(sizeof(struct usbi_transfer))))
+#define LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer)	\
+	((struct usbi_transfer *)			\
+	 ((unsigned char *)(transfer)			\
+	  - PTR_ALIGN(sizeof(struct usbi_transfer))))
+
+#ifdef _MSC_VER
+#pragma pack(push, 1)
+#endif
+
+/* All standard descriptors have these 2 fields in common */
+struct usbi_descriptor_header {
+	uint8_t  bLength;
+	uint8_t  bDescriptorType;
+} LIBUSB_PACKED;
+
+struct usbi_device_descriptor {
+	uint8_t  bLength;
+	uint8_t  bDescriptorType;
+	uint16_t bcdUSB;
+	uint8_t  bDeviceClass;
+	uint8_t  bDeviceSubClass;
+	uint8_t  bDeviceProtocol;
+	uint8_t  bMaxPacketSize0;
+	uint16_t idVendor;
+	uint16_t idProduct;
+	uint16_t bcdDevice;
+	uint8_t  iManufacturer;
+	uint8_t  iProduct;
+	uint8_t  iSerialNumber;
+	uint8_t  bNumConfigurations;
+} LIBUSB_PACKED;
+
+struct usbi_configuration_descriptor {
+	uint8_t  bLength;
+	uint8_t  bDescriptorType;
+	uint16_t wTotalLength;
+	uint8_t  bNumInterfaces;
+	uint8_t  bConfigurationValue;
+	uint8_t  iConfiguration;
+	uint8_t  bmAttributes;
+	uint8_t  bMaxPower;
+} LIBUSB_PACKED;
+
+struct usbi_interface_descriptor {
+	uint8_t  bLength;
+	uint8_t  bDescriptorType;
+	uint8_t  bInterfaceNumber;
+	uint8_t  bAlternateSetting;
+	uint8_t  bNumEndpoints;
+	uint8_t  bInterfaceClass;
+	uint8_t  bInterfaceSubClass;
+	uint8_t  bInterfaceProtocol;
+	uint8_t  iInterface;
+} LIBUSB_PACKED;
+
+struct usbi_string_descriptor {
+	uint8_t  bLength;
+	uint8_t  bDescriptorType;
+	uint16_t wData[ZERO_SIZED_ARRAY];
+} LIBUSB_PACKED;
+
+struct usbi_bos_descriptor {
+	uint8_t  bLength;
+	uint8_t  bDescriptorType;
+	uint16_t wTotalLength;
+	uint8_t  bNumDeviceCaps;
+} LIBUSB_PACKED;
+
+#ifdef _MSC_VER
+#pragma pack(pop)
+#endif
+
+union usbi_config_desc_buf {
+        struct usbi_configuration_descriptor desc;
+        uint8_t buf[LIBUSB_DT_CONFIG_SIZE];
+        uint16_t align;         /* Force 2-byte alignment */
+};
+
+union usbi_string_desc_buf {
+        struct usbi_string_descriptor desc;
+        uint8_t buf[255];       /* Some devices choke on size > 255 */
+        uint16_t align;         /* Force 2-byte alignment */
+};
+
+union usbi_bos_desc_buf {
+        struct usbi_bos_descriptor desc;
+        uint8_t buf[LIBUSB_DT_BOS_SIZE];
+        uint16_t align;         /* Force 2-byte alignment */
+};
+
+enum usbi_hotplug_flags {
+	/* This callback is interested in device arrivals */
+	USBI_HOTPLUG_DEVICE_ARRIVED = LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED,
+
+	/* This callback is interested in device removals */
+	USBI_HOTPLUG_DEVICE_LEFT = LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT,
+
+	/* IMPORTANT: The values for the below entries must start *after*
+	 * the highest value of the above entries!!!
+	 */
+
+	/* The vendor_id field is valid for matching */
+	USBI_HOTPLUG_VENDOR_ID_VALID = (1U << 3),
+
+	/* The product_id field is valid for matching */
+	USBI_HOTPLUG_PRODUCT_ID_VALID = (1U << 4),
+
+	/* The dev_class field is valid for matching */
+	USBI_HOTPLUG_DEV_CLASS_VALID = (1U << 5),
+
+	/* This callback has been unregistered and needs to be freed */
+	USBI_HOTPLUG_NEEDS_FREE = (1U << 6),
+};
+
+struct usbi_hotplug_callback {
+	/* Flags that control how this callback behaves */
+	uint8_t flags;
+
+	/* Vendor ID to match (if flags says this is valid) */
+	uint16_t vendor_id;
+
+	/* Product ID to match (if flags says this is valid) */
+	uint16_t product_id;
+
+	/* Device class to match (if flags says this is valid) */
+	uint8_t dev_class;
+
+	/* Callback function to invoke for matching event/device */
+	libusb_hotplug_callback_fn cb;
+
+	/* Handle for this callback (used to match on deregister) */
+	libusb_hotplug_callback_handle handle;
+
+	/* User data that will be passed to the callback function */
+	void *user_data;
+
+	/* List this callback is registered in (ctx->hotplug_cbs) */
+	struct list_head list;
+};
+
+struct usbi_hotplug_message {
+	/* The hotplug event that occurred */
+	libusb_hotplug_event event;
+
+	/* The device for which this hotplug event occurred */
+	struct libusb_device *device;
+
+	/* List this message is contained in (ctx->hotplug_msgs) */
+	struct list_head list;
+};
+
+/* shared data and functions */
+
+void usbi_hotplug_init(struct libusb_context *ctx);
+void usbi_hotplug_exit(struct libusb_context *ctx);
+void usbi_hotplug_notification(struct libusb_context *ctx, struct libusb_device *dev,
+	libusb_hotplug_event event);
+void usbi_hotplug_process(struct libusb_context *ctx, struct list_head *hotplug_msgs);
+
+int usbi_io_init(struct libusb_context *ctx);
+void usbi_io_exit(struct libusb_context *ctx);
+
+struct libusb_device *usbi_alloc_device(struct libusb_context *ctx,
+	unsigned long session_id);
+struct libusb_device *usbi_get_device_by_session_id(struct libusb_context *ctx,
+	unsigned long session_id);
+int usbi_sanitize_device(struct libusb_device *dev);
+void usbi_handle_disconnect(struct libusb_device_handle *dev_handle);
+
+int usbi_handle_transfer_completion(struct usbi_transfer *itransfer,
+	enum libusb_transfer_status status);
+int usbi_handle_transfer_cancellation(struct usbi_transfer *itransfer);
+void usbi_signal_transfer_completion(struct usbi_transfer *itransfer);
+
+void usbi_connect_device(struct libusb_device *dev);
+void usbi_disconnect_device(struct libusb_device *dev);
+
+struct usbi_event_source {
+	struct usbi_event_source_data {
+		usbi_os_handle_t os_handle;
+		short poll_events;
+	} data;
+	struct list_head list;
+};
+
+int usbi_add_event_source(struct libusb_context *ctx, usbi_os_handle_t os_handle,
+	short poll_events);
+void usbi_remove_event_source(struct libusb_context *ctx, usbi_os_handle_t os_handle);
+
+struct usbi_option {
+  int is_set;
+  union {
+    int ival;
+  } arg;
+};
+
+/* OS event abstraction */
+
+int usbi_create_event(usbi_event_t *event);
+void usbi_destroy_event(usbi_event_t *event);
+void usbi_signal_event(usbi_event_t *event);
+void usbi_clear_event(usbi_event_t *event);
+
+#ifdef HAVE_OS_TIMER
+int usbi_create_timer(usbi_timer_t *timer);
+void usbi_destroy_timer(usbi_timer_t *timer);
+int usbi_arm_timer(usbi_timer_t *timer, const struct timespec *timeout);
+int usbi_disarm_timer(usbi_timer_t *timer);
+#endif
+
+static inline int usbi_using_timer(struct libusb_context *ctx)
+{
+#ifdef HAVE_OS_TIMER
+	return usbi_timer_valid(&ctx->timer);
+#else
+	UNUSED(ctx);
+	return 0;
+#endif
+}
+
+struct usbi_reported_events {
+	union {
+		struct {
+			unsigned int event_triggered:1;
+#ifdef HAVE_OS_TIMER
+			unsigned int timer_triggered:1;
+#endif
+		};
+		unsigned int event_bits;
+	};
+	void *event_data;
+	unsigned int event_data_count;
+	unsigned int num_ready;
+};
+
+int usbi_alloc_event_data(struct libusb_context *ctx);
+int usbi_wait_for_events(struct libusb_context *ctx,
+	struct usbi_reported_events *reported_events, int timeout_ms);
+
+/* accessor functions for structure private data */
+
+static inline void *usbi_get_context_priv(struct libusb_context *ctx)
+{
+	return (unsigned char *)ctx + PTR_ALIGN(sizeof(*ctx));
+}
+
+static inline void *usbi_get_device_priv(struct libusb_device *dev)
+{
+	return (unsigned char *)dev + PTR_ALIGN(sizeof(*dev));
+}
+
+static inline void *usbi_get_device_handle_priv(struct libusb_device_handle *dev_handle)
+{
+	return (unsigned char *)dev_handle + PTR_ALIGN(sizeof(*dev_handle));
+}
+
+static inline void *usbi_get_transfer_priv(struct usbi_transfer *itransfer)
+{
+	return itransfer->priv;
+}
+
+/* device discovery */
+
+/* we traverse usbfs without knowing how many devices we are going to find.
+ * so we create this discovered_devs model which is similar to a linked-list
+ * which grows when required. it can be freed once discovery has completed,
+ * eliminating the need for a list node in the libusb_device structure
+ * itself. */
+struct discovered_devs {
+	size_t len;
+	size_t capacity;
+	struct libusb_device *devices[ZERO_SIZED_ARRAY];
+};
+
+struct discovered_devs *discovered_devs_append(
+	struct discovered_devs *discdevs, struct libusb_device *dev);
+
+/* OS abstraction */
+
+/* This is the interface that OS backends need to implement.
+ * All fields are mandatory, except ones explicitly noted as optional. */
+struct usbi_os_backend {
+	/* A human-readable name for your backend, e.g. "Linux usbfs" */
+	const char *name;
+
+	/* Binary mask for backend specific capabilities */
+	uint32_t caps;
+
+	/* Perform initialization of your backend. You might use this function
+	 * to determine specific capabilities of the system, allocate required
+	 * data structures for later, etc.
+	 *
+	 * This function is called when a libusb user initializes the library
+	 * prior to use. Mutual exclusion with other init and exit calls is
+	 * guaranteed when this function is called.
+	 *
+	 * Return 0 on success, or a LIBUSB_ERROR code on failure.
+	 */
+	int (*init)(struct libusb_context *ctx);
+
+	/* Deinitialization. Optional. This function should destroy anything
+	 * that was set up by init.
+	 *
+	 * This function is called when the user deinitializes the library.
+	 * Mutual exclusion with other init and exit calls is guaranteed when
+	 * this function is called.
+	 */
+	void (*exit)(struct libusb_context *ctx);
+
+	/* Set a backend-specific option. Optional.
+	 *
+	 * This function is called when the user calls libusb_set_option() and
+	 * the option is not handled by the core library.
+	 *
+	 * Return 0 on success, or a LIBUSB_ERROR code on failure.
+	 */
+	int (*set_option)(struct libusb_context *ctx, enum libusb_option option,
+		va_list args);
+
+	/* Enumerate all the USB devices on the system, returning them in a list
+	 * of discovered devices.
+	 *
+	 * Your implementation should enumerate all devices on the system,
+	 * regardless of whether they have been seen before or not.
+	 *
+	 * When you have found a device, compute a session ID for it. The session
+	 * ID should uniquely represent that particular device for that particular
+	 * connection session since boot (i.e. if you disconnect and reconnect a
+	 * device immediately after, it should be assigned a different session ID).
+	 * If your OS cannot provide a unique session ID as described above,
+	 * presenting a session ID of (bus_number << 8 | device_address) should
+	 * be sufficient. Bus numbers and device addresses wrap and get reused,
+	 * but that is an unlikely case.
+	 *
+	 * After computing a session ID for a device, call
+	 * usbi_get_device_by_session_id(). This function checks if libusb already
+	 * knows about the device, and if so, it provides you with a reference
+	 * to a libusb_device structure for it.
+	 *
+	 * If usbi_get_device_by_session_id() returns NULL, it is time to allocate
+	 * a new device structure for the device. Call usbi_alloc_device() to
+	 * obtain a new libusb_device structure with reference count 1. Populate
+	 * the bus_number and device_address attributes of the new device, and
+	 * perform any other internal backend initialization you need to do. At
+	 * this point, you should be ready to provide device descriptors and so
+	 * on through the get_*_descriptor functions. Finally, call
+	 * usbi_sanitize_device() to perform some final sanity checks on the
+	 * device. Assuming all of the above succeeded, we can now continue.
+	 * If any of the above failed, remember to unreference the device that
+	 * was returned by usbi_alloc_device().
+	 *
+	 * At this stage we have a populated libusb_device structure (either one
+	 * that was found earlier, or one that we have just allocated and
+	 * populated). This can now be added to the discovered devices list
+	 * using discovered_devs_append(). Note that discovered_devs_append()
+	 * may reallocate the list, returning a new location for it, and also
+	 * note that reallocation can fail. Your backend should handle these
+	 * error conditions appropriately.
+	 *
+	 * This function should not generate any bus I/O and should not block.
+	 * If I/O is required (e.g. reading the active configuration value), it is
+	 * OK to ignore these suggestions :)
+	 *
+	 * This function is executed when the user wishes to retrieve a list
+	 * of USB devices connected to the system.
+	 *
+	 * If the backend has hotplug support, this function is not used!
+	 *
+	 * Return 0 on success, or a LIBUSB_ERROR code on failure.
+	 */
+	int (*get_device_list)(struct libusb_context *ctx,
+		struct discovered_devs **discdevs);
+
+	/* Apps which were written before hotplug support, may listen for
+	 * hotplug events on their own and call libusb_get_device_list on
+	 * device addition. In this case libusb_get_device_list will likely
+	 * return a list without the new device in there, as the hotplug
+	 * event thread will still be busy enumerating the device, which may
+	 * take a while, or may not even have seen the event yet.
+	 *
+	 * To avoid this libusb_get_device_list will call this optional
+	 * function for backends with hotplug support before copying
+	 * ctx->usb_devs to the user. In this function the backend should
+	 * ensure any pending hotplug events are fully processed before
+	 * returning.
+	 *
+	 * Optional, should be implemented by backends with hotplug support.
+	 */
+	void (*hotplug_poll)(void);
+
+	/* Wrap a platform-specific device handle for I/O and other USB
+	 * operations. The device handle is preallocated for you.
+	 *
+	 * Your backend should allocate any internal resources required for I/O
+	 * and other operations so that those operations can happen (hopefully)
+	 * without hiccup. This is also a good place to inform libusb that it
+	 * should monitor certain file descriptors related to this device -
+	 * see the usbi_add_event_source() function.
+	 *
+	 * Your backend should also initialize the device structure
+	 * (dev_handle->dev), which is NULL at the beginning of the call.
+	 *
+	 * This function should not generate any bus I/O and should not block.
+	 *
+	 * This function is called when the user attempts to wrap an existing
+	 * platform-specific device handle for a device.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_ACCESS if the user has insufficient permissions
+	 * - another LIBUSB_ERROR code on other failure
+	 *
+	 * Do not worry about freeing the handle on failed open, the upper layers
+	 * do this for you.
+	 */
+	int (*wrap_sys_device)(struct libusb_context *ctx,
+		struct libusb_device_handle *dev_handle, intptr_t sys_dev);
+
+	/* Open a device for I/O and other USB operations. The device handle
+	 * is preallocated for you, you can retrieve the device in question
+	 * through handle->dev.
+	 *
+	 * Your backend should allocate any internal resources required for I/O
+	 * and other operations so that those operations can happen (hopefully)
+	 * without hiccup. This is also a good place to inform libusb that it
+	 * should monitor certain file descriptors related to this device -
+	 * see the usbi_add_event_source() function.
+	 *
+	 * This function should not generate any bus I/O and should not block.
+	 *
+	 * This function is called when the user attempts to obtain a device
+	 * handle for a device.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_ACCESS if the user has insufficient permissions
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since
+	 *   discovery
+	 * - another LIBUSB_ERROR code on other failure
+	 *
+	 * Do not worry about freeing the handle on failed open, the upper layers
+	 * do this for you.
+	 */
+	int (*open)(struct libusb_device_handle *dev_handle);
+
+	/* Close a device such that the handle cannot be used again. Your backend
+	 * should destroy any resources that were allocated in the open path.
+	 * This may also be a good place to call usbi_remove_event_source() to
+	 * inform libusb of any event sources associated with this device that
+	 * should no longer be monitored.
+	 *
+	 * This function is called when the user closes a device handle.
+	 */
+	void (*close)(struct libusb_device_handle *dev_handle);
+
+	/* Get the ACTIVE configuration descriptor for a device.
+	 *
+	 * The descriptor should be retrieved from memory, NOT via bus I/O to the
+	 * device. This means that you may have to cache it in a private structure
+	 * during get_device_list enumeration. You may also have to keep track
+	 * of which configuration is active when the user changes it.
+	 *
+	 * This function is expected to write len bytes of data into buffer, which
+	 * is guaranteed to be big enough. If you can only do a partial write,
+	 * return an error code.
+	 *
+	 * This function is expected to return the descriptor in bus-endian format
+	 * (LE).
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if the device is in unconfigured state
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*get_active_config_descriptor)(struct libusb_device *device,
+		void *buffer, size_t len);
+
+	/* Get a specific configuration descriptor for a device.
+	 *
+	 * The descriptor should be retrieved from memory, NOT via bus I/O to the
+	 * device. This means that you may have to cache it in a private structure
+	 * during get_device_list enumeration.
+	 *
+	 * The requested descriptor is expressed as a zero-based index (i.e. 0
+	 * indicates that we are requesting the first descriptor). The index does
+	 * not (necessarily) equal the bConfigurationValue of the configuration
+	 * being requested.
+	 *
+	 * This function is expected to write len bytes of data into buffer, which
+	 * is guaranteed to be big enough. If you can only do a partial write,
+	 * return an error code.
+	 *
+	 * This function is expected to return the descriptor in bus-endian format
+	 * (LE).
+	 *
+	 * Return the length read on success or a LIBUSB_ERROR code on failure.
+	 */
+	int (*get_config_descriptor)(struct libusb_device *device,
+		uint8_t config_index, void *buffer, size_t len);
+
+	/* Like get_config_descriptor but then by bConfigurationValue instead
+	 * of by index.
+	 *
+	 * Optional, if not present the core will call get_config_descriptor
+	 * for all configs until it finds the desired bConfigurationValue.
+	 *
+	 * Returns a pointer to the raw-descriptor in *buffer, this memory
+	 * is valid as long as device is valid.
+	 *
+	 * Returns the length of the returned raw-descriptor on success,
+	 * or a LIBUSB_ERROR code on failure.
+	 */
+	int (*get_config_descriptor_by_value)(struct libusb_device *device,
+		uint8_t bConfigurationValue, void **buffer);
+
+	/* Get the bConfigurationValue for the active configuration for a device.
+	 * Optional. This should only be implemented if you can retrieve it from
+	 * cache (don't generate I/O).
+	 *
+	 * If you cannot retrieve this from cache, either do not implement this
+	 * function, or return LIBUSB_ERROR_NOT_SUPPORTED. This will cause
+	 * libusb to retrieve the information through a standard control transfer.
+	 *
+	 * This function must be non-blocking.
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - LIBUSB_ERROR_NOT_SUPPORTED if the value cannot be retrieved without
+	 *   blocking
+	 * - another LIBUSB_ERROR code on other failure.
+	 */
+	int (*get_configuration)(struct libusb_device_handle *dev_handle, uint8_t *config);
+
+	/* Set the active configuration for a device.
+	 *
+	 * A configuration value of -1 should put the device in unconfigured state.
+	 *
+	 * This function can block.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
+	 * - LIBUSB_ERROR_BUSY if interfaces are currently claimed (and hence
+	 *   configuration cannot be changed)
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - another LIBUSB_ERROR code on other failure.
+	 */
+	int (*set_configuration)(struct libusb_device_handle *dev_handle, int config);
+
+	/* Claim an interface. When claimed, the application can then perform
+	 * I/O to an interface's endpoints.
+	 *
+	 * This function should not generate any bus I/O and should not block.
+	 * Interface claiming is a logical operation that simply ensures that
+	 * no other drivers/applications are using the interface, and after
+	 * claiming, no other drivers/applications can use the interface because
+	 * we now "own" it.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if the interface does not exist
+	 * - LIBUSB_ERROR_BUSY if the interface is in use by another driver/app
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*claim_interface)(struct libusb_device_handle *dev_handle, uint8_t interface_number);
+
+	/* Release a previously claimed interface.
+	 *
+	 * This function should also generate a SET_INTERFACE control request,
+	 * resetting the alternate setting of that interface to 0. It's OK for
+	 * this function to block as a result.
+	 *
+	 * You will only ever be asked to release an interface which was
+	 * successfully claimed earlier.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*release_interface)(struct libusb_device_handle *dev_handle, uint8_t interface_number);
+
+	/* Set the alternate setting for an interface.
+	 *
+	 * You will only ever be asked to set the alternate setting for an
+	 * interface which was successfully claimed earlier.
+	 *
+	 * It's OK for this function to block.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if the alternate setting does not exist
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*set_interface_altsetting)(struct libusb_device_handle *dev_handle,
+		uint8_t interface_number, uint8_t altsetting);
+
+	/* Clear a halt/stall condition on an endpoint.
+	 *
+	 * It's OK for this function to block.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*clear_halt)(struct libusb_device_handle *dev_handle,
+		unsigned char endpoint);
+
+	/* Perform a USB port reset to reinitialize a device. Optional.
+	 *
+	 * If possible, the device handle should still be usable after the reset
+	 * completes, assuming that the device descriptors did not change during
+	 * reset and all previous interface state can be restored.
+	 *
+	 * If something changes, or you cannot easily locate/verify the reset
+	 * device, return LIBUSB_ERROR_NOT_FOUND. This prompts the application
+	 * to close the old handle and re-enumerate the device.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if re-enumeration is required, or if the device
+	 *   has been disconnected since it was opened
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*reset_device)(struct libusb_device_handle *dev_handle);
+
+	/* Alloc num_streams usb3 bulk streams on the passed in endpoints */
+	int (*alloc_streams)(struct libusb_device_handle *dev_handle,
+		uint32_t num_streams, unsigned char *endpoints, int num_endpoints);
+
+	/* Free usb3 bulk streams allocated with alloc_streams */
+	int (*free_streams)(struct libusb_device_handle *dev_handle,
+		unsigned char *endpoints, int num_endpoints);
+
+	/* Allocate persistent DMA memory for the given device, suitable for
+	 * zerocopy. May return NULL on failure. Optional to implement.
+	 */
+	void *(*dev_mem_alloc)(struct libusb_device_handle *handle, size_t len);
+
+	/* Free memory allocated by dev_mem_alloc. */
+	int (*dev_mem_free)(struct libusb_device_handle *handle, void *buffer,
+		size_t len);
+
+	/* Determine if a kernel driver is active on an interface. Optional.
+	 *
+	 * The presence of a kernel driver on an interface indicates that any
+	 * calls to claim_interface would fail with the LIBUSB_ERROR_BUSY code.
+	 *
+	 * Return:
+	 * - 0 if no driver is active
+	 * - 1 if a driver is active
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*kernel_driver_active)(struct libusb_device_handle *dev_handle,
+		uint8_t interface_number);
+
+	/* Detach a kernel driver from an interface. Optional.
+	 *
+	 * After detaching a kernel driver, the interface should be available
+	 * for claim.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
+	 * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*detach_kernel_driver)(struct libusb_device_handle *dev_handle,
+		uint8_t interface_number);
+
+	/* Attach a kernel driver to an interface. Optional.
+	 *
+	 * Reattach a kernel driver to the device.
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
+	 * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - LIBUSB_ERROR_BUSY if a program or driver has claimed the interface,
+	 *   preventing reattachment
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*attach_kernel_driver)(struct libusb_device_handle *dev_handle,
+		uint8_t interface_number);
+
+	/* Destroy a device. Optional.
+	 *
+	 * This function is called when the last reference to a device is
+	 * destroyed. It should free any resources allocated in the get_device_list
+	 * path.
+	 */
+	void (*destroy_device)(struct libusb_device *dev);
+
+	/* Submit a transfer. Your implementation should take the transfer,
+	 * morph it into whatever form your platform requires, and submit it
+	 * asynchronously.
+	 *
+	 * This function must not block.
+	 *
+	 * This function gets called with the flying_transfers_lock locked!
+	 *
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+	 * - another LIBUSB_ERROR code on other failure
+	 */
+	int (*submit_transfer)(struct usbi_transfer *itransfer);
+
+	/* Cancel a previously submitted transfer.
+	 *
+	 * This function must not block. The transfer cancellation must complete
+	 * later, resulting in a call to usbi_handle_transfer_cancellation()
+	 * from the context of handle_events.
+	 */
+	int (*cancel_transfer)(struct usbi_transfer *itransfer);
+
+	/* Clear a transfer as if it has completed or cancelled, but do not
+	 * report any completion/cancellation to the library. You should free
+	 * all private data from the transfer as if you were just about to report
+	 * completion or cancellation.
+	 *
+	 * This function might seem a bit out of place. It is used when libusb
+	 * detects a disconnected device - it calls this function for all pending
+	 * transfers before reporting completion (with the disconnect code) to
+	 * the user. Maybe we can improve upon this internal interface in future.
+	 */
+	void (*clear_transfer_priv)(struct usbi_transfer *itransfer);
+
+	/* Handle any pending events on event sources. Optional.
+	 *
+	 * Provide this function when event sources directly indicate device
+	 * or transfer activity. If your backend does not have such event sources,
+	 * implement the handle_transfer_completion function below.
+	 *
+	 * This involves monitoring any active transfers and processing their
+	 * completion or cancellation.
+	 *
+	 * The function is passed a pointer that represents platform-specific
+	 * data for monitoring event sources (size count). This data is to be
+	 * (re)allocated as necessary when event sources are modified.
+	 * The num_ready parameter indicates the number of event sources that
+	 * have reported events. This should be enough information for you to
+	 * determine which actions need to be taken on the currently active
+	 * transfers.
+	 *
+	 * For any cancelled transfers, call usbi_handle_transfer_cancellation().
+	 * For completed transfers, call usbi_handle_transfer_completion().
+	 * For control/bulk/interrupt transfers, populate the "transferred"
+	 * element of the appropriate usbi_transfer structure before calling the
+	 * above functions. For isochronous transfers, populate the status and
+	 * transferred fields of the iso packet descriptors of the transfer.
+	 *
+	 * This function should also be able to detect disconnection of the
+	 * device, reporting that situation with usbi_handle_disconnect().
+	 *
+	 * When processing an event related to a transfer, you probably want to
+	 * take usbi_transfer.lock to prevent races. See the documentation for
+	 * the usbi_transfer structure.
+	 *
+	 * Return 0 on success, or a LIBUSB_ERROR code on failure.
+	 */
+	int (*handle_events)(struct libusb_context *ctx,
+		void *event_data, unsigned int count, unsigned int num_ready);
+
+	/* Handle transfer completion. Optional.
+	 *
+	 * Provide this function when there are no event sources available that
+	 * directly indicate device or transfer activity. If your backend does
+	 * have such event sources, implement the handle_events function above.
+	 *
+	 * Your backend must tell the library when a transfer has completed by
+	 * calling usbi_signal_transfer_completion(). You should store any private
+	 * information about the transfer and its completion status in the transfer's
+	 * private backend data.
+	 *
+	 * During event handling, this function will be called on each transfer for
+	 * which usbi_signal_transfer_completion() was called.
+	 *
+	 * For any cancelled transfers, call usbi_handle_transfer_cancellation().
+	 * For completed transfers, call usbi_handle_transfer_completion().
+	 * For control/bulk/interrupt transfers, populate the "transferred"
+	 * element of the appropriate usbi_transfer structure before calling the
+	 * above functions. For isochronous transfers, populate the status and
+	 * transferred fields of the iso packet descriptors of the transfer.
+	 *
+	 * Return 0 on success, or a LIBUSB_ERROR code on failure.
+	 */
+	int (*handle_transfer_completion)(struct usbi_transfer *itransfer);
+
+	/* Number of bytes to reserve for per-context private backend data.
+	 * This private data area is accessible by calling
+	 * usbi_get_context_priv() on the libusb_context instance.
+	 */
+	size_t context_priv_size;
+
+	/* Number of bytes to reserve for per-device private backend data.
+	 * This private data area is accessible by calling
+	 * usbi_get_device_priv() on the libusb_device instance.
+	 */
+	size_t device_priv_size;
+
+	/* Number of bytes to reserve for per-handle private backend data.
+	 * This private data area is accessible by calling
+	 * usbi_get_device_handle_priv() on the libusb_device_handle instance.
+	 */
+	size_t device_handle_priv_size;
+
+	/* Number of bytes to reserve for per-transfer private backend data.
+	 * This private data area is accessible by calling
+	 * usbi_get_transfer_priv() on the usbi_transfer instance.
+	 */
+	size_t transfer_priv_size;
+};
+
+extern const struct usbi_os_backend usbi_backend;
+
+#define for_each_context(c) \
+	for_each_helper(c, &active_contexts_list, struct libusb_context)
+
+#define for_each_device(ctx, d) \
+	for_each_helper(d, &(ctx)->usb_devs, struct libusb_device)
+
+#define for_each_device_safe(ctx, d, n) \
+	for_each_safe_helper(d, n, &(ctx)->usb_devs, struct libusb_device)
+
+#define for_each_open_device(ctx, h) \
+	for_each_helper(h, &(ctx)->open_devs, struct libusb_device_handle)
+
+#define __for_each_transfer(list, t) \
+	for_each_helper(t, (list), struct usbi_transfer)
+
+#define for_each_transfer(ctx, t) \
+	__for_each_transfer(&(ctx)->flying_transfers, t)
+
+#define __for_each_transfer_safe(list, t, n) \
+	for_each_safe_helper(t, n, (list), struct usbi_transfer)
+
+#define for_each_transfer_safe(ctx, t, n) \
+	__for_each_transfer_safe(&(ctx)->flying_transfers, t, n)
+
+#define __for_each_completed_transfer_safe(list, t, n) \
+	list_for_each_entry_safe(t, n, (list), completed_list, struct usbi_transfer)
+
+#define for_each_event_source(ctx, e) \
+	for_each_helper(e, &(ctx)->event_sources, struct usbi_event_source)
+
+#define for_each_removed_event_source(ctx, e) \
+	for_each_helper(e, &(ctx)->removed_event_sources, struct usbi_event_source)
+
+#define for_each_removed_event_source_safe(ctx, e, n) \
+	for_each_safe_helper(e, n, &(ctx)->removed_event_sources, struct usbi_event_source)
+
+#define for_each_hotplug_cb(ctx, c) \
+	for_each_helper(c, &(ctx)->hotplug_cbs, struct usbi_hotplug_callback)
+
+#define for_each_hotplug_cb_safe(ctx, c, n) \
+	for_each_safe_helper(c, n, &(ctx)->hotplug_cbs, struct usbi_hotplug_callback)
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/hardware/src/libusb/os/darwin_usb.c b/hardware/src/libusb/os/darwin_usb.c
new file mode 100644
index 0000000..903422c
--- /dev/null
+++ b/hardware/src/libusb/os/darwin_usb.c
@@ -0,0 +1,2611 @@
+/* -*- Mode: C; indent-tabs-mode:nil -*- */
+/*
+ * darwin backend for libusb 1.0
+ * Copyright © 2008-2021 Nathan Hjelm <hjelmn@cs.unm.edu>
+ * Copyright © 2019-2021 Google LLC. All rights reserved.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <config.h>
+#include <assert.h>
+#include <time.h>
+#include <ctype.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/sysctl.h>
+
+#include <mach/clock.h>
+#include <mach/clock_types.h>
+#include <mach/mach_host.h>
+#include <mach/mach_port.h>
+
+/* Suppress warnings about the use of the deprecated objc_registerThreadWithCollector
+ * function. Its use is also conditionalized to only older deployment targets. */
+#define OBJC_SILENCE_GC_DEPRECATIONS 1
+
+/* Default timeout to 10s for reenumerate. This is needed because USBDeviceReEnumerate
+ * does not return error status on macOS. */
+#define DARWIN_REENUMERATE_TIMEOUT_US 10000000
+
+#include <AvailabilityMacros.h>
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 1060 && MAC_OS_X_VERSION_MIN_REQUIRED < 101200
+  #include <objc/objc-auto.h>
+#endif
+
+#include "darwin_usb.h"
+
+static int init_count = 0;
+
+/* Both kIOMasterPortDefault or kIOMainPortDefault are synonyms for 0. */
+static const mach_port_t darwin_default_master_port = 0;
+
+/* async event thread */
+static pthread_mutex_t libusb_darwin_at_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t  libusb_darwin_at_cond = PTHREAD_COND_INITIALIZER;
+
+#if !defined(HAVE_CLOCK_GETTIME)
+static clock_serv_t clock_realtime;
+static clock_serv_t clock_monotonic;
+#endif
+
+#define LIBUSB_DARWIN_STARTUP_FAILURE ((CFRunLoopRef) -1)
+
+static CFRunLoopRef libusb_darwin_acfl = NULL; /* event cf loop */
+static CFRunLoopSourceRef libusb_darwin_acfls = NULL; /* shutdown signal for event cf loop */
+
+static usbi_mutex_t darwin_cached_devices_lock = PTHREAD_MUTEX_INITIALIZER;
+static struct list_head darwin_cached_devices;
+static const char *darwin_device_class = "IOUSBDevice";
+
+#define DARWIN_CACHED_DEVICE(a) (((struct darwin_device_priv *)usbi_get_device_priv((a)))->dev)
+
+/* async event thread */
+static pthread_t libusb_darwin_at;
+
+static int darwin_get_config_descriptor(struct libusb_device *dev, uint8_t config_index, void *buffer, size_t len);
+static int darwin_claim_interface(struct libusb_device_handle *dev_handle, uint8_t iface);
+static int darwin_release_interface(struct libusb_device_handle *dev_handle, uint8_t iface);
+static int darwin_reenumerate_device(struct libusb_device_handle *dev_handle, bool capture);
+static int darwin_clear_halt(struct libusb_device_handle *dev_handle, unsigned char endpoint);
+static int darwin_reset_device(struct libusb_device_handle *dev_handle);
+static void darwin_async_io_callback (void *refcon, IOReturn result, void *arg0);
+
+static enum libusb_error darwin_scan_devices(struct libusb_context *ctx);
+static enum libusb_error process_new_device (struct libusb_context *ctx, struct darwin_cached_device *cached_device,
+                                             UInt64 old_session_id);
+
+static enum libusb_error darwin_get_cached_device(struct libusb_context *ctx, io_service_t service, struct darwin_cached_device **cached_out,
+                                                  UInt64 *old_session_id);
+
+#if defined(ENABLE_LOGGING)
+static const char *darwin_error_str (IOReturn result) {
+  static char string_buffer[50];
+  switch (result) {
+  case kIOReturnSuccess:
+    return "no error";
+  case kIOReturnNotOpen:
+    return "device not opened for exclusive access";
+  case kIOReturnNoDevice:
+    return "no connection to an IOService";
+  case kIOUSBNoAsyncPortErr:
+    return "no async port has been opened for interface";
+  case kIOReturnExclusiveAccess:
+    return "another process has device opened for exclusive access";
+  case kIOUSBPipeStalled:
+#if defined(kUSBHostReturnPipeStalled)
+  case kUSBHostReturnPipeStalled:
+#endif
+    return "pipe is stalled";
+  case kIOReturnError:
+    return "could not establish a connection to the Darwin kernel";
+  case kIOUSBTransactionTimeout:
+    return "transaction timed out";
+  case kIOReturnBadArgument:
+    return "invalid argument";
+  case kIOReturnAborted:
+    return "transaction aborted";
+  case kIOReturnNotResponding:
+    return "device not responding";
+  case kIOReturnOverrun:
+    return "data overrun";
+  case kIOReturnCannotWire:
+    return "physical memory can not be wired down";
+  case kIOReturnNoResources:
+    return "out of resources";
+  case kIOUSBHighSpeedSplitError:
+    return "high speed split error";
+  case kIOUSBUnknownPipeErr:
+    return "pipe ref not recognized";
+  default:
+    snprintf(string_buffer, sizeof(string_buffer), "unknown error (0x%x)", result);
+    return string_buffer;
+  }
+}
+#endif
+
+static enum libusb_error darwin_to_libusb (IOReturn result) {
+  switch (result) {
+  case kIOReturnUnderrun:
+  case kIOReturnSuccess:
+    return LIBUSB_SUCCESS;
+  case kIOReturnNotOpen:
+  case kIOReturnNoDevice:
+    return LIBUSB_ERROR_NO_DEVICE;
+  case kIOReturnExclusiveAccess:
+    return LIBUSB_ERROR_ACCESS;
+  case kIOUSBPipeStalled:
+#if defined(kUSBHostReturnPipeStalled)
+  case kUSBHostReturnPipeStalled:
+#endif
+    return LIBUSB_ERROR_PIPE;
+  case kIOReturnBadArgument:
+    return LIBUSB_ERROR_INVALID_PARAM;
+  case kIOUSBTransactionTimeout:
+    return LIBUSB_ERROR_TIMEOUT;
+  case kIOUSBUnknownPipeErr:
+    return LIBUSB_ERROR_NOT_FOUND;
+  case kIOReturnNotResponding:
+  case kIOReturnAborted:
+  case kIOReturnError:
+  case kIOUSBNoAsyncPortErr:
+  default:
+    return LIBUSB_ERROR_OTHER;
+  }
+}
+
+/* this function must be called with the darwin_cached_devices_lock held */
+static void darwin_deref_cached_device(struct darwin_cached_device *cached_dev) {
+  cached_dev->refcount--;
+  /* free the device and remove it from the cache */
+  if (0 == cached_dev->refcount) {
+    list_del(&cached_dev->list);
+
+    if (cached_dev->device) {
+      (*(cached_dev->device))->Release(cached_dev->device);
+      cached_dev->device = NULL;
+    }
+    IOObjectRelease (cached_dev->service);
+    free (cached_dev);
+  }
+}
+
+static void darwin_ref_cached_device(struct darwin_cached_device *cached_dev) {
+  cached_dev->refcount++;
+}
+
+static int ep_to_pipeRef(struct libusb_device_handle *dev_handle, uint8_t ep, uint8_t *pipep, uint8_t *ifcp, struct darwin_interface **interface_out) {
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+
+  /* current interface */
+  struct darwin_interface *cInterface;
+
+  uint8_t i, iface;
+
+  struct libusb_context *ctx = HANDLE_CTX(dev_handle);
+
+  usbi_dbg (ctx, "converting ep address 0x%02x to pipeRef and interface", ep);
+
+  for (iface = 0 ; iface < USB_MAXINTERFACES ; iface++) {
+    cInterface = &priv->interfaces[iface];
+
+    if (dev_handle->claimed_interfaces & (1U << iface)) {
+      for (i = 0 ; i < cInterface->num_endpoints ; i++) {
+        if (cInterface->endpoint_addrs[i] == ep) {
+          *pipep = i + 1;
+
+          if (ifcp)
+            *ifcp = iface;
+
+          if (interface_out)
+            *interface_out = cInterface;
+
+          usbi_dbg (ctx, "pipe %d on interface %d matches", *pipep, iface);
+          return LIBUSB_SUCCESS;
+        }
+      }
+    }
+  }
+
+  /* No pipe found with the correct endpoint address */
+  usbi_warn (HANDLE_CTX(dev_handle), "no pipeRef found with endpoint address 0x%02x.", ep);
+
+  return LIBUSB_ERROR_NOT_FOUND;
+}
+
+static IOReturn usb_setup_device_iterator (io_iterator_t *deviceIterator, UInt32 location) {
+  CFMutableDictionaryRef matchingDict = IOServiceMatching(darwin_device_class);
+
+  if (!matchingDict)
+    return kIOReturnError;
+
+  if (location) {
+    CFMutableDictionaryRef propertyMatchDict = CFDictionaryCreateMutable(kCFAllocatorDefault, 0,
+                                                                         &kCFTypeDictionaryKeyCallBacks,
+                                                                         &kCFTypeDictionaryValueCallBacks);
+
+    /* there are no unsigned CFNumber types so treat the value as signed. the OS seems to do this
+         internally (CFNumberType of locationID is kCFNumberSInt32Type) */
+    CFTypeRef locationCF = CFNumberCreate (NULL, kCFNumberSInt32Type, &location);
+
+    if (propertyMatchDict && locationCF) {
+      CFDictionarySetValue (propertyMatchDict, CFSTR(kUSBDevicePropertyLocationID), locationCF);
+      CFDictionarySetValue (matchingDict, CFSTR(kIOPropertyMatchKey), propertyMatchDict);
+    }
+    /* else we can still proceed as long as the caller accounts for the possibility of other devices in the iterator */
+
+    /* release our references as per the Create Rule */
+    if (propertyMatchDict)
+      CFRelease (propertyMatchDict);
+    if (locationCF)
+      CFRelease (locationCF);
+  }
+
+  return IOServiceGetMatchingServices(darwin_default_master_port, matchingDict, deviceIterator);
+}
+
+/* Returns 1 on success, 0 on failure. */
+static bool get_ioregistry_value_number (io_service_t service, CFStringRef property, CFNumberType type, void *p) {
+  CFTypeRef cfNumber = IORegistryEntryCreateCFProperty (service, property, kCFAllocatorDefault, 0);
+  Boolean success = 0;
+
+  if (cfNumber) {
+    if (CFGetTypeID(cfNumber) == CFNumberGetTypeID()) {
+      success = CFNumberGetValue(cfNumber, type, p);
+    }
+
+    CFRelease (cfNumber);
+  }
+
+  return (success != 0);
+}
+
+/* Returns 1 on success, 0 on failure. */
+static bool get_ioregistry_value_data (io_service_t service, CFStringRef property, ssize_t size, void *p) {
+  CFTypeRef cfData = IORegistryEntryCreateCFProperty (service, property, kCFAllocatorDefault, 0);
+  bool success = false;
+
+  if (cfData) {
+    if (CFGetTypeID (cfData) == CFDataGetTypeID ()) {
+      CFIndex length = CFDataGetLength (cfData);
+      if (length < size) {
+        size = length;
+      }
+
+      CFDataGetBytes (cfData, CFRangeMake(0, size), p);
+      success = true;
+    }
+
+    CFRelease (cfData);
+  }
+
+  return success;
+}
+
+static usb_device_t **darwin_device_from_service (struct libusb_context *ctx, io_service_t service)
+{
+  io_cf_plugin_ref_t *plugInInterface = NULL;
+  usb_device_t **device;
+  IOReturn kresult;
+  SInt32 score;
+  const int max_retries = 5;
+
+  /* The IOCreatePlugInInterfaceForService function might consistently return
+     an "out of resources" error with certain USB devices the first time we run 
+     it. The reason is still unclear, but retrying fixes the problem */
+  for (int count = 0; count < max_retries; count++) {
+    kresult = IOCreatePlugInInterfaceForService(service, kIOUSBDeviceUserClientTypeID,
+                                                kIOCFPlugInInterfaceID, &plugInInterface,
+                                                &score);
+    if (kIOReturnSuccess == kresult && plugInInterface) {
+      break;
+    }
+
+    usbi_dbg (ctx, "set up plugin for service retry: %s", darwin_error_str (kresult));
+
+    /* sleep for a little while before trying again */
+    nanosleep(&(struct timespec){.tv_sec = 0, .tv_nsec = 1000}, NULL);
+  }
+
+  if (kIOReturnSuccess != kresult || !plugInInterface) {
+    usbi_dbg (ctx, "could not set up plugin for service: %s", darwin_error_str (kresult));
+    return NULL;
+  }
+
+  (void)(*plugInInterface)->QueryInterface(plugInInterface, CFUUIDGetUUIDBytes(DeviceInterfaceID),
+                                           (LPVOID)&device);
+  /* Use release instead of IODestroyPlugInInterface to avoid stopping IOServices associated with this device */
+  (*plugInInterface)->Release (plugInInterface);
+
+  return device;
+}
+
+static void darwin_devices_attached (void *ptr, io_iterator_t add_devices) {
+  UNUSED(ptr);
+  struct darwin_cached_device *cached_device;
+  UInt64 old_session_id;
+  struct libusb_context *ctx;
+  io_service_t service;
+  int ret;
+
+  usbi_mutex_lock(&active_contexts_lock);
+
+  while ((service = IOIteratorNext(add_devices))) {
+    ret = darwin_get_cached_device (NULL, service, &cached_device, &old_session_id);
+    if (ret < 0 || !cached_device->can_enumerate) {
+      continue;
+    }
+
+    /* add this device to each active context's device list */
+    for_each_context(ctx) {
+      process_new_device (ctx, cached_device, old_session_id);
+    }
+
+    if (cached_device->in_reenumerate) {
+      usbi_dbg (NULL, "cached device in reset state. reset complete...");
+      cached_device->in_reenumerate = false;
+    }
+
+    IOObjectRelease(service);
+  }
+
+  usbi_mutex_unlock(&active_contexts_lock);
+}
+
+static void darwin_devices_detached (void *ptr, io_iterator_t rem_devices) {
+  UNUSED(ptr);
+  struct libusb_device *dev = NULL;
+  struct libusb_context *ctx;
+  struct darwin_cached_device *old_device;
+
+  io_service_t device;
+  UInt64 session, locationID;
+  int ret;
+
+  usbi_mutex_lock(&active_contexts_lock);
+
+  while ((device = IOIteratorNext (rem_devices)) != 0) {
+    bool is_reenumerating = false;
+
+    /* get the location from the i/o registry */
+    ret = get_ioregistry_value_number (device, CFSTR("sessionID"), kCFNumberSInt64Type, &session);
+    (void) get_ioregistry_value_number (device, CFSTR("locationID"), kCFNumberSInt32Type, &locationID);
+    IOObjectRelease (device);
+    if (!ret)
+      continue;
+
+    /* we need to match darwin_ref_cached_device call made in darwin_get_cached_device function
+       otherwise no cached device will ever get freed */
+    usbi_mutex_lock(&darwin_cached_devices_lock);
+    list_for_each_entry(old_device, &darwin_cached_devices, list, struct darwin_cached_device) {
+      if (old_device->session == session) {
+        if (old_device->in_reenumerate) {
+          /* device is re-enumerating. do not dereference the device at this time. libusb_reset_device()
+           * will deref if needed. */
+          usbi_dbg (NULL, "detected device detached due to re-enumeration. sessionID: 0x%" PRIx64 ", locationID: 0x%" PRIx64,
+                    session, locationID);
+
+          /* the device object is no longer usable so go ahead and release it */
+          if (old_device->device) {
+            (*(old_device->device))->Release(old_device->device);
+            old_device->device = NULL;
+          }
+
+          is_reenumerating = true;
+        } else {
+          darwin_deref_cached_device (old_device);
+        }
+
+        break;
+      }
+    }
+
+    usbi_mutex_unlock(&darwin_cached_devices_lock);
+    if (is_reenumerating) {
+      continue;
+    }
+
+    for_each_context(ctx) {
+      usbi_dbg (ctx, "notifying context %p of device disconnect", ctx);
+
+      dev = usbi_get_device_by_session_id(ctx, (unsigned long) session);
+      if (dev) {
+        /* signal the core that this device has been disconnected. the core will tear down this device
+           when the reference count reaches 0 */
+        usbi_disconnect_device(dev);
+        libusb_unref_device(dev);
+      }
+    }
+  }
+
+  usbi_mutex_unlock(&active_contexts_lock);
+}
+
+static void darwin_hotplug_poll (void)
+{
+  /* not sure if 1 ms will be too long/short but it should work ok */
+  mach_timespec_t timeout = {.tv_sec = 0, .tv_nsec = 1000000ul};
+
+  /* since a kernel thread may notify the IOIterators used for
+   * hotplug notification we can't just clear the iterators.
+   * instead just wait until all IOService providers are quiet */
+  (void) IOKitWaitQuiet (darwin_default_master_port, &timeout);
+}
+
+static void darwin_clear_iterator (io_iterator_t iter) {
+  io_service_t device;
+
+  while ((device = IOIteratorNext (iter)) != 0)
+    IOObjectRelease (device);
+}
+
+static void darwin_fail_startup(void) {
+  pthread_mutex_lock (&libusb_darwin_at_mutex);
+  libusb_darwin_acfl = LIBUSB_DARWIN_STARTUP_FAILURE;
+  pthread_cond_signal (&libusb_darwin_at_cond);
+  pthread_mutex_unlock (&libusb_darwin_at_mutex);
+  pthread_exit (NULL);
+}
+
+static void *darwin_event_thread_main (void *arg0) {
+  IOReturn kresult;
+  struct libusb_context *ctx = (struct libusb_context *)arg0;
+  CFRunLoopRef runloop;
+  CFRunLoopSourceRef libusb_shutdown_cfsource;
+  CFRunLoopSourceContext libusb_shutdown_cfsourcectx;
+
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
+  /* Set this thread's name, so it can be seen in the debugger
+     and crash reports. */
+  pthread_setname_np ("org.libusb.device-hotplug");
+#endif
+
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 1060 && MAC_OS_X_VERSION_MIN_REQUIRED < 101200
+  /* Tell the Objective-C garbage collector about this thread.
+     This is required because, unlike NSThreads, pthreads are
+     not automatically registered. Although we don't use
+     Objective-C, we use CoreFoundation, which does.
+     Garbage collection support was entirely removed in 10.12,
+     so don't bother there. */
+  objc_registerThreadWithCollector();
+#endif
+
+  /* hotplug (device arrival/removal) sources */
+  CFRunLoopSourceRef     libusb_notification_cfsource;
+  io_notification_port_t libusb_notification_port;
+  io_iterator_t          libusb_rem_device_iterator;
+  io_iterator_t          libusb_add_device_iterator;
+
+  usbi_dbg (ctx, "creating hotplug event source");
+
+  runloop = CFRunLoopGetCurrent ();
+  CFRetain (runloop);
+
+  /* add the shutdown cfsource to the run loop */
+  memset(&libusb_shutdown_cfsourcectx, 0, sizeof(libusb_shutdown_cfsourcectx));
+  libusb_shutdown_cfsourcectx.info = runloop;
+  libusb_shutdown_cfsourcectx.perform = (void (*)(void *))CFRunLoopStop;
+  libusb_shutdown_cfsource = CFRunLoopSourceCreate(NULL, 0, &libusb_shutdown_cfsourcectx);
+  CFRunLoopAddSource(runloop, libusb_shutdown_cfsource, kCFRunLoopDefaultMode);
+
+  /* add the notification port to the run loop */
+  libusb_notification_port     = IONotificationPortCreate (darwin_default_master_port);
+  libusb_notification_cfsource = IONotificationPortGetRunLoopSource (libusb_notification_port);
+  CFRunLoopAddSource(runloop, libusb_notification_cfsource, kCFRunLoopDefaultMode);
+
+  /* create notifications for removed devices */
+  kresult = IOServiceAddMatchingNotification (libusb_notification_port, kIOTerminatedNotification,
+                                              IOServiceMatching(darwin_device_class),
+                                              darwin_devices_detached,
+                                              ctx, &libusb_rem_device_iterator);
+
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (ctx, "could not add hotplug event source: %s", darwin_error_str (kresult));
+    CFRelease (libusb_shutdown_cfsource);
+    CFRelease (runloop);
+    darwin_fail_startup ();
+  }
+
+  /* create notifications for attached devices */
+  kresult = IOServiceAddMatchingNotification(libusb_notification_port, kIOFirstMatchNotification,
+                                              IOServiceMatching(darwin_device_class),
+                                              darwin_devices_attached,
+                                              ctx, &libusb_add_device_iterator);
+
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (ctx, "could not add hotplug event source: %s", darwin_error_str (kresult));
+    CFRelease (libusb_shutdown_cfsource);
+    CFRelease (runloop);
+    darwin_fail_startup ();
+  }
+
+  /* arm notifiers */
+  darwin_clear_iterator (libusb_rem_device_iterator);
+  darwin_clear_iterator (libusb_add_device_iterator);
+
+  usbi_dbg (ctx, "darwin event thread ready to receive events");
+
+  /* signal the main thread that the hotplug runloop has been created. */
+  pthread_mutex_lock (&libusb_darwin_at_mutex);
+  libusb_darwin_acfl = runloop;
+  libusb_darwin_acfls = libusb_shutdown_cfsource;
+  pthread_cond_signal (&libusb_darwin_at_cond);
+  pthread_mutex_unlock (&libusb_darwin_at_mutex);
+
+  /* run the runloop */
+  CFRunLoopRun();
+
+  usbi_dbg (ctx, "darwin event thread exiting");
+
+  /* signal the main thread that the hotplug runloop has finished. */
+  pthread_mutex_lock (&libusb_darwin_at_mutex);
+  libusb_darwin_acfls = NULL;
+  libusb_darwin_acfl = NULL;
+  pthread_cond_signal (&libusb_darwin_at_cond);
+  pthread_mutex_unlock (&libusb_darwin_at_mutex);
+
+  /* remove the notification cfsource */
+  CFRunLoopRemoveSource(runloop, libusb_notification_cfsource, kCFRunLoopDefaultMode);
+
+  /* remove the shutdown cfsource */
+  CFRunLoopRemoveSource(runloop, libusb_shutdown_cfsource, kCFRunLoopDefaultMode);
+
+  /* delete notification port */
+  IONotificationPortDestroy (libusb_notification_port);
+
+  /* delete iterators */
+  IOObjectRelease (libusb_rem_device_iterator);
+  IOObjectRelease (libusb_add_device_iterator);
+
+  CFRelease (libusb_shutdown_cfsource);
+  CFRelease (runloop);
+
+  pthread_exit (NULL);
+}
+
+/* cleanup function to destroy cached devices */
+static void darwin_cleanup_devices(void) {
+  struct darwin_cached_device *dev, *next;
+
+  list_for_each_entry_safe(dev, next, &darwin_cached_devices, list, struct darwin_cached_device) {
+    darwin_deref_cached_device(dev);
+  }
+}
+
+static int darwin_init(struct libusb_context *ctx) {
+  bool first_init;
+  int rc;
+
+  first_init = (1 == ++init_count);
+
+  do {
+    if (first_init) {
+      if (NULL == darwin_cached_devices.next) {
+        list_init (&darwin_cached_devices);
+      }
+      assert(list_empty(&darwin_cached_devices));
+#if !defined(HAVE_CLOCK_GETTIME)
+      /* create the clocks that will be used if clock_gettime() is not available */
+      host_name_port_t host_self;
+
+      host_self = mach_host_self();
+      host_get_clock_service(host_self, CALENDAR_CLOCK, &clock_realtime);
+      host_get_clock_service(host_self, SYSTEM_CLOCK, &clock_monotonic);
+      mach_port_deallocate(mach_task_self(), host_self);
+#endif
+    }
+
+    rc = darwin_scan_devices (ctx);
+    if (LIBUSB_SUCCESS != rc)
+      break;
+
+    if (first_init) {
+      rc = pthread_create (&libusb_darwin_at, NULL, darwin_event_thread_main, ctx);
+      if (0 != rc) {
+        usbi_err (ctx, "could not create event thread, error %d", rc);
+        rc = LIBUSB_ERROR_OTHER;
+        break;
+      }
+
+      pthread_mutex_lock (&libusb_darwin_at_mutex);
+      while (!libusb_darwin_acfl)
+        pthread_cond_wait (&libusb_darwin_at_cond, &libusb_darwin_at_mutex);
+      if (libusb_darwin_acfl == LIBUSB_DARWIN_STARTUP_FAILURE) {
+        libusb_darwin_acfl = NULL;
+        rc = LIBUSB_ERROR_OTHER;
+      }
+      pthread_mutex_unlock (&libusb_darwin_at_mutex);
+
+      if (0 != rc)
+        pthread_join (libusb_darwin_at, NULL);
+    }
+  } while (0);
+
+  if (LIBUSB_SUCCESS != rc) {
+    if (first_init) {
+      darwin_cleanup_devices ();
+#if !defined(HAVE_CLOCK_GETTIME)
+      mach_port_deallocate(mach_task_self(), clock_realtime);
+      mach_port_deallocate(mach_task_self(), clock_monotonic);
+#endif
+    }
+    --init_count;
+  }
+
+  return rc;
+}
+
+static void darwin_exit (struct libusb_context *ctx) {
+  UNUSED(ctx);
+
+  if (0 == --init_count) {
+    /* stop the event runloop and wait for the thread to terminate. */
+    pthread_mutex_lock (&libusb_darwin_at_mutex);
+    CFRunLoopSourceSignal (libusb_darwin_acfls);
+    CFRunLoopWakeUp (libusb_darwin_acfl);
+    while (libusb_darwin_acfl)
+      pthread_cond_wait (&libusb_darwin_at_cond, &libusb_darwin_at_mutex);
+    pthread_mutex_unlock (&libusb_darwin_at_mutex);
+    pthread_join (libusb_darwin_at, NULL);
+
+    darwin_cleanup_devices ();
+
+#if !defined(HAVE_CLOCK_GETTIME)
+    mach_port_deallocate(mach_task_self(), clock_realtime);
+    mach_port_deallocate(mach_task_self(), clock_monotonic);
+#endif
+  }
+}
+
+static int get_configuration_index (struct libusb_device *dev, UInt8 config_value) {
+  struct darwin_cached_device *priv = DARWIN_CACHED_DEVICE(dev);
+  UInt8 i, numConfig;
+  IOUSBConfigurationDescriptorPtr desc;
+  IOReturn kresult;
+
+  /* is there a simpler way to determine the index? */
+  kresult = (*(priv->device))->GetNumberOfConfigurations (priv->device, &numConfig);
+  if (kresult != kIOReturnSuccess)
+    return darwin_to_libusb (kresult);
+
+  for (i = 0 ; i < numConfig ; i++) {
+    (*(priv->device))->GetConfigurationDescriptorPtr (priv->device, i, &desc);
+
+    if (desc->bConfigurationValue == config_value)
+      return i;
+  }
+
+  /* configuration not found */
+  return LIBUSB_ERROR_NOT_FOUND;
+}
+
+static int darwin_get_active_config_descriptor(struct libusb_device *dev, void *buffer, size_t len) {
+  struct darwin_cached_device *priv = DARWIN_CACHED_DEVICE(dev);
+  int config_index;
+
+  if (0 == priv->active_config)
+    return LIBUSB_ERROR_NOT_FOUND;
+
+  config_index = get_configuration_index (dev, priv->active_config);
+  if (config_index < 0)
+    return config_index;
+
+  assert(config_index >= 0 && config_index <= UINT8_MAX);
+  return darwin_get_config_descriptor (dev, (UInt8)config_index, buffer, len);
+}
+
+static int darwin_get_config_descriptor(struct libusb_device *dev, uint8_t config_index, void *buffer, size_t len) {
+  struct darwin_cached_device *priv = DARWIN_CACHED_DEVICE(dev);
+  IOUSBConfigurationDescriptorPtr desc;
+  IOReturn kresult;
+  int ret;
+
+  if (!priv || !priv->device)
+    return LIBUSB_ERROR_OTHER;
+
+  kresult = (*priv->device)->GetConfigurationDescriptorPtr (priv->device, config_index, &desc);
+  if (kresult == kIOReturnSuccess) {
+    /* copy descriptor */
+    if (libusb_le16_to_cpu(desc->wTotalLength) < len)
+      len = libusb_le16_to_cpu(desc->wTotalLength);
+
+    memmove (buffer, desc, len);
+  }
+
+  ret = darwin_to_libusb (kresult);
+  if (ret != LIBUSB_SUCCESS)
+    return ret;
+
+  return (int) len;
+}
+
+/* check whether the os has configured the device */
+static enum libusb_error darwin_check_configuration (struct libusb_context *ctx, struct darwin_cached_device *dev) {
+  usb_device_t **darwin_device = dev->device;
+
+  IOUSBConfigurationDescriptorPtr configDesc;
+  IOUSBFindInterfaceRequest request;
+  IOReturn                  kresult;
+  io_iterator_t             interface_iterator;
+  io_service_t              firstInterface;
+
+  if (dev->dev_descriptor.bNumConfigurations < 1) {
+    usbi_err (ctx, "device has no configurations");
+    return LIBUSB_ERROR_OTHER; /* no configurations at this speed so we can't use it */
+  }
+
+  /* checking the configuration of a root hub simulation takes ~1 s in 10.11. the device is
+     not usable anyway */
+  if (0x05ac == libusb_le16_to_cpu (dev->dev_descriptor.idVendor) &&
+      0x8005 == libusb_le16_to_cpu (dev->dev_descriptor.idProduct)) {
+    usbi_dbg (ctx, "ignoring configuration on root hub simulation");
+    dev->active_config = 0;
+    return LIBUSB_SUCCESS;
+  }
+
+  /* find the first configuration */
+  kresult = (*darwin_device)->GetConfigurationDescriptorPtr (darwin_device, 0, &configDesc);
+  dev->first_config = (kIOReturnSuccess == kresult) ? configDesc->bConfigurationValue : 1;
+
+  /* check if the device is already configured. there is probably a better way than iterating over the
+     to accomplish this (the trick is we need to avoid a call to GetConfigurations since buggy devices
+     might lock up on the device request) */
+
+  /* Setup the Interface Request */
+  request.bInterfaceClass    = kIOUSBFindInterfaceDontCare;
+  request.bInterfaceSubClass = kIOUSBFindInterfaceDontCare;
+  request.bInterfaceProtocol = kIOUSBFindInterfaceDontCare;
+  request.bAlternateSetting  = kIOUSBFindInterfaceDontCare;
+
+  kresult = (*(darwin_device))->CreateInterfaceIterator(darwin_device, &request, &interface_iterator);
+  if (kresult != kIOReturnSuccess)
+    return darwin_to_libusb (kresult);
+
+  /* iterate once */
+  firstInterface = IOIteratorNext(interface_iterator);
+
+  /* done with the interface iterator */
+  IOObjectRelease(interface_iterator);
+
+  if (firstInterface) {
+    IOObjectRelease (firstInterface);
+
+    /* device is configured */
+    if (dev->dev_descriptor.bNumConfigurations == 1)
+      /* to avoid problems with some devices get the configurations value from the configuration descriptor */
+      dev->active_config = dev->first_config;
+    else
+      /* devices with more than one configuration should work with GetConfiguration */
+      (*darwin_device)->GetConfiguration (darwin_device, &dev->active_config);
+  } else
+    /* not configured */
+    dev->active_config = 0;
+
+  usbi_dbg (ctx, "active config: %u, first config: %u", dev->active_config, dev->first_config);
+
+  return LIBUSB_SUCCESS;
+}
+
+static IOReturn darwin_request_descriptor (usb_device_t **device, UInt8 desc, UInt8 desc_index, void *buffer, size_t buffer_size) {
+  IOUSBDevRequestTO req;
+
+  assert(buffer_size <= UINT16_MAX);
+
+  memset (buffer, 0, buffer_size);
+
+  /* Set up request for descriptor/ */
+  req.bmRequestType = USBmakebmRequestType(kUSBIn, kUSBStandard, kUSBDevice);
+  req.bRequest      = kUSBRqGetDescriptor;
+  req.wValue        = (UInt16)(desc << 8);
+  req.wIndex        = desc_index;
+  req.wLength       = (UInt16)buffer_size;
+  req.pData         = buffer;
+  req.noDataTimeout = 20;
+  req.completionTimeout = 100;
+
+  return (*device)->DeviceRequestTO (device, &req);
+}
+
+static enum libusb_error darwin_cache_device_descriptor (struct libusb_context *ctx, struct darwin_cached_device *dev) {
+  usb_device_t **device = dev->device;
+  int retries = 1;
+  long delay = 30000; // microseconds
+  int unsuspended = 0, try_unsuspend = 1, try_reconfigure = 1;
+  int is_open = 0;
+  IOReturn ret = 0, ret2;
+  UInt8 bDeviceClass;
+  UInt16 idProduct, idVendor;
+
+  dev->can_enumerate = 0;
+
+  (*device)->GetDeviceClass (device, &bDeviceClass);
+  (*device)->GetDeviceProduct (device, &idProduct);
+  (*device)->GetDeviceVendor (device, &idVendor);
+
+  /* According to Apple's documentation the device must be open for DeviceRequest but we may not be able to open some
+   * devices and Apple's USB Prober doesn't bother to open the device before issuing a descriptor request.  Still,
+   * to follow the spec as closely as possible, try opening the device */
+  is_open = ((*device)->USBDeviceOpenSeize(device) == kIOReturnSuccess);
+
+  do {
+    /**** retrieve device descriptor ****/
+    ret = darwin_request_descriptor (device, kUSBDeviceDesc, 0, &dev->dev_descriptor, sizeof(dev->dev_descriptor));
+
+    if (kIOReturnOverrun == ret && kUSBDeviceDesc == dev->dev_descriptor.bDescriptorType)
+      /* received an overrun error but we still received a device descriptor */
+      ret = kIOReturnSuccess;
+
+    if (kIOUSBVendorIDAppleComputer == idVendor) {
+      /* NTH: don't bother retrying or unsuspending Apple devices */
+      break;
+    }
+
+    if (kIOReturnSuccess == ret && (0 == dev->dev_descriptor.bNumConfigurations ||
+                                    0 == dev->dev_descriptor.bcdUSB)) {
+      /* work around for incorrectly configured devices */
+      if (try_reconfigure && is_open) {
+        usbi_dbg(ctx, "descriptor appears to be invalid. resetting configuration before trying again...");
+
+        /* set the first configuration */
+        (*device)->SetConfiguration(device, 1);
+
+        /* don't try to reconfigure again */
+        try_reconfigure = 0;
+      }
+
+      ret = kIOUSBPipeStalled;
+    }
+
+    if (kIOReturnSuccess != ret && is_open && try_unsuspend) {
+      /* device may be suspended. unsuspend it and try again */
+#if DeviceVersion >= 320
+      UInt32 info = 0;
+
+      /* IOUSBFamily 320+ provides a way to detect device suspension but earlier versions do not */
+      (void)(*device)->GetUSBDeviceInformation (device, &info);
+
+      /* note that the device was suspended */
+      if (info & (1U << kUSBInformationDeviceIsSuspendedBit) || 0 == info)
+        try_unsuspend = 1;
+#endif
+
+      if (try_unsuspend) {
+        /* try to unsuspend the device */
+        ret2 = (*device)->USBDeviceSuspend (device, 0);
+        if (kIOReturnSuccess != ret2) {
+          /* prevent log spew from poorly behaving devices.  this indicates the
+             os actually had trouble communicating with the device */
+          usbi_dbg(ctx, "could not retrieve device descriptor. failed to unsuspend: %s",darwin_error_str(ret2));
+        } else
+          unsuspended = 1;
+
+        try_unsuspend = 0;
+      }
+    }
+
+    if (kIOReturnSuccess != ret) {
+      usbi_dbg(ctx, "kernel responded with code: 0x%08x. sleeping for %ld ms before trying again", ret, delay/1000);
+      /* sleep for a little while before trying again */
+      nanosleep(&(struct timespec){delay / 1000000, (delay * 1000) % 1000000000}, NULL);
+    }
+  } while (kIOReturnSuccess != ret && retries--);
+
+  if (unsuspended)
+    /* resuspend the device */
+    (void)(*device)->USBDeviceSuspend (device, 1);
+
+  if (is_open)
+    (void) (*device)->USBDeviceClose (device);
+
+  if (ret != kIOReturnSuccess) {
+    /* a debug message was already printed out for this error */
+    if (LIBUSB_CLASS_HUB == bDeviceClass)
+      usbi_dbg (ctx, "could not retrieve device descriptor %.4x:%.4x: %s (%x). skipping device",
+                idVendor, idProduct, darwin_error_str (ret), ret);
+    else
+      usbi_warn (ctx, "could not retrieve device descriptor %.4x:%.4x: %s (%x). skipping device",
+                 idVendor, idProduct, darwin_error_str (ret), ret);
+    return darwin_to_libusb (ret);
+  }
+
+  /* catch buggy hubs (which appear to be virtual). Apple's own USB prober has problems with these devices. */
+  if (libusb_le16_to_cpu (dev->dev_descriptor.idProduct) != idProduct) {
+    /* not a valid device */
+    usbi_warn (NULL, "idProduct from iokit (%04x) does not match idProduct in descriptor (%04x). skipping device",
+               idProduct, libusb_le16_to_cpu (dev->dev_descriptor.idProduct));
+    return LIBUSB_ERROR_NO_DEVICE;
+  }
+
+  usbi_dbg (ctx, "cached device descriptor:");
+  usbi_dbg (ctx, "  bDescriptorType:    0x%02x", dev->dev_descriptor.bDescriptorType);
+  usbi_dbg (ctx, "  bcdUSB:             0x%04x", libusb_le16_to_cpu (dev->dev_descriptor.bcdUSB));
+  usbi_dbg (ctx, "  bDeviceClass:       0x%02x", dev->dev_descriptor.bDeviceClass);
+  usbi_dbg (ctx, "  bDeviceSubClass:    0x%02x", dev->dev_descriptor.bDeviceSubClass);
+  usbi_dbg (ctx, "  bDeviceProtocol:    0x%02x", dev->dev_descriptor.bDeviceProtocol);
+  usbi_dbg (ctx, "  bMaxPacketSize0:    0x%02x", dev->dev_descriptor.bMaxPacketSize0);
+  usbi_dbg (ctx, "  idVendor:           0x%04x", libusb_le16_to_cpu (dev->dev_descriptor.idVendor));
+  usbi_dbg (ctx, "  idProduct:          0x%04x", libusb_le16_to_cpu (dev->dev_descriptor.idProduct));
+  usbi_dbg (ctx, "  bcdDevice:          0x%04x", libusb_le16_to_cpu (dev->dev_descriptor.bcdDevice));
+  usbi_dbg (ctx, "  iManufacturer:      0x%02x", dev->dev_descriptor.iManufacturer);
+  usbi_dbg (ctx, "  iProduct:           0x%02x", dev->dev_descriptor.iProduct);
+  usbi_dbg (ctx, "  iSerialNumber:      0x%02x", dev->dev_descriptor.iSerialNumber);
+  usbi_dbg (ctx, "  bNumConfigurations: 0x%02x", dev->dev_descriptor.bNumConfigurations);
+
+  dev->can_enumerate = 1;
+
+  return LIBUSB_SUCCESS;
+}
+
+/* Returns 1 on success, 0 on failure. */
+static bool get_device_port (io_service_t service, UInt8 *port) {
+  IOReturn kresult;
+  io_service_t parent;
+  bool ret = false;
+
+  if (get_ioregistry_value_number (service, CFSTR("PortNum"), kCFNumberSInt8Type, port)) {
+    return true;
+  }
+
+  kresult = IORegistryEntryGetParentEntry (service, kIOServicePlane, &parent);
+  if (kIOReturnSuccess == kresult) {
+    ret = get_ioregistry_value_data (parent, CFSTR("port"), 1, port);
+    IOObjectRelease (parent);
+  }
+
+  return ret;
+}
+
+/* Returns 1 on success, 0 on failure. */
+static bool get_device_parent_sessionID(io_service_t service, UInt64 *parent_sessionID) {
+  IOReturn kresult;
+  io_service_t parent;
+
+  /* Walk up the tree in the IOService plane until we find a parent that has a sessionID */
+  parent = service;
+  while((kresult = IORegistryEntryGetParentEntry (parent, kIOUSBPlane, &parent)) == kIOReturnSuccess) {
+    if (get_ioregistry_value_number (parent, CFSTR("sessionID"), kCFNumberSInt64Type, parent_sessionID)) {
+        /* Success */
+        return true;
+    }
+  }
+
+  /* We ran out of parents */
+  return false;
+}
+
+static enum libusb_error darwin_get_cached_device(struct libusb_context *ctx, io_service_t service, struct darwin_cached_device **cached_out,
+                                                  UInt64 *old_session_id) {
+  struct darwin_cached_device *new_device;
+  UInt64 sessionID = 0, parent_sessionID = 0;
+  UInt32 locationID = 0;
+  enum libusb_error ret = LIBUSB_SUCCESS;
+  usb_device_t **device;
+  UInt8 port = 0;
+
+  /* assuming sessionID != 0 normally (never seen it be 0) */
+  *old_session_id = 0;
+  *cached_out = NULL;
+
+  /* get some info from the io registry */
+  (void) get_ioregistry_value_number (service, CFSTR("sessionID"), kCFNumberSInt64Type, &sessionID);
+  (void) get_ioregistry_value_number (service, CFSTR("locationID"), kCFNumberSInt32Type, &locationID);
+  if (!get_device_port (service, &port)) {
+    usbi_dbg(ctx, "could not get connected port number");
+  }
+
+  usbi_dbg(ctx, "finding cached device for sessionID 0x%" PRIx64, sessionID);
+
+  if (get_device_parent_sessionID(service, &parent_sessionID)) {
+    usbi_dbg(ctx, "parent sessionID: 0x%" PRIx64, parent_sessionID);
+  }
+
+  usbi_mutex_lock(&darwin_cached_devices_lock);
+  do {
+    list_for_each_entry(new_device, &darwin_cached_devices, list, struct darwin_cached_device) {
+      usbi_dbg(ctx, "matching sessionID/locationID 0x%" PRIx64 "/0x%x against cached device with sessionID/locationID 0x%" PRIx64 "/0x%x",
+               sessionID, locationID, new_device->session, new_device->location);
+      if (new_device->location == locationID && new_device->in_reenumerate) {
+        usbi_dbg (ctx, "found cached device with matching location that is being re-enumerated");
+        *old_session_id = new_device->session;
+        break;
+      }
+
+      if (new_device->session == sessionID) {
+        usbi_dbg(ctx, "using cached device for device");
+        *cached_out = new_device;
+        break;
+      }
+    }
+
+    if (*cached_out)
+      break;
+
+    usbi_dbg(ctx, "caching new device with sessionID 0x%" PRIx64, sessionID);
+
+    device = darwin_device_from_service (ctx, service);
+    if (!device) {
+      ret = LIBUSB_ERROR_NO_DEVICE;
+      break;
+    }
+
+    if (!(*old_session_id)) {
+      new_device = calloc (1, sizeof (*new_device));
+      if (!new_device) {
+        ret = LIBUSB_ERROR_NO_MEM;
+        break;
+      }
+
+      /* add this device to the cached device list */
+      list_add(&new_device->list, &darwin_cached_devices);
+
+      (*device)->GetDeviceAddress (device, (USBDeviceAddress *)&new_device->address);
+
+      /* keep a reference to this device */
+      darwin_ref_cached_device(new_device);
+
+      (*device)->GetLocationID (device, &new_device->location);
+      new_device->port = port;
+      new_device->parent_session = parent_sessionID;
+    } else {
+      /* release the ref to old device's service */
+      IOObjectRelease (new_device->service);
+    }
+
+    /* keep track of devices regardless of if we successfully enumerate them to
+       prevent them from being enumerated multiple times */
+    *cached_out = new_device;
+
+    new_device->session = sessionID;
+    new_device->device = device;
+    new_device->service = service;
+
+    /* retain the service */
+    IOObjectRetain (service);
+
+    /* cache the device descriptor */
+    ret = darwin_cache_device_descriptor(ctx, new_device);
+    if (ret)
+      break;
+
+    if (new_device->can_enumerate) {
+      snprintf(new_device->sys_path, 20, "%03i-%04x-%04x-%02x-%02x", new_device->address,
+               libusb_le16_to_cpu (new_device->dev_descriptor.idVendor),
+               libusb_le16_to_cpu (new_device->dev_descriptor.idProduct),
+               new_device->dev_descriptor.bDeviceClass, new_device->dev_descriptor.bDeviceSubClass);
+    }
+  } while (0);
+
+  usbi_mutex_unlock(&darwin_cached_devices_lock);
+
+  return ret;
+}
+
+static enum libusb_error process_new_device (struct libusb_context *ctx, struct darwin_cached_device *cached_device,
+                                             UInt64 old_session_id) {
+  struct darwin_device_priv *priv;
+  struct libusb_device *dev = NULL;
+  UInt8 devSpeed;
+  enum libusb_error ret = LIBUSB_SUCCESS;
+
+  do {
+    /* check current active configuration (and cache the first configuration value--
+       which may be used by claim_interface) */
+    ret = darwin_check_configuration (ctx, cached_device);
+    if (ret)
+      break;
+
+    if (0 != old_session_id) {
+      usbi_dbg (ctx, "re-using existing device from context %p for with session 0x%" PRIx64 " new session 0x%" PRIx64,
+                ctx, old_session_id, cached_device->session);
+      /* save the libusb device before the session id is updated */
+      dev = usbi_get_device_by_session_id (ctx, (unsigned long) old_session_id);
+    }
+
+    if (!dev) {
+      usbi_dbg (ctx, "allocating new device in context %p for with session 0x%" PRIx64,
+                ctx, cached_device->session);
+
+      dev = usbi_alloc_device(ctx, (unsigned long) cached_device->session);
+      if (!dev) {
+        return LIBUSB_ERROR_NO_MEM;
+      }
+
+      priv = usbi_get_device_priv(dev);
+
+      priv->dev = cached_device;
+      darwin_ref_cached_device (priv->dev);
+      dev->port_number    = cached_device->port;
+      /* the location ID encodes the path to the device. the top byte of the location ID contains the bus number
+         (numbered from 0). the remaining bytes can be used to construct the device tree for that bus. */
+      dev->bus_number     = cached_device->location >> 24;
+      assert(cached_device->address <= UINT8_MAX);
+      dev->device_address = (uint8_t)cached_device->address;
+    } else {
+      priv = usbi_get_device_priv(dev);
+    }
+
+    static_assert(sizeof(dev->device_descriptor) == sizeof(cached_device->dev_descriptor),
+                  "mismatch between libusb and IOKit device descriptor sizes");
+    memcpy(&dev->device_descriptor, &cached_device->dev_descriptor, LIBUSB_DT_DEVICE_SIZE);
+    usbi_localize_device_descriptor(&dev->device_descriptor);
+    dev->session_data = cached_device->session;
+
+    if (NULL != dev->parent_dev) {
+      libusb_unref_device(dev->parent_dev);
+      dev->parent_dev = NULL;
+    }
+
+    if (cached_device->parent_session > 0) {
+      dev->parent_dev = usbi_get_device_by_session_id (ctx, (unsigned long) cached_device->parent_session);
+    }
+
+    (*(priv->dev->device))->GetDeviceSpeed (priv->dev->device, &devSpeed);
+
+    switch (devSpeed) {
+    case kUSBDeviceSpeedLow: dev->speed = LIBUSB_SPEED_LOW; break;
+    case kUSBDeviceSpeedFull: dev->speed = LIBUSB_SPEED_FULL; break;
+    case kUSBDeviceSpeedHigh: dev->speed = LIBUSB_SPEED_HIGH; break;
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 1080
+    case kUSBDeviceSpeedSuper: dev->speed = LIBUSB_SPEED_SUPER; break;
+#endif
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 101200
+    case kUSBDeviceSpeedSuperPlus: dev->speed = LIBUSB_SPEED_SUPER_PLUS; break;
+#endif
+    default:
+      usbi_warn (ctx, "Got unknown device speed %d", devSpeed);
+    }
+
+    ret = usbi_sanitize_device (dev);
+    if (ret < 0)
+      break;
+
+    usbi_dbg (ctx, "found device with address %d port = %d parent = %p at %p", dev->device_address,
+              dev->port_number, (void *) dev->parent_dev, priv->dev->sys_path);
+
+  } while (0);
+
+  if (!cached_device->in_reenumerate && 0 == ret) {
+    usbi_connect_device (dev);
+  } else {
+    libusb_unref_device (dev);
+  }
+
+  return ret;
+}
+
+static enum libusb_error darwin_scan_devices(struct libusb_context *ctx) {
+  struct darwin_cached_device *cached_device;
+  UInt64 old_session_id;
+  io_iterator_t deviceIterator;
+  io_service_t service;
+  IOReturn kresult;
+  int ret;
+
+  kresult = usb_setup_device_iterator (&deviceIterator, 0);
+  if (kresult != kIOReturnSuccess)
+    return darwin_to_libusb (kresult);
+
+  while ((service = IOIteratorNext (deviceIterator))) {
+    ret = darwin_get_cached_device (ctx, service, &cached_device, &old_session_id);
+    if (ret < 0 || !cached_device->can_enumerate) {
+      continue;
+    }
+
+    (void) process_new_device (ctx, cached_device, old_session_id);
+
+    IOObjectRelease(service);
+  }
+
+  IOObjectRelease(deviceIterator);
+
+  return LIBUSB_SUCCESS;
+}
+
+static int darwin_open (struct libusb_device_handle *dev_handle) {
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  IOReturn kresult;
+
+  if (0 == dpriv->open_count) {
+    /* try to open the device */
+    kresult = (*(dpriv->device))->USBDeviceOpenSeize (dpriv->device);
+    if (kresult != kIOReturnSuccess) {
+      usbi_warn (HANDLE_CTX (dev_handle), "USBDeviceOpen: %s", darwin_error_str(kresult));
+
+      if (kIOReturnExclusiveAccess != kresult) {
+        return darwin_to_libusb (kresult);
+      }
+
+      /* it is possible to perform some actions on a device that is not open so do not return an error */
+      priv->is_open = false;
+    } else {
+      priv->is_open = true;
+    }
+
+    /* create async event source */
+    kresult = (*(dpriv->device))->CreateDeviceAsyncEventSource (dpriv->device, &priv->cfSource);
+    if (kresult != kIOReturnSuccess) {
+      usbi_err (HANDLE_CTX (dev_handle), "CreateDeviceAsyncEventSource: %s", darwin_error_str(kresult));
+
+      if (priv->is_open) {
+        (*(dpriv->device))->USBDeviceClose (dpriv->device);
+      }
+
+      priv->is_open = false;
+
+      return darwin_to_libusb (kresult);
+    }
+
+    CFRetain (libusb_darwin_acfl);
+
+    /* add the cfSource to the async run loop */
+    CFRunLoopAddSource(libusb_darwin_acfl, priv->cfSource, kCFRunLoopCommonModes);
+  }
+
+  /* device opened successfully */
+  dpriv->open_count++;
+
+  usbi_dbg (HANDLE_CTX(dev_handle), "device open for access");
+
+  return 0;
+}
+
+static void darwin_close (struct libusb_device_handle *dev_handle) {
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  IOReturn kresult;
+  int i;
+
+  if (dpriv->open_count == 0) {
+    /* something is probably very wrong if this is the case */
+    usbi_err (HANDLE_CTX (dev_handle), "Close called on a device that was not open!");
+    return;
+  }
+
+  dpriv->open_count--;
+  if (NULL == dpriv->device) {
+    usbi_warn (HANDLE_CTX (dev_handle), "darwin_close device missing IOService");
+    return;
+  }
+
+  /* make sure all interfaces are released */
+  for (i = 0 ; i < USB_MAXINTERFACES ; i++)
+    if (dev_handle->claimed_interfaces & (1U << i))
+      libusb_release_interface (dev_handle, i);
+
+  if (0 == dpriv->open_count) {
+    /* delete the device's async event source */
+    if (priv->cfSource) {
+      CFRunLoopRemoveSource (libusb_darwin_acfl, priv->cfSource, kCFRunLoopDefaultMode);
+      CFRelease (priv->cfSource);
+      priv->cfSource = NULL;
+      CFRelease (libusb_darwin_acfl);
+    }
+
+    if (priv->is_open) {
+      /* close the device */
+      kresult = (*(dpriv->device))->USBDeviceClose(dpriv->device);
+      if (kresult != kIOReturnSuccess) {
+        /* Log the fact that we had a problem closing the file, however failing a
+         * close isn't really an error, so return success anyway */
+        usbi_warn (HANDLE_CTX (dev_handle), "USBDeviceClose: %s", darwin_error_str(kresult));
+      }
+    }
+  }
+}
+
+static int darwin_get_configuration(struct libusb_device_handle *dev_handle, uint8_t *config) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+
+  *config = dpriv->active_config;
+
+  return LIBUSB_SUCCESS;
+}
+
+static enum libusb_error darwin_set_configuration(struct libusb_device_handle *dev_handle, int config) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  IOReturn kresult;
+  uint8_t i;
+
+  if (config == -1)
+    config = 0;
+
+  /* Setting configuration will invalidate the interface, so we need
+     to reclaim it. First, dispose of existing interfaces, if any. */
+  for (i = 0 ; i < USB_MAXINTERFACES ; i++)
+    if (dev_handle->claimed_interfaces & (1U << i))
+      darwin_release_interface (dev_handle, i);
+
+  kresult = (*(dpriv->device))->SetConfiguration (dpriv->device, (UInt8)config);
+  if (kresult != kIOReturnSuccess)
+    return darwin_to_libusb (kresult);
+
+  /* Reclaim any interfaces. */
+  for (i = 0 ; i < USB_MAXINTERFACES ; i++)
+    if (dev_handle->claimed_interfaces & (1U << i))
+      darwin_claim_interface (dev_handle, i);
+
+  dpriv->active_config = (UInt8)config;
+
+  return LIBUSB_SUCCESS;
+}
+
+static IOReturn darwin_get_interface (usb_device_t **darwin_device, uint8_t ifc, io_service_t *usbInterfacep) {
+  IOUSBFindInterfaceRequest request;
+  IOReturn                  kresult;
+  io_iterator_t             interface_iterator;
+  UInt8                     bInterfaceNumber;
+  bool                      ret;
+
+  *usbInterfacep = IO_OBJECT_NULL;
+
+  /* Setup the Interface Request */
+  request.bInterfaceClass    = kIOUSBFindInterfaceDontCare;
+  request.bInterfaceSubClass = kIOUSBFindInterfaceDontCare;
+  request.bInterfaceProtocol = kIOUSBFindInterfaceDontCare;
+  request.bAlternateSetting  = kIOUSBFindInterfaceDontCare;
+
+  kresult = (*(darwin_device))->CreateInterfaceIterator(darwin_device, &request, &interface_iterator);
+  if (kresult != kIOReturnSuccess)
+    return kresult;
+
+  while ((*usbInterfacep = IOIteratorNext(interface_iterator))) {
+    /* find the interface number */
+    ret = get_ioregistry_value_number (*usbInterfacep, CFSTR("bInterfaceNumber"), kCFNumberSInt8Type,
+                                       &bInterfaceNumber);
+
+    if (ret && bInterfaceNumber == ifc) {
+      break;
+    }
+
+    (void) IOObjectRelease (*usbInterfacep);
+  }
+
+  /* done with the interface iterator */
+  IOObjectRelease(interface_iterator);
+
+  return kIOReturnSuccess;
+}
+
+static enum libusb_error get_endpoints (struct libusb_device_handle *dev_handle, uint8_t iface) {
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+
+  /* current interface */
+  struct darwin_interface *cInterface = &priv->interfaces[iface];
+
+  IOReturn kresult;
+
+  UInt8 numep, direction, number;
+  UInt8 dont_care1, dont_care3;
+  UInt16 dont_care2;
+  int rc;
+  struct libusb_context *ctx = HANDLE_CTX (dev_handle);
+
+
+  usbi_dbg (ctx, "building table of endpoints.");
+
+  /* retrieve the total number of endpoints on this interface */
+  kresult = (*(cInterface->interface))->GetNumEndpoints(cInterface->interface, &numep);
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (ctx, "can't get number of endpoints for interface: %s", darwin_error_str(kresult));
+    return darwin_to_libusb (kresult);
+  }
+
+  /* iterate through pipe references */
+  for (UInt8 i = 1 ; i <= numep ; i++) {
+    kresult = (*(cInterface->interface))->GetPipeProperties(cInterface->interface, i, &direction, &number, &dont_care1,
+                                                            &dont_care2, &dont_care3);
+
+    if (kresult != kIOReturnSuccess) {
+      /* probably a buggy device. try to get the endpoint address from the descriptors */
+      struct libusb_config_descriptor *config;
+      const struct libusb_endpoint_descriptor *endpoint_desc;
+      UInt8 alt_setting;
+
+      kresult = (*(cInterface->interface))->GetAlternateSetting (cInterface->interface, &alt_setting);
+      if (kresult != kIOReturnSuccess) {
+        usbi_err (HANDLE_CTX (dev_handle), "can't get alternate setting for interface");
+        return darwin_to_libusb (kresult);
+      }
+
+      rc = libusb_get_active_config_descriptor (dev_handle->dev, &config);
+      if (LIBUSB_SUCCESS != rc) {
+        return rc;
+      }
+
+      endpoint_desc = config->interface[iface].altsetting[alt_setting].endpoint + i - 1;
+
+      cInterface->endpoint_addrs[i - 1] = endpoint_desc->bEndpointAddress;
+    } else {
+      cInterface->endpoint_addrs[i - 1] = (UInt8)(((kUSBIn == direction) << kUSBRqDirnShift) | (number & LIBUSB_ENDPOINT_ADDRESS_MASK));
+    }
+
+    usbi_dbg (ctx, "interface: %i pipe %i: dir: %i number: %i", iface, i, cInterface->endpoint_addrs[i - 1] >> kUSBRqDirnShift,
+              cInterface->endpoint_addrs[i - 1] & LIBUSB_ENDPOINT_ADDRESS_MASK);
+  }
+
+  cInterface->num_endpoints = numep;
+
+  return LIBUSB_SUCCESS;
+}
+
+static int darwin_claim_interface(struct libusb_device_handle *dev_handle, uint8_t iface) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+  io_service_t          usbInterface = IO_OBJECT_NULL;
+  IOReturn              kresult;
+  enum libusb_error     ret;
+  IOCFPlugInInterface **plugInInterface = NULL;
+  SInt32                score;
+
+  /* current interface */
+  struct darwin_interface *cInterface = &priv->interfaces[iface];
+
+  struct libusb_context *ctx = HANDLE_CTX (dev_handle);
+
+  kresult = darwin_get_interface (dpriv->device, iface, &usbInterface);
+  if (kresult != kIOReturnSuccess)
+    return darwin_to_libusb (kresult);
+
+  /* make sure we have an interface */
+  if (!usbInterface && dpriv->first_config != 0) {
+    usbi_info (ctx, "no interface found; setting configuration: %d", dpriv->first_config);
+
+    /* set the configuration */
+    ret = darwin_set_configuration (dev_handle, (int) dpriv->first_config);
+    if (ret != LIBUSB_SUCCESS) {
+      usbi_err (ctx, "could not set configuration");
+      return ret;
+    }
+
+    kresult = darwin_get_interface (dpriv->device, iface, &usbInterface);
+    if (kresult != kIOReturnSuccess) {
+      usbi_err (ctx, "darwin_get_interface: %s", darwin_error_str(kresult));
+      return darwin_to_libusb (kresult);
+    }
+  }
+
+  if (!usbInterface) {
+    usbi_info (ctx, "interface not found");
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  /* get an interface to the device's interface */
+  kresult = IOCreatePlugInInterfaceForService (usbInterface, kIOUSBInterfaceUserClientTypeID,
+                                               kIOCFPlugInInterfaceID, &plugInInterface, &score);
+
+  /* ignore release error */
+  (void)IOObjectRelease (usbInterface);
+
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (ctx, "IOCreatePlugInInterfaceForService: %s", darwin_error_str(kresult));
+    return darwin_to_libusb (kresult);
+  }
+
+  if (!plugInInterface) {
+    usbi_err (ctx, "plugin interface not found");
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  /* Do the actual claim */
+  kresult = (*plugInInterface)->QueryInterface(plugInInterface,
+                                               CFUUIDGetUUIDBytes(InterfaceInterfaceID),
+                                               (LPVOID)&cInterface->interface);
+  /* We no longer need the intermediate plug-in */
+  /* Use release instead of IODestroyPlugInInterface to avoid stopping IOServices associated with this device */
+  (*plugInInterface)->Release (plugInInterface);
+  if (kresult != kIOReturnSuccess || !cInterface->interface) {
+    usbi_err (ctx, "QueryInterface: %s", darwin_error_str(kresult));
+    return darwin_to_libusb (kresult);
+  }
+
+  /* claim the interface */
+  kresult = (*(cInterface->interface))->USBInterfaceOpen(cInterface->interface);
+  if (kresult != kIOReturnSuccess) {
+    usbi_info (ctx, "USBInterfaceOpen: %s", darwin_error_str(kresult));
+    return darwin_to_libusb (kresult);
+  }
+
+  /* update list of endpoints */
+  ret = get_endpoints (dev_handle, iface);
+  if (ret) {
+    /* this should not happen */
+    darwin_release_interface (dev_handle, iface);
+    usbi_err (ctx, "could not build endpoint table");
+    return ret;
+  }
+
+  cInterface->cfSource = NULL;
+
+  /* create async event source */
+  kresult = (*(cInterface->interface))->CreateInterfaceAsyncEventSource (cInterface->interface, &cInterface->cfSource);
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (ctx, "could not create async event source");
+
+    /* can't continue without an async event source */
+    (void)darwin_release_interface (dev_handle, iface);
+
+    return darwin_to_libusb (kresult);
+  }
+
+  /* add the cfSource to the async thread's run loop */
+  CFRunLoopAddSource(libusb_darwin_acfl, cInterface->cfSource, kCFRunLoopDefaultMode);
+
+  usbi_dbg (ctx, "interface opened");
+
+  return LIBUSB_SUCCESS;
+}
+
+static int darwin_release_interface(struct libusb_device_handle *dev_handle, uint8_t iface) {
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+  IOReturn kresult;
+
+  /* current interface */
+  struct darwin_interface *cInterface = &priv->interfaces[iface];
+
+  /* Check to see if an interface is open */
+  if (!cInterface->interface)
+    return LIBUSB_SUCCESS;
+
+  /* clean up endpoint data */
+  cInterface->num_endpoints = 0;
+
+  /* delete the interface's async event source */
+  if (cInterface->cfSource) {
+    CFRunLoopRemoveSource (libusb_darwin_acfl, cInterface->cfSource, kCFRunLoopDefaultMode);
+    CFRelease (cInterface->cfSource);
+    cInterface->cfSource = NULL;
+  }
+
+  kresult = (*(cInterface->interface))->USBInterfaceClose(cInterface->interface);
+  if (kresult != kIOReturnSuccess)
+    usbi_warn (HANDLE_CTX (dev_handle), "USBInterfaceClose: %s", darwin_error_str(kresult));
+
+  kresult = (*(cInterface->interface))->Release(cInterface->interface);
+  if (kresult != kIOReturnSuccess)
+    usbi_warn (HANDLE_CTX (dev_handle), "Release: %s", darwin_error_str(kresult));
+
+  cInterface->interface = (usb_interface_t **) IO_OBJECT_NULL;
+
+  return darwin_to_libusb (kresult);
+}
+
+static int check_alt_setting_and_clear_halt(struct libusb_device_handle *dev_handle, uint8_t altsetting, struct darwin_interface *cInterface) {
+  enum libusb_error ret;
+  IOReturn kresult;
+  uint8_t current_alt_setting;
+
+  kresult = (*(cInterface->interface))->GetAlternateSetting (cInterface->interface, &current_alt_setting);
+  if (kresult == kIOReturnSuccess && altsetting != current_alt_setting) {
+    return LIBUSB_ERROR_PIPE;
+  }
+
+  for (int i = 0 ; i < cInterface->num_endpoints ; i++) {
+    ret = darwin_clear_halt(dev_handle, cInterface->endpoint_addrs[i]);
+    if (LIBUSB_SUCCESS != ret) {
+      usbi_warn(HANDLE_CTX (dev_handle), "error clearing pipe halt for endpoint %d", i);
+      if (LIBUSB_ERROR_NOT_FOUND == ret) {
+        /* may need to re-open the interface */
+        return ret;
+      }
+    }
+  }
+
+  return LIBUSB_SUCCESS;
+}
+
+static int darwin_set_interface_altsetting(struct libusb_device_handle *dev_handle, uint8_t iface, uint8_t altsetting) {
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+  IOReturn kresult;
+  enum libusb_error ret;
+
+  /* current interface */
+  struct darwin_interface *cInterface = &priv->interfaces[iface];
+
+  if (!cInterface->interface)
+    return LIBUSB_ERROR_NO_DEVICE;
+
+  kresult = (*(cInterface->interface))->SetAlternateInterface (cInterface->interface, altsetting);
+  if (kresult == kIOReturnSuccess) {
+    /* update the list of endpoints */
+    ret = get_endpoints (dev_handle, iface);
+    if (ret) {
+      /* this should not happen */
+      darwin_release_interface (dev_handle, iface);
+      usbi_err (HANDLE_CTX (dev_handle), "could not build endpoint table");
+    }
+    return ret;
+  }
+
+  usbi_warn (HANDLE_CTX (dev_handle), "SetAlternateInterface: %s", darwin_error_str(kresult));
+
+  ret = darwin_to_libusb(kresult);
+  if (ret != LIBUSB_ERROR_PIPE) {
+    return ret;
+  }
+
+  /* If a device only supports a default setting for the specified interface, then a STALL
+     (kIOUSBPipeStalled) may be returned. Ref: USB 2.0 specs 9.4.10.
+     Mimic the behaviour in e.g. the Linux kernel: in such case, reset all endpoints
+     of the interface (as would have been done per 9.1.1.5) and return success. */
+
+  ret = check_alt_setting_and_clear_halt(dev_handle, altsetting, cInterface);
+  if (LIBUSB_ERROR_NOT_FOUND == ret) {
+    /* For some reason we need to reclaim the interface after the pipe error with some versions of macOS */
+    ret = darwin_claim_interface (dev_handle, iface);
+    if (LIBUSB_SUCCESS != ret) {
+      darwin_release_interface (dev_handle, iface);
+      usbi_err (HANDLE_CTX (dev_handle), "could not reclaim interface: %s", darwin_error_str(kresult));
+    }
+    ret = check_alt_setting_and_clear_halt(dev_handle, altsetting, cInterface);
+  }
+
+  return ret;
+}
+
+static int darwin_clear_halt(struct libusb_device_handle *dev_handle, unsigned char endpoint) {
+  /* current interface */
+  struct darwin_interface *cInterface;
+  IOReturn kresult;
+  uint8_t pipeRef;
+
+  /* determine the interface/endpoint to use */
+  if (ep_to_pipeRef (dev_handle, endpoint, &pipeRef, NULL, &cInterface) != 0) {
+    usbi_err (HANDLE_CTX (dev_handle), "endpoint not found on any open interface");
+
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  /* newer versions of darwin support clearing additional bits on the device's endpoint */
+  kresult = (*(cInterface->interface))->ClearPipeStallBothEnds(cInterface->interface, pipeRef);
+  if (kresult != kIOReturnSuccess)
+    usbi_warn (HANDLE_CTX (dev_handle), "ClearPipeStall: %s", darwin_error_str (kresult));
+
+  return darwin_to_libusb (kresult);
+}
+
+static int darwin_restore_state (struct libusb_device_handle *dev_handle, int8_t active_config,
+                                 unsigned long claimed_interfaces) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  struct darwin_device_handle_priv *priv = usbi_get_device_handle_priv(dev_handle);
+  int open_count = dpriv->open_count;
+  int ret;
+
+  struct libusb_context *ctx = HANDLE_CTX (dev_handle);
+
+  /* clear claimed interfaces temporarily */
+  dev_handle->claimed_interfaces = 0;
+
+  /* close and re-open the device */
+  priv->is_open = false;
+  dpriv->open_count = 1;
+
+  /* clean up open interfaces */
+  (void) darwin_close (dev_handle);
+
+  /* re-open the device */
+  ret = darwin_open (dev_handle);
+  dpriv->open_count = open_count;
+  if (LIBUSB_SUCCESS != ret) {
+    /* could not restore configuration */
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  if (dpriv->active_config != active_config) {
+    usbi_dbg (ctx, "darwin/restore_state: restoring configuration %d...", active_config);
+
+    ret = darwin_set_configuration (dev_handle, active_config);
+    if (LIBUSB_SUCCESS != ret) {
+      usbi_dbg (ctx, "darwin/restore_state: could not restore configuration");
+      return LIBUSB_ERROR_NOT_FOUND;
+    }
+  }
+
+  usbi_dbg (ctx, "darwin/restore_state: reclaiming interfaces");
+
+  if (claimed_interfaces) {
+    for (uint8_t iface = 0 ; iface < USB_MAXINTERFACES ; ++iface) {
+      if (!(claimed_interfaces & (1U << iface))) {
+        continue;
+      }
+
+      usbi_dbg (ctx, "darwin/restore_state: re-claiming interface %u", iface);
+
+      ret = darwin_claim_interface (dev_handle, iface);
+      if (LIBUSB_SUCCESS != ret) {
+        usbi_dbg (ctx, "darwin/restore_state: could not claim interface %u", iface);
+        return LIBUSB_ERROR_NOT_FOUND;
+      }
+
+      dev_handle->claimed_interfaces |= 1U << iface;
+    }
+  }
+
+  usbi_dbg (ctx, "darwin/restore_state: device state restored");
+
+  return LIBUSB_SUCCESS;
+}
+
+static int darwin_reenumerate_device (struct libusb_device_handle *dev_handle, bool capture) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  unsigned long claimed_interfaces = dev_handle->claimed_interfaces;
+  int8_t active_config = dpriv->active_config;
+  UInt32 options = 0;
+  IOUSBDeviceDescriptor descriptor;
+  IOUSBConfigurationDescriptorPtr cached_configuration;
+  IOUSBConfigurationDescriptor *cached_configurations;
+  IOReturn kresult;
+  UInt8 i;
+
+  struct libusb_context *ctx = HANDLE_CTX (dev_handle);
+
+  if (dpriv->in_reenumerate) {
+    /* ack, two (or more) threads are trying to reset the device! abort! */
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  dpriv->in_reenumerate = true;
+
+  /* store copies of descriptors so they can be compared after the reset */
+  memcpy (&descriptor, &dpriv->dev_descriptor, sizeof (descriptor));
+  cached_configurations = alloca (sizeof (*cached_configurations) * descriptor.bNumConfigurations);
+
+  for (i = 0 ; i < descriptor.bNumConfigurations ; ++i) {
+    (*(dpriv->device))->GetConfigurationDescriptorPtr (dpriv->device, i, &cached_configuration);
+    memcpy (cached_configurations + i, cached_configuration, sizeof (cached_configurations[i]));
+  }
+
+  /* if we need to release capture */
+  if (HAS_CAPTURE_DEVICE()) {
+    if (capture) {
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 101000
+      options |= kUSBReEnumerateCaptureDeviceMask;
+#endif
+    }
+  } else {
+    capture = false;
+  }
+
+  /* from macOS 10.11 ResetDevice no longer does anything so just use USBDeviceReEnumerate */
+  kresult = (*(dpriv->device))->USBDeviceReEnumerate (dpriv->device, options);
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (ctx, "USBDeviceReEnumerate: %s", darwin_error_str (kresult));
+    dpriv->in_reenumerate = false;
+    return darwin_to_libusb (kresult);
+  }
+
+  /* capture mode does not re-enumerate but it does require re-open */
+  if (capture) {
+    usbi_dbg (ctx, "darwin/reenumerate_device: restoring state...");
+    dpriv->in_reenumerate = false;
+    return darwin_restore_state (dev_handle, active_config, claimed_interfaces);
+  }
+
+  usbi_dbg (ctx, "darwin/reenumerate_device: waiting for re-enumeration to complete...");
+
+  struct timespec start;
+  clock_gettime(CLOCK_MONOTONIC, &start);
+
+  while (dpriv->in_reenumerate) {
+    struct timespec delay = {.tv_sec = 0, .tv_nsec = 1000};
+    nanosleep (&delay, NULL);
+
+    struct timespec now;
+    clock_gettime(CLOCK_MONOTONIC, &now);
+    UInt32 elapsed = (now.tv_sec - start.tv_sec) * 1000000 + (now.tv_nsec - start.tv_nsec) / 1000;
+
+    if (elapsed >= DARWIN_REENUMERATE_TIMEOUT_US) {
+      usbi_err (ctx, "darwin/reenumerate_device: timeout waiting for reenumerate");
+      dpriv->in_reenumerate = false;
+      return LIBUSB_ERROR_TIMEOUT;
+    }
+  }
+
+  /* compare descriptors */
+  usbi_dbg (ctx, "darwin/reenumerate_device: checking whether descriptors changed");
+
+  if (memcmp (&descriptor, &dpriv->dev_descriptor, sizeof (descriptor))) {
+    /* device descriptor changed. need to return not found. */
+    usbi_dbg (ctx, "darwin/reenumerate_device: device descriptor changed");
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  for (i = 0 ; i < descriptor.bNumConfigurations ; ++i) {
+    (void) (*(dpriv->device))->GetConfigurationDescriptorPtr (dpriv->device, i, &cached_configuration);
+    if (memcmp (cached_configuration, cached_configurations + i, sizeof (cached_configurations[i]))) {
+      usbi_dbg (ctx, "darwin/reenumerate_device: configuration descriptor %d changed", i);
+      return LIBUSB_ERROR_NOT_FOUND;
+    }
+  }
+
+  usbi_dbg (ctx, "darwin/reenumerate_device: device reset complete. restoring state...");
+
+  return darwin_restore_state (dev_handle, active_config, claimed_interfaces);
+}
+
+static int darwin_reset_device (struct libusb_device_handle *dev_handle) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  IOReturn kresult;
+
+  if (dpriv->capture_count > 0) {
+    /* we have to use ResetDevice as USBDeviceReEnumerate() loses the authorization for capture */
+    kresult = (*(dpriv->device))->ResetDevice (dpriv->device);
+    return darwin_to_libusb (kresult);
+  } else {
+    return darwin_reenumerate_device (dev_handle, false);
+  }
+}
+
+static io_service_t usb_find_interface_matching_location (const io_name_t class_name, UInt8 interface_number, UInt32 location) {
+  CFMutableDictionaryRef matchingDict = IOServiceMatching (class_name);
+  CFMutableDictionaryRef propertyMatchDict = CFDictionaryCreateMutable (kCFAllocatorDefault, 0,
+                                                                        &kCFTypeDictionaryKeyCallBacks,
+                                                                        &kCFTypeDictionaryValueCallBacks);
+  CFTypeRef locationCF = CFNumberCreate (NULL, kCFNumberSInt32Type, &location);
+  CFTypeRef interfaceCF =  CFNumberCreate (NULL, kCFNumberSInt8Type, &interface_number);
+
+  CFDictionarySetValue (matchingDict, CFSTR(kIOPropertyMatchKey), propertyMatchDict);
+  CFDictionarySetValue (propertyMatchDict, CFSTR(kUSBDevicePropertyLocationID), locationCF);
+  CFDictionarySetValue (propertyMatchDict, CFSTR(kUSBHostMatchingPropertyInterfaceNumber), interfaceCF);
+
+  CFRelease (interfaceCF);
+  CFRelease (locationCF);
+  CFRelease (propertyMatchDict);
+
+  return IOServiceGetMatchingService (darwin_default_master_port, matchingDict);
+}
+
+static int darwin_kernel_driver_active(struct libusb_device_handle *dev_handle, uint8_t interface) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  io_service_t usb_interface, child = IO_OBJECT_NULL;
+
+  /* locate the IO registry entry for this interface */
+  usb_interface = usb_find_interface_matching_location (kIOUSBHostInterfaceClassName, interface, dpriv->location);
+  if (0 == usb_interface) {
+    /* check for the legacy class entry */
+    usb_interface = usb_find_interface_matching_location (kIOUSBInterfaceClassName, interface, dpriv->location);
+    if (0 == usb_interface) {
+      return LIBUSB_ERROR_NOT_FOUND;
+    }
+  }
+
+  /* if the IO object has a child entry in the IO Registry it has a kernel driver attached */
+  (void) IORegistryEntryGetChildEntry (usb_interface, kIOServicePlane, &child);
+  IOObjectRelease (usb_interface);
+  if (IO_OBJECT_NULL != child) {
+    IOObjectRelease (child);
+    return 1;
+  }
+
+  /* no driver */
+  return 0;
+}
+
+static void darwin_destroy_device(struct libusb_device *dev) {
+  struct darwin_device_priv *dpriv = usbi_get_device_priv(dev);
+
+  if (dpriv->dev) {
+    /* need to hold the lock in case this is the last reference to the device */
+    usbi_mutex_lock(&darwin_cached_devices_lock);
+    darwin_deref_cached_device (dpriv->dev);
+    dpriv->dev = NULL;
+    usbi_mutex_unlock(&darwin_cached_devices_lock);
+  }
+}
+
+static int submit_bulk_transfer(struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+
+  IOReturn               ret;
+  uint8_t                transferType;
+  uint8_t                pipeRef;
+  uint16_t               maxPacketSize;
+
+  struct darwin_interface *cInterface;
+#if InterfaceVersion >= 550
+  IOUSBEndpointProperties pipeProperties = {.bVersion = kUSBEndpointPropertiesVersion3};
+#else
+  /* None of the values below are used in libusb for bulk transfers */
+  uint8_t                 direction, number, interval;
+#endif
+
+  if (ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, NULL, &cInterface) != 0) {
+    usbi_err (TRANSFER_CTX (transfer), "endpoint not found on any open interface");
+
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+#if InterfaceVersion >= 550
+  ret = (*(cInterface->interface))->GetPipePropertiesV3 (cInterface->interface, pipeRef, &pipeProperties);
+
+  transferType = pipeProperties.bTransferType;
+  maxPacketSize = pipeProperties.wMaxPacketSize;
+#else
+  ret = (*(cInterface->interface))->GetPipeProperties (cInterface->interface, pipeRef, &direction, &number,
+                                                       &transferType, &maxPacketSize, &interval);
+#endif
+
+  if (ret) {
+    usbi_err (TRANSFER_CTX (transfer), "bulk transfer failed (dir = %s): %s (code = 0x%08x)", IS_XFERIN(transfer) ? "In" : "Out",
+              darwin_error_str(ret), ret);
+    return darwin_to_libusb (ret);
+  }
+
+  if (0 != (transfer->length % maxPacketSize)) {
+    /* do not need a zero packet */
+    transfer->flags &= ~LIBUSB_TRANSFER_ADD_ZERO_PACKET;
+  }
+
+  /* submit the request */
+  /* timeouts are unavailable on interrupt endpoints */
+  if (transferType == kUSBInterrupt) {
+    if (IS_XFERIN(transfer))
+      ret = (*(cInterface->interface))->ReadPipeAsync(cInterface->interface, pipeRef, transfer->buffer,
+                                                      (UInt32)transfer->length, darwin_async_io_callback, itransfer);
+    else
+      ret = (*(cInterface->interface))->WritePipeAsync(cInterface->interface, pipeRef, transfer->buffer,
+                                                       (UInt32)transfer->length, darwin_async_io_callback, itransfer);
+  } else {
+    itransfer->timeout_flags |= USBI_TRANSFER_OS_HANDLES_TIMEOUT;
+
+    if (IS_XFERIN(transfer))
+      ret = (*(cInterface->interface))->ReadPipeAsyncTO(cInterface->interface, pipeRef, transfer->buffer,
+                                                        (UInt32)transfer->length, transfer->timeout, transfer->timeout,
+                                                        darwin_async_io_callback, itransfer);
+    else
+      ret = (*(cInterface->interface))->WritePipeAsyncTO(cInterface->interface, pipeRef, transfer->buffer,
+                                                         (UInt32)transfer->length, transfer->timeout, transfer->timeout,
+                                                         darwin_async_io_callback, itransfer);
+  }
+
+  if (ret)
+    usbi_err (TRANSFER_CTX (transfer), "bulk transfer failed (dir = %s): %s (code = 0x%08x)", IS_XFERIN(transfer) ? "In" : "Out",
+               darwin_error_str(ret), ret);
+
+  return darwin_to_libusb (ret);
+}
+
+#if InterfaceVersion >= 550
+static int submit_stream_transfer(struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+  struct darwin_interface *cInterface;
+  uint8_t pipeRef;
+  IOReturn ret;
+
+  if (ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, NULL, &cInterface) != 0) {
+    usbi_err (TRANSFER_CTX (transfer), "endpoint not found on any open interface");
+
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  itransfer->timeout_flags |= USBI_TRANSFER_OS_HANDLES_TIMEOUT;
+
+  if (IS_XFERIN(transfer))
+    ret = (*(cInterface->interface))->ReadStreamsPipeAsyncTO(cInterface->interface, pipeRef, itransfer->stream_id,
+                                                             transfer->buffer, (UInt32)transfer->length, transfer->timeout,
+                                                             transfer->timeout, darwin_async_io_callback, itransfer);
+  else
+    ret = (*(cInterface->interface))->WriteStreamsPipeAsyncTO(cInterface->interface, pipeRef, itransfer->stream_id,
+                                                              transfer->buffer, (UInt32)transfer->length, transfer->timeout,
+                                                              transfer->timeout, darwin_async_io_callback, itransfer);
+
+  if (ret)
+    usbi_err (TRANSFER_CTX (transfer), "bulk stream transfer failed (dir = %s): %s (code = 0x%08x)", IS_XFERIN(transfer) ? "In" : "Out",
+               darwin_error_str(ret), ret);
+
+  return darwin_to_libusb (ret);
+}
+#endif
+
+static int submit_iso_transfer(struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+  struct darwin_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+
+  IOReturn kresult;
+  uint8_t direction, number, interval, pipeRef, transferType;
+  uint16_t maxPacketSize;
+  UInt64 frame;
+  AbsoluteTime atTime;
+  int i;
+
+  struct darwin_interface *cInterface;
+
+  /* construct an array of IOUSBIsocFrames, reuse the old one if the sizes are the same */
+  if (tpriv->num_iso_packets != transfer->num_iso_packets) {
+    free(tpriv->isoc_framelist);
+    tpriv->isoc_framelist = NULL;
+  }
+
+  if (!tpriv->isoc_framelist) {
+    tpriv->num_iso_packets = transfer->num_iso_packets;
+    tpriv->isoc_framelist = (IOUSBIsocFrame*) calloc ((size_t)transfer->num_iso_packets, sizeof(IOUSBIsocFrame));
+    if (!tpriv->isoc_framelist)
+      return LIBUSB_ERROR_NO_MEM;
+  }
+
+  /* copy the frame list from the libusb descriptor (the structures differ only is member order) */
+  for (i = 0 ; i < transfer->num_iso_packets ; i++) {
+    unsigned int length = transfer->iso_packet_desc[i].length;
+    assert(length <= UINT16_MAX);
+    tpriv->isoc_framelist[i].frReqCount = (UInt16)length;
+  }
+
+  /* determine the interface/endpoint to use */
+  if (ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, NULL, &cInterface) != 0) {
+    usbi_err (TRANSFER_CTX (transfer), "endpoint not found on any open interface");
+
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  /* determine the properties of this endpoint and the speed of the device */
+  (*(cInterface->interface))->GetPipeProperties (cInterface->interface, pipeRef, &direction, &number,
+                                                 &transferType, &maxPacketSize, &interval);
+
+  /* Last but not least we need the bus frame number */
+  kresult = (*(cInterface->interface))->GetBusFrameNumber(cInterface->interface, &frame, &atTime);
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (TRANSFER_CTX (transfer), "failed to get bus frame number: %d", kresult);
+    free(tpriv->isoc_framelist);
+    tpriv->isoc_framelist = NULL;
+
+    return darwin_to_libusb (kresult);
+  }
+
+  (*(cInterface->interface))->GetPipeProperties (cInterface->interface, pipeRef, &direction, &number,
+                                                 &transferType, &maxPacketSize, &interval);
+
+  /* schedule for a frame a little in the future */
+  frame += 4;
+
+  if (cInterface->frames[transfer->endpoint] && frame < cInterface->frames[transfer->endpoint])
+    frame = cInterface->frames[transfer->endpoint];
+
+  /* submit the request */
+  if (IS_XFERIN(transfer))
+    kresult = (*(cInterface->interface))->ReadIsochPipeAsync(cInterface->interface, pipeRef, transfer->buffer, frame,
+                                                             (UInt32)transfer->num_iso_packets, tpriv->isoc_framelist, darwin_async_io_callback,
+                                                             itransfer);
+  else
+    kresult = (*(cInterface->interface))->WriteIsochPipeAsync(cInterface->interface, pipeRef, transfer->buffer, frame,
+                                                              (UInt32)transfer->num_iso_packets, tpriv->isoc_framelist, darwin_async_io_callback,
+                                                              itransfer);
+
+  if (LIBUSB_SPEED_FULL == transfer->dev_handle->dev->speed)
+    /* Full speed */
+    cInterface->frames[transfer->endpoint] = frame + (UInt32)transfer->num_iso_packets * (1U << (interval - 1));
+  else
+    /* High/super speed */
+    cInterface->frames[transfer->endpoint] = frame + (UInt32)transfer->num_iso_packets * (1U << (interval - 1)) / 8;
+
+  if (kresult != kIOReturnSuccess) {
+    usbi_err (TRANSFER_CTX (transfer), "isochronous transfer failed (dir: %s): %s", IS_XFERIN(transfer) ? "In" : "Out",
+               darwin_error_str(kresult));
+    free (tpriv->isoc_framelist);
+    tpriv->isoc_framelist = NULL;
+  }
+
+  return darwin_to_libusb (kresult);
+}
+
+static int submit_control_transfer(struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+  struct libusb_control_setup *setup = (struct libusb_control_setup *) transfer->buffer;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(transfer->dev_handle->dev);
+  struct darwin_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+
+  IOReturn               kresult;
+
+  memset(&tpriv->req, 0, sizeof(tpriv->req));
+
+  /* IOUSBDeviceInterface expects the request in cpu endianness */
+  tpriv->req.bmRequestType     = setup->bmRequestType;
+  tpriv->req.bRequest          = setup->bRequest;
+  /* these values should be in bus order from libusb_fill_control_setup */
+  tpriv->req.wValue            = OSSwapLittleToHostInt16 (setup->wValue);
+  tpriv->req.wIndex            = OSSwapLittleToHostInt16 (setup->wIndex);
+  tpriv->req.wLength           = OSSwapLittleToHostInt16 (setup->wLength);
+  /* data is stored after the libusb control block */
+  tpriv->req.pData             = transfer->buffer + LIBUSB_CONTROL_SETUP_SIZE;
+  tpriv->req.completionTimeout = transfer->timeout;
+  tpriv->req.noDataTimeout     = transfer->timeout;
+
+  itransfer->timeout_flags |= USBI_TRANSFER_OS_HANDLES_TIMEOUT;
+
+  /* all transfers in libusb-1.0 are async */
+
+  if (transfer->endpoint) {
+    struct darwin_interface *cInterface;
+    uint8_t                 pipeRef;
+
+    if (ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, NULL, &cInterface) != 0) {
+      usbi_err (TRANSFER_CTX (transfer), "endpoint not found on any open interface");
+
+      return LIBUSB_ERROR_NOT_FOUND;
+    }
+
+    kresult = (*(cInterface->interface))->ControlRequestAsyncTO (cInterface->interface, pipeRef, &(tpriv->req), darwin_async_io_callback, itransfer);
+  } else
+    /* control request on endpoint 0 */
+    kresult = (*(dpriv->device))->DeviceRequestAsyncTO(dpriv->device, &(tpriv->req), darwin_async_io_callback, itransfer);
+
+  if (kresult != kIOReturnSuccess)
+    usbi_err (TRANSFER_CTX (transfer), "control request failed: %s", darwin_error_str(kresult));
+
+  return darwin_to_libusb (kresult);
+}
+
+static int darwin_submit_transfer(struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+
+  switch (transfer->type) {
+  case LIBUSB_TRANSFER_TYPE_CONTROL:
+    return submit_control_transfer(itransfer);
+  case LIBUSB_TRANSFER_TYPE_BULK:
+  case LIBUSB_TRANSFER_TYPE_INTERRUPT:
+    return submit_bulk_transfer(itransfer);
+  case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
+    return submit_iso_transfer(itransfer);
+  case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
+#if InterfaceVersion >= 550
+    return submit_stream_transfer(itransfer);
+#else
+    usbi_err (TRANSFER_CTX(transfer), "IOUSBFamily version does not support bulk stream transfers");
+    return LIBUSB_ERROR_NOT_SUPPORTED;
+#endif
+  default:
+    usbi_err (TRANSFER_CTX(transfer), "unknown endpoint type %d", transfer->type);
+    return LIBUSB_ERROR_INVALID_PARAM;
+  }
+}
+
+static int cancel_control_transfer(struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(transfer->dev_handle->dev);
+  IOReturn kresult;
+
+  usbi_warn (ITRANSFER_CTX (itransfer), "aborting all transactions control pipe");
+
+  if (!dpriv->device)
+    return LIBUSB_ERROR_NO_DEVICE;
+
+  kresult = (*(dpriv->device))->USBDeviceAbortPipeZero (dpriv->device);
+
+  return darwin_to_libusb (kresult);
+}
+
+static int darwin_abort_transfers (struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(transfer->dev_handle->dev);
+  struct darwin_interface *cInterface;
+  uint8_t pipeRef, iface;
+  IOReturn kresult;
+
+  struct libusb_context *ctx = ITRANSFER_CTX (itransfer);
+
+  if (ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, &iface, &cInterface) != 0) {
+    usbi_err (ctx, "endpoint not found on any open interface");
+
+    return LIBUSB_ERROR_NOT_FOUND;
+  }
+
+  if (!dpriv->device)
+    return LIBUSB_ERROR_NO_DEVICE;
+
+  usbi_warn (ctx, "aborting all transactions on interface %d pipe %d", iface, pipeRef);
+
+  /* abort transactions */
+#if InterfaceVersion >= 550
+  if (LIBUSB_TRANSFER_TYPE_BULK_STREAM == transfer->type)
+    (*(cInterface->interface))->AbortStreamsPipe (cInterface->interface, pipeRef, itransfer->stream_id);
+  else
+#endif
+    (*(cInterface->interface))->AbortPipe (cInterface->interface, pipeRef);
+
+  usbi_dbg (ctx, "calling clear pipe stall to clear the data toggle bit");
+
+  /* newer versions of darwin support clearing additional bits on the device's endpoint */
+  kresult = (*(cInterface->interface))->ClearPipeStallBothEnds(cInterface->interface, pipeRef);
+
+  return darwin_to_libusb (kresult);
+}
+
+static int darwin_cancel_transfer(struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+
+  switch (transfer->type) {
+  case LIBUSB_TRANSFER_TYPE_CONTROL:
+    return cancel_control_transfer(itransfer);
+  case LIBUSB_TRANSFER_TYPE_BULK:
+  case LIBUSB_TRANSFER_TYPE_INTERRUPT:
+  case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
+    return darwin_abort_transfers (itransfer);
+  default:
+    usbi_err (TRANSFER_CTX(transfer), "unknown endpoint type %d", transfer->type);
+    return LIBUSB_ERROR_INVALID_PARAM;
+  }
+}
+
+static void darwin_async_io_callback (void *refcon, IOReturn result, void *arg0) {
+  struct usbi_transfer *itransfer = (struct usbi_transfer *)refcon;
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+  struct darwin_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+
+  usbi_dbg (TRANSFER_CTX(transfer), "an async io operation has completed");
+
+  /* if requested write a zero packet */
+  if (kIOReturnSuccess == result && IS_XFEROUT(transfer) && transfer->flags & LIBUSB_TRANSFER_ADD_ZERO_PACKET) {
+    struct darwin_interface *cInterface;
+    uint8_t pipeRef;
+
+    (void) ep_to_pipeRef (transfer->dev_handle, transfer->endpoint, &pipeRef, NULL, &cInterface);
+
+    (*(cInterface->interface))->WritePipe (cInterface->interface, pipeRef, transfer->buffer, 0);
+  }
+
+  tpriv->result = result;
+  tpriv->size = (UInt32) (uintptr_t) arg0;
+
+  /* signal the core that this transfer is complete */
+  usbi_signal_transfer_completion(itransfer);
+}
+
+static enum libusb_transfer_status darwin_transfer_status (struct usbi_transfer *itransfer, IOReturn result) {
+  if (itransfer->timeout_flags & USBI_TRANSFER_TIMED_OUT)
+    result = kIOUSBTransactionTimeout;
+
+  struct libusb_context *ctx = ITRANSFER_CTX (itransfer);
+
+  switch (result) {
+  case kIOReturnUnderrun:
+  case kIOReturnSuccess:
+    return LIBUSB_TRANSFER_COMPLETED;
+  case kIOReturnAborted:
+    return LIBUSB_TRANSFER_CANCELLED;
+  case kIOUSBPipeStalled:
+    usbi_dbg (ctx, "transfer error: pipe is stalled");
+    return LIBUSB_TRANSFER_STALL;
+  case kIOReturnOverrun:
+    usbi_warn (ctx, "transfer error: data overrun");
+    return LIBUSB_TRANSFER_OVERFLOW;
+  case kIOUSBTransactionTimeout:
+    usbi_warn (ctx, "transfer error: timed out");
+    itransfer->timeout_flags |= USBI_TRANSFER_TIMED_OUT;
+    return LIBUSB_TRANSFER_TIMED_OUT;
+  default:
+    usbi_warn (ctx, "transfer error: %s (value = 0x%08x)", darwin_error_str (result), result);
+    return LIBUSB_TRANSFER_ERROR;
+  }
+}
+
+static int darwin_handle_transfer_completion (struct usbi_transfer *itransfer) {
+  struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+  struct darwin_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+  const unsigned char max_transfer_type = LIBUSB_TRANSFER_TYPE_BULK_STREAM;
+  const char *transfer_types[] = {"control", "isoc", "bulk", "interrupt", "bulk-stream", NULL};
+  bool is_isoc = LIBUSB_TRANSFER_TYPE_ISOCHRONOUS == transfer->type;
+  struct libusb_context *ctx = ITRANSFER_CTX (itransfer);
+
+  if (transfer->type > max_transfer_type) {
+    usbi_err (ctx, "unknown endpoint type %d", transfer->type);
+    return LIBUSB_ERROR_INVALID_PARAM;
+  }
+
+  if (NULL == tpriv) {
+    usbi_err (ctx, "malformed request is missing transfer priv");
+    return LIBUSB_ERROR_INVALID_PARAM;
+  }
+
+  usbi_dbg (ctx, "handling transfer completion type %s with kernel status %d", transfer_types[transfer->type], tpriv->result);
+
+  if (kIOReturnSuccess == tpriv->result || kIOReturnUnderrun == tpriv->result || kIOUSBTransactionTimeout == tpriv->result) {
+    if (is_isoc && tpriv->isoc_framelist) {
+      /* copy isochronous results back */
+
+      for (int i = 0; i < transfer->num_iso_packets ; i++) {
+        struct libusb_iso_packet_descriptor *lib_desc = &transfer->iso_packet_desc[i];
+        lib_desc->status = darwin_transfer_status (itransfer, tpriv->isoc_framelist[i].frStatus);
+        lib_desc->actual_length = tpriv->isoc_framelist[i].frActCount;
+      }
+    } else if (!is_isoc) {
+      itransfer->transferred += tpriv->size;
+    }
+  }
+
+  /* it is ok to handle cancelled transfers without calling usbi_handle_transfer_cancellation (we catch timeout transfers) */
+  return usbi_handle_transfer_completion (itransfer, darwin_transfer_status (itransfer, tpriv->result));
+}
+
+#if !defined(HAVE_CLOCK_GETTIME)
+void usbi_get_monotonic_time(struct timespec *tp) {
+  mach_timespec_t sys_time;
+
+  /* use system boot time as reference for the monotonic clock */
+  clock_get_time (clock_monotonic, &sys_time);
+
+  tp->tv_sec  = sys_time.tv_sec;
+  tp->tv_nsec = sys_time.tv_nsec;
+}
+
+void usbi_get_real_time(struct timespec *tp) {
+  mach_timespec_t sys_time;
+
+  /* CLOCK_REALTIME represents time since the epoch */
+  clock_get_time (clock_realtime, &sys_time);
+
+  tp->tv_sec  = sys_time.tv_sec;
+  tp->tv_nsec = sys_time.tv_nsec;
+}
+#endif
+
+#if InterfaceVersion >= 550
+static int darwin_alloc_streams (struct libusb_device_handle *dev_handle, uint32_t num_streams, unsigned char *endpoints,
+                                 int num_endpoints) {
+  struct darwin_interface *cInterface;
+  UInt32 supportsStreams;
+  uint8_t pipeRef;
+  int rc, i;
+
+  /* find the minimum number of supported streams on the endpoint list */
+  for (i = 0 ; i < num_endpoints ; ++i) {
+    if (0 != (rc = ep_to_pipeRef (dev_handle, endpoints[i], &pipeRef, NULL, &cInterface))) {
+      return rc;
+    }
+
+    (*(cInterface->interface))->SupportsStreams (cInterface->interface, pipeRef, &supportsStreams);
+    if (num_streams > supportsStreams)
+      num_streams = supportsStreams;
+  }
+
+  /* it is an error if any endpoint in endpoints does not support streams */
+  if (0 == num_streams)
+    return LIBUSB_ERROR_INVALID_PARAM;
+
+  /* create the streams */
+  for (i = 0 ; i < num_endpoints ; ++i) {
+    (void) ep_to_pipeRef (dev_handle, endpoints[i], &pipeRef, NULL, &cInterface);
+
+    rc = (*(cInterface->interface))->CreateStreams (cInterface->interface, pipeRef, num_streams);
+    if (kIOReturnSuccess != rc)
+      return darwin_to_libusb(rc);
+  }
+
+  assert(num_streams <= INT_MAX);
+  return (int)num_streams;
+}
+
+static int darwin_free_streams (struct libusb_device_handle *dev_handle, unsigned char *endpoints, int num_endpoints) {
+  struct darwin_interface *cInterface;
+  UInt32 supportsStreams;
+  uint8_t pipeRef;
+  int rc;
+
+  for (int i = 0 ; i < num_endpoints ; ++i) {
+    if (0 != (rc = ep_to_pipeRef (dev_handle, endpoints[i], &pipeRef, NULL, &cInterface)))
+      return rc;
+
+    (*(cInterface->interface))->SupportsStreams (cInterface->interface, pipeRef, &supportsStreams);
+    if (0 == supportsStreams)
+      return LIBUSB_ERROR_INVALID_PARAM;
+
+    rc = (*(cInterface->interface))->CreateStreams (cInterface->interface, pipeRef, 0);
+    if (kIOReturnSuccess != rc)
+      return darwin_to_libusb(rc);
+  }
+
+  return LIBUSB_SUCCESS;
+}
+#endif
+
+#if InterfaceVersion >= 700
+
+/* macOS APIs for getting entitlement values */
+
+#if TARGET_OS_OSX
+#include <Security/Security.h>
+#else
+typedef struct __SecTask *SecTaskRef;
+extern SecTaskRef SecTaskCreateFromSelf(CFAllocatorRef allocator);
+extern CFTypeRef SecTaskCopyValueForEntitlement(SecTaskRef task, CFStringRef entitlement, CFErrorRef *error);
+#endif
+
+static bool darwin_has_capture_entitlements (void) {
+  SecTaskRef task;
+  CFTypeRef value;
+  bool entitled;
+
+  task = SecTaskCreateFromSelf (kCFAllocatorDefault);
+  if (task == NULL) {
+    return false;
+  }
+  value = SecTaskCopyValueForEntitlement(task, CFSTR("com.apple.vm.device-access"), NULL);
+  CFRelease (task);
+  entitled = value && (CFGetTypeID (value) == CFBooleanGetTypeID ()) && CFBooleanGetValue (value);
+  if (value) {
+    CFRelease (value);
+  }
+  return entitled;
+}
+
+static int darwin_reload_device (struct libusb_device_handle *dev_handle) {
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  enum libusb_error err;
+
+  usbi_mutex_lock(&darwin_cached_devices_lock);
+  (*(dpriv->device))->Release(dpriv->device);
+  dpriv->device = darwin_device_from_service (HANDLE_CTX (dev_handle), dpriv->service);
+  if (!dpriv->device) {
+    err = LIBUSB_ERROR_NO_DEVICE;
+  } else {
+    err = LIBUSB_SUCCESS;
+  }
+  usbi_mutex_unlock(&darwin_cached_devices_lock);
+
+  return err;
+}
+
+/* On macOS, we capture an entire device at once, not individual interfaces. */
+
+static int darwin_detach_kernel_driver (struct libusb_device_handle *dev_handle, uint8_t interface) {
+  UNUSED(interface);
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+  IOReturn kresult;
+  enum libusb_error err;
+  struct libusb_context *ctx = HANDLE_CTX (dev_handle);
+
+  if (HAS_CAPTURE_DEVICE()) {
+  } else {
+    return LIBUSB_ERROR_NOT_SUPPORTED;
+  }
+
+  if (dpriv->capture_count == 0) {
+    usbi_dbg (ctx, "attempting to detach kernel driver from device");
+
+    if (darwin_has_capture_entitlements ()) {
+      /* request authorization */
+      kresult = IOServiceAuthorize (dpriv->service, kIOServiceInteractionAllowed);
+      if (kresult != kIOReturnSuccess) {
+        usbi_warn (ctx, "IOServiceAuthorize: %s", darwin_error_str(kresult));
+        return darwin_to_libusb (kresult);
+      }
+
+      /* we need start() to be called again for authorization status to refresh */
+      err = darwin_reload_device (dev_handle);
+      if (err != LIBUSB_SUCCESS) {
+        return err;
+      }
+    } else {
+      usbi_info (ctx, "no capture entitlements. may not be able to detach the kernel driver for this device");
+      if (0 != geteuid()) {
+        usbi_warn (ctx, "USB device capture requires either an entitlement (com.apple.vm.device-access) or root privilege");
+        return LIBUSB_ERROR_ACCESS;
+      }
+    }
+
+    /* reset device to release existing drivers */
+    err = darwin_reenumerate_device (dev_handle, true);
+    if (err != LIBUSB_SUCCESS) {
+      return err;
+    }
+  }
+  dpriv->capture_count++;
+  return LIBUSB_SUCCESS;
+}
+
+
+static int darwin_attach_kernel_driver (struct libusb_device_handle *dev_handle, uint8_t interface) {
+  UNUSED(interface);
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+
+  if (HAS_CAPTURE_DEVICE()) {
+  } else {
+    return LIBUSB_ERROR_NOT_SUPPORTED;
+  }
+
+  dpriv->capture_count--;
+  if (dpriv->capture_count > 0) {
+    return LIBUSB_SUCCESS;
+  }
+
+  usbi_dbg (HANDLE_CTX (dev_handle), "reenumerating device for kernel driver attach");
+
+  /* reset device to attach kernel drivers */
+  return darwin_reenumerate_device (dev_handle, false);
+}
+
+static int darwin_capture_claim_interface(struct libusb_device_handle *dev_handle, uint8_t iface) {
+  enum libusb_error ret;
+  if (dev_handle->auto_detach_kernel_driver && darwin_kernel_driver_active(dev_handle, iface)) {
+    ret = darwin_detach_kernel_driver (dev_handle, iface);
+    if (ret != LIBUSB_SUCCESS) {
+      usbi_info (HANDLE_CTX (dev_handle), "failed to auto-detach the kernel driver for this device, ret=%d", ret);
+    }
+  }
+
+  return darwin_claim_interface (dev_handle, iface);
+}
+
+static int darwin_capture_release_interface(struct libusb_device_handle *dev_handle, uint8_t iface) {
+  enum libusb_error ret;
+  struct darwin_cached_device *dpriv = DARWIN_CACHED_DEVICE(dev_handle->dev);
+
+  ret = darwin_release_interface (dev_handle, iface);
+  if (ret != LIBUSB_SUCCESS) {
+    return ret;
+  }
+
+  if (dev_handle->auto_detach_kernel_driver && dpriv->capture_count > 0) {
+    ret = darwin_attach_kernel_driver (dev_handle, iface);
+    if (LIBUSB_SUCCESS != ret) {
+      usbi_info (HANDLE_CTX (dev_handle), "on attempt to reattach the kernel driver got ret=%d", ret);
+    }
+    /* ignore the error as the interface was successfully released */
+  }
+
+  return LIBUSB_SUCCESS;
+}
+
+#endif
+
+const struct usbi_os_backend usbi_backend = {
+        .name = "Darwin",
+        .caps = USBI_CAP_SUPPORTS_DETACH_KERNEL_DRIVER,
+        .init = darwin_init,
+        .exit = darwin_exit,
+        .get_active_config_descriptor = darwin_get_active_config_descriptor,
+        .get_config_descriptor = darwin_get_config_descriptor,
+        .hotplug_poll = darwin_hotplug_poll,
+
+        .open = darwin_open,
+        .close = darwin_close,
+        .get_configuration = darwin_get_configuration,
+        .set_configuration = darwin_set_configuration,
+
+        .set_interface_altsetting = darwin_set_interface_altsetting,
+        .clear_halt = darwin_clear_halt,
+        .reset_device = darwin_reset_device,
+
+#if InterfaceVersion >= 550
+        .alloc_streams = darwin_alloc_streams,
+        .free_streams = darwin_free_streams,
+#endif
+
+        .kernel_driver_active = darwin_kernel_driver_active,
+
+#if InterfaceVersion >= 700
+        .detach_kernel_driver = darwin_detach_kernel_driver,
+        .attach_kernel_driver = darwin_attach_kernel_driver,
+        .claim_interface = darwin_capture_claim_interface,
+        .release_interface = darwin_capture_release_interface,
+#else
+        .claim_interface = darwin_claim_interface,
+        .release_interface = darwin_release_interface,
+#endif
+
+        .destroy_device = darwin_destroy_device,
+
+        .submit_transfer = darwin_submit_transfer,
+        .cancel_transfer = darwin_cancel_transfer,
+
+        .handle_transfer_completion = darwin_handle_transfer_completion,
+
+        .device_priv_size = sizeof(struct darwin_device_priv),
+        .device_handle_priv_size = sizeof(struct darwin_device_handle_priv),
+        .transfer_priv_size = sizeof(struct darwin_transfer_priv),
+};
diff --git a/hardware/src/libusb/os/darwin_usb.h b/hardware/src/libusb/os/darwin_usb.h
new file mode 100644
index 0000000..7b72fff
--- /dev/null
+++ b/hardware/src/libusb/os/darwin_usb.h
@@ -0,0 +1,227 @@
+/*
+ * darwin backend for libusb 1.0
+ * Copyright © 2008-2019 Nathan Hjelm <hjelmn@users.sourceforge.net>
+ * Copyright © 2019      Google LLC. All rights reserved.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#if !defined(LIBUSB_DARWIN_H)
+#define LIBUSB_DARWIN_H
+
+#include <stdbool.h>
+
+#include "libusbi.h"
+
+#include <IOKit/IOTypes.h>
+#include <IOKit/IOCFBundle.h>
+#include <IOKit/usb/IOUSBLib.h>
+#include <IOKit/IOCFPlugIn.h>
+
+#if defined(HAVE_IOKIT_USB_IOUSBHOSTFAMILYDEFINITIONS_H)
+#include <IOKit/usb/IOUSBHostFamilyDefinitions.h>
+#endif
+
+/* IOUSBInterfaceInferface */
+
+/* New in OS 10.12.0. */
+#if defined (kIOUSBInterfaceInterfaceID800)
+
+#define usb_interface_t IOUSBInterfaceInterface800
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID800
+#define InterfaceVersion 800
+
+/* New in OS 10.10.0. */
+#elif defined (kIOUSBInterfaceInterfaceID700)
+
+#define usb_interface_t IOUSBInterfaceInterface700
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID700
+#define InterfaceVersion 700
+
+/* New in OS 10.9.0. */
+#elif defined (kIOUSBInterfaceInterfaceID650)
+
+#define usb_interface_t IOUSBInterfaceInterface650
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID650
+#define InterfaceVersion 650
+
+/* New in OS 10.8.2 but can't test deployment target to that granularity, so round up. */
+#elif defined (kIOUSBInterfaceInterfaceID550)
+
+#define usb_interface_t IOUSBInterfaceInterface550
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID550
+#define InterfaceVersion 550
+
+/* New in OS 10.7.3 but can't test deployment target to that granularity, so round up. */
+#elif defined (kIOUSBInterfaceInterfaceID500)
+
+#define usb_interface_t IOUSBInterfaceInterface500
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID500
+#define InterfaceVersion 500
+
+/* New in OS 10.5.0. */
+#elif defined (kIOUSBInterfaceInterfaceID300)
+
+#define usb_interface_t IOUSBInterfaceInterface300
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID300
+#define InterfaceVersion 300
+
+/* New in OS 10.4.5 (or 10.4.6?) but can't test deployment target to that granularity, so round up. */
+#elif defined (kIOUSBInterfaceInterfaceID245)
+
+#define usb_interface_t IOUSBInterfaceInterface245
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID245
+#define InterfaceVersion 245
+
+/* New in OS 10.4.0. */
+#elif defined (kIOUSBInterfaceInterfaceID220)
+
+#define usb_interface_t IOUSBInterfaceInterface220
+#define InterfaceInterfaceID kIOUSBInterfaceInterfaceID220
+#define InterfaceVersion 220
+
+#else
+
+#error "IOUSBFamily is too old. Please upgrade your SDK and/or deployment target"
+
+#endif
+
+/* IOUSBDeviceInterface */
+
+/* New in OS 10.9.0. */
+#if defined (kIOUSBDeviceInterfaceID650)
+
+#define usb_device_t    IOUSBDeviceInterface650
+#define DeviceInterfaceID kIOUSBDeviceInterfaceID650
+#define DeviceVersion 650
+
+/* New in OS 10.7.3 but can't test deployment target to that granularity, so round up. */
+#elif defined (kIOUSBDeviceInterfaceID500)
+
+#define usb_device_t    IOUSBDeviceInterface500
+#define DeviceInterfaceID kIOUSBDeviceInterfaceID500
+#define DeviceVersion 500
+
+/* New in OS 10.5.4 but can't test deployment target to that granularity, so round up. */
+#elif defined (kIOUSBDeviceInterfaceID320)
+
+#define usb_device_t    IOUSBDeviceInterface320
+#define DeviceInterfaceID kIOUSBDeviceInterfaceID320
+#define DeviceVersion 320
+
+/* New in OS 10.5.0. */
+#elif defined (kIOUSBDeviceInterfaceID300)
+
+#define usb_device_t    IOUSBDeviceInterface300
+#define DeviceInterfaceID kIOUSBDeviceInterfaceID300
+#define DeviceVersion 300
+
+/* New in OS 10.4.5 (or 10.4.6?) but can't test deployment target to that granularity, so round up. */
+#elif defined (kIOUSBDeviceInterfaceID245)
+
+#define usb_device_t    IOUSBDeviceInterface245
+#define DeviceInterfaceID kIOUSBDeviceInterfaceID245
+#define DeviceVersion 245
+
+/* New in OS 10.2.3 but can't test deployment target to that granularity, so round up. */
+#elif defined (kIOUSBDeviceInterfaceID197)
+
+#define usb_device_t    IOUSBDeviceInterface197
+#define DeviceInterfaceID kIOUSBDeviceInterfaceID197
+#define DeviceVersion 197
+
+#else
+
+#error "IOUSBFamily is too old. Please upgrade your SDK and/or deployment target"
+
+#endif
+
+#if !defined(kIOUSBHostInterfaceClassName)
+#define kIOUSBHostInterfaceClassName "IOUSBHostInterface"
+#endif
+
+#if !defined(kUSBHostMatchingPropertyInterfaceNumber)
+#define kUSBHostMatchingPropertyInterfaceNumber "bInterfaceNumber"
+#endif
+
+#if !defined(IO_OBJECT_NULL)
+#define IO_OBJECT_NULL ((io_object_t) 0)
+#endif
+
+/* Testing availability */
+#ifndef __has_builtin
+  #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
+#endif
+#if __has_builtin(__builtin_available)
+  #define HAS_CAPTURE_DEVICE() __builtin_available(macOS 10.10, *)
+#else
+  #define HAS_CAPTURE_DEVICE() 0
+#endif
+
+typedef IOCFPlugInInterface *io_cf_plugin_ref_t;
+typedef IONotificationPortRef io_notification_port_t;
+
+/* private structures */
+struct darwin_cached_device {
+  struct list_head      list;
+  IOUSBDeviceDescriptor dev_descriptor;
+  UInt32                location;
+  UInt64                parent_session;
+  UInt64                session;
+  USBDeviceAddress      address;
+  char                  sys_path[21];
+  usb_device_t        **device;
+  io_service_t          service;
+  int                   open_count;
+  UInt8                 first_config, active_config, port;
+  int                   can_enumerate;
+  int                   refcount;
+  bool                  in_reenumerate;
+  int                   capture_count;
+};
+
+struct darwin_device_priv {
+  struct darwin_cached_device *dev;
+};
+
+struct darwin_device_handle_priv {
+  bool                 is_open;
+  CFRunLoopSourceRef   cfSource;
+
+  struct darwin_interface {
+    usb_interface_t    **interface;
+    uint8_t              num_endpoints;
+    CFRunLoopSourceRef   cfSource;
+    uint64_t             frames[256];
+    uint8_t              endpoint_addrs[USB_MAXENDPOINTS];
+  } interfaces[USB_MAXINTERFACES];
+};
+
+struct darwin_transfer_priv {
+  /* Isoc */
+  IOUSBIsocFrame *isoc_framelist;
+  int num_iso_packets;
+
+  /* Control */
+  IOUSBDevRequestTO req;
+
+  /* Bulk */
+
+  /* Completion status */
+  IOReturn result;
+  UInt32 size;
+};
+
+#endif
diff --git a/hardware/src/libusb/os/events_posix.c b/hardware/src/libusb/os/events_posix.c
new file mode 100644
index 0000000..715a2d5
--- /dev/null
+++ b/hardware/src/libusb/os/events_posix.c
@@ -0,0 +1,300 @@
+/*
+ * libusb event abstraction on POSIX platforms
+ *
+ * Copyright © 2020 Chris Dickens <christopher.a.dickens@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#ifdef HAVE_EVENTFD
+#include <sys/eventfd.h>
+#endif
+#ifdef HAVE_TIMERFD
+#include <sys/timerfd.h>
+#endif
+#include <unistd.h>
+
+#ifdef HAVE_EVENTFD
+#define EVENT_READ_FD(e)	((e)->eventfd)
+#define EVENT_WRITE_FD(e)	((e)->eventfd)
+#else
+#define EVENT_READ_FD(e)	((e)->pipefd[0])
+#define EVENT_WRITE_FD(e)	((e)->pipefd[1])
+#endif
+
+#ifdef HAVE_NFDS_T
+typedef nfds_t usbi_nfds_t;
+#else
+typedef unsigned int usbi_nfds_t;
+#endif
+
+int usbi_create_event(usbi_event_t *event)
+{
+#ifdef HAVE_EVENTFD
+	event->eventfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
+	if (event->eventfd == -1) {
+		usbi_err(NULL, "failed to create eventfd, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return 0;
+#else
+#if defined(HAVE_PIPE2)
+	int ret = pipe2(event->pipefd, O_CLOEXEC);
+#else
+	int ret = pipe(event->pipefd);
+#endif
+
+	if (ret != 0) {
+		usbi_err(NULL, "failed to create pipe, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+#if !defined(HAVE_PIPE2) && defined(FD_CLOEXEC)
+	ret = fcntl(event->pipefd[0], F_GETFD);
+	if (ret == -1) {
+		usbi_err(NULL, "failed to get pipe fd flags, errno=%d", errno);
+		goto err_close_pipe;
+	}
+	ret = fcntl(event->pipefd[0], F_SETFD, ret | FD_CLOEXEC);
+	if (ret == -1) {
+		usbi_err(NULL, "failed to set pipe fd flags, errno=%d", errno);
+		goto err_close_pipe;
+	}
+
+	ret = fcntl(event->pipefd[1], F_GETFD);
+	if (ret == -1) {
+		usbi_err(NULL, "failed to get pipe fd flags, errno=%d", errno);
+		goto err_close_pipe;
+	}
+	ret = fcntl(event->pipefd[1], F_SETFD, ret | FD_CLOEXEC);
+	if (ret == -1) {
+		usbi_err(NULL, "failed to set pipe fd flags, errno=%d", errno);
+		goto err_close_pipe;
+	}
+#endif
+
+	ret = fcntl(event->pipefd[1], F_GETFL);
+	if (ret == -1) {
+		usbi_err(NULL, "failed to get pipe fd status flags, errno=%d", errno);
+		goto err_close_pipe;
+	}
+	ret = fcntl(event->pipefd[1], F_SETFL, ret | O_NONBLOCK);
+	if (ret == -1) {
+		usbi_err(NULL, "failed to set pipe fd status flags, errno=%d", errno);
+		goto err_close_pipe;
+	}
+
+	return 0;
+
+err_close_pipe:
+	close(event->pipefd[1]);
+	close(event->pipefd[0]);
+	return LIBUSB_ERROR_OTHER;
+#endif
+}
+
+void usbi_destroy_event(usbi_event_t *event)
+{
+#ifdef HAVE_EVENTFD
+	if (close(event->eventfd) == -1)
+		usbi_warn(NULL, "failed to close eventfd, errno=%d", errno);
+#else
+	if (close(event->pipefd[1]) == -1)
+		usbi_warn(NULL, "failed to close pipe write end, errno=%d", errno);
+	if (close(event->pipefd[0]) == -1)
+		usbi_warn(NULL, "failed to close pipe read end, errno=%d", errno);
+#endif
+}
+
+void usbi_signal_event(usbi_event_t *event)
+{
+	uint64_t dummy = 1;
+	ssize_t r;
+
+	r = write(EVENT_WRITE_FD(event), &dummy, sizeof(dummy));
+	if (r != sizeof(dummy))
+		usbi_warn(NULL, "event write failed");
+}
+
+void usbi_clear_event(usbi_event_t *event)
+{
+	uint64_t dummy;
+	ssize_t r;
+
+	r = read(EVENT_READ_FD(event), &dummy, sizeof(dummy));
+	if (r != sizeof(dummy))
+		usbi_warn(NULL, "event read failed");
+}
+
+#ifdef HAVE_TIMERFD
+int usbi_create_timer(usbi_timer_t *timer)
+{
+	timer->timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK | TFD_CLOEXEC);
+	if (timer->timerfd == -1) {
+		usbi_warn(NULL, "failed to create timerfd, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return 0;
+}
+
+void usbi_destroy_timer(usbi_timer_t *timer)
+{
+	if (close(timer->timerfd) == -1)
+		usbi_warn(NULL, "failed to close timerfd, errno=%d", errno);
+}
+
+int usbi_arm_timer(usbi_timer_t *timer, const struct timespec *timeout)
+{
+	const struct itimerspec it = { { 0, 0 }, { timeout->tv_sec, timeout->tv_nsec } };
+
+	if (timerfd_settime(timer->timerfd, TFD_TIMER_ABSTIME, &it, NULL) == -1) {
+		usbi_warn(NULL, "failed to arm timerfd, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return 0;
+}
+
+int usbi_disarm_timer(usbi_timer_t *timer)
+{
+	const struct itimerspec it = { { 0, 0 }, { 0, 0 } };
+
+	if (timerfd_settime(timer->timerfd, 0, &it, NULL) == -1) {
+		usbi_warn(NULL, "failed to disarm timerfd, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return 0;
+}
+#endif
+
+int usbi_alloc_event_data(struct libusb_context *ctx)
+{
+	struct usbi_event_source *ievent_source;
+	struct pollfd *fds;
+	size_t i = 0;
+
+	if (ctx->event_data) {
+		free(ctx->event_data);
+		ctx->event_data = NULL;
+	}
+
+	ctx->event_data_cnt = 0;
+	for_each_event_source(ctx, ievent_source)
+		ctx->event_data_cnt++;
+
+	fds = calloc(ctx->event_data_cnt, sizeof(*fds));
+	if (!fds)
+		return LIBUSB_ERROR_NO_MEM;
+
+	for_each_event_source(ctx, ievent_source) {
+		fds[i].fd = ievent_source->data.os_handle;
+		fds[i].events = ievent_source->data.poll_events;
+		i++;
+	}
+
+	ctx->event_data = fds;
+	return 0;
+}
+
+int usbi_wait_for_events(struct libusb_context *ctx,
+	struct usbi_reported_events *reported_events, int timeout_ms)
+{
+	struct pollfd *fds = ctx->event_data;
+	usbi_nfds_t nfds = (usbi_nfds_t)ctx->event_data_cnt;
+	int internal_fds, num_ready;
+
+	usbi_dbg(ctx, "poll() %u fds with timeout in %dms", (unsigned int)nfds, timeout_ms);
+	num_ready = poll(fds, nfds, timeout_ms);
+	usbi_dbg(ctx, "poll() returned %d", num_ready);
+	if (num_ready == 0) {
+		if (usbi_using_timer(ctx))
+			goto done;
+		return LIBUSB_ERROR_TIMEOUT;
+	} else if (num_ready == -1) {
+		if (errno == EINTR)
+			return LIBUSB_ERROR_INTERRUPTED;
+		usbi_err(ctx, "poll() failed, errno=%d", errno);
+		return LIBUSB_ERROR_IO;
+	}
+
+	/* fds[0] is always the internal signalling event */
+	if (fds[0].revents) {
+		reported_events->event_triggered = 1;
+		num_ready--;
+	} else {
+		reported_events->event_triggered = 0;
+	}
+
+#ifdef HAVE_OS_TIMER
+	/* on timer configurations, fds[1] is the timer */
+	if (usbi_using_timer(ctx) && fds[1].revents) {
+		reported_events->timer_triggered = 1;
+		num_ready--;
+	} else {
+		reported_events->timer_triggered = 0;
+	}
+#endif
+
+	if (!num_ready)
+		goto done;
+
+	/* the backend will never need to attempt to handle events on the
+	 * library's internal file descriptors, so we determine how many are
+	 * in use internally for this context and skip these when passing any
+	 * remaining pollfds to the backend. */
+	internal_fds = usbi_using_timer(ctx) ? 2 : 1;
+	fds += internal_fds;
+	nfds -= internal_fds;
+
+	usbi_mutex_lock(&ctx->event_data_lock);
+	if (ctx->event_flags & USBI_EVENT_EVENT_SOURCES_MODIFIED) {
+		struct usbi_event_source *ievent_source;
+
+		for_each_removed_event_source(ctx, ievent_source) {
+			usbi_nfds_t n;
+
+			for (n = 0; n < nfds; n++) {
+				if (ievent_source->data.os_handle != fds[n].fd)
+					continue;
+				if (!fds[n].revents)
+					continue;
+				/* pollfd was removed between the creation of the fds array and
+				 * here. remove triggered revent as it is no longer relevant. */
+				usbi_dbg(ctx, "fd %d was removed, ignoring raised events", fds[n].fd);
+				fds[n].revents = 0;
+				num_ready--;
+				break;
+			}
+		}
+	}
+	usbi_mutex_unlock(&ctx->event_data_lock);
+
+	if (num_ready) {
+		assert(num_ready > 0);
+		reported_events->event_data = fds;
+		reported_events->event_data_count = (unsigned int)nfds;
+	}
+
+done:
+	reported_events->num_ready = num_ready;
+	return LIBUSB_SUCCESS;
+}
diff --git a/hardware/src/libusb/os/events_posix.h b/hardware/src/libusb/os/events_posix.h
new file mode 100644
index 0000000..d81b5c4
--- /dev/null
+++ b/hardware/src/libusb/os/events_posix.h
@@ -0,0 +1,59 @@
+/*
+ * libusb event abstraction on POSIX platforms
+ *
+ * Copyright © 2020 Chris Dickens <christopher.a.dickens@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef LIBUSB_EVENTS_POSIX_H
+#define LIBUSB_EVENTS_POSIX_H
+
+#include <poll.h>
+
+typedef int usbi_os_handle_t;
+#define USBI_OS_HANDLE_FORMAT_STRING	"fd %d"
+
+#ifdef HAVE_EVENTFD
+typedef struct usbi_event {
+	int eventfd;
+} usbi_event_t;
+#define USBI_EVENT_OS_HANDLE(e)	((e)->eventfd)
+#define USBI_EVENT_POLL_EVENTS	POLLIN
+#define USBI_INVALID_EVENT	{ -1 }
+#else
+typedef struct usbi_event {
+	int pipefd[2];
+} usbi_event_t;
+#define USBI_EVENT_OS_HANDLE(e)	((e)->pipefd[0])
+#define USBI_EVENT_POLL_EVENTS	POLLIN
+#define USBI_INVALID_EVENT	{ { -1, -1 } }
+#endif
+
+#ifdef HAVE_TIMERFD
+#define HAVE_OS_TIMER 1
+typedef struct usbi_timer {
+	int timerfd;
+} usbi_timer_t;
+#define USBI_TIMER_OS_HANDLE(t)	((t)->timerfd)
+#define USBI_TIMER_POLL_EVENTS	POLLIN
+
+static inline int usbi_timer_valid(usbi_timer_t *timer)
+{
+	return timer->timerfd >= 0;
+}
+#endif
+
+#endif
diff --git a/hardware/src/libusb/os/linux_udev.c b/hardware/src/libusb/os/linux_udev.c
new file mode 100644
index 0000000..9ec9eb1
--- /dev/null
+++ b/hardware/src/libusb/os/linux_udev.c
@@ -0,0 +1,321 @@
+/* -*- Mode: C; c-basic-offset:8 ; indent-tabs-mode:t -*- */
+/*
+ * Linux usbfs backend for libusb
+ * Copyright (C) 2007-2009 Daniel Drake <dsd@gentoo.org>
+ * Copyright (c) 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ * Copyright (c) 2012-2013 Nathan Hjelm <hjelmn@mac.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+#include "linux_usbfs.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#include <libudev.h>
+#include <poll.h>
+#include <pthread.h>
+#include <string.h>
+#include <unistd.h>
+
+/* udev context */
+static struct udev *udev_ctx = NULL;
+static int udev_monitor_fd = -1;
+static usbi_event_t udev_control_event = USBI_INVALID_EVENT;
+static struct udev_monitor *udev_monitor = NULL;
+static pthread_t linux_event_thread;
+
+static void udev_hotplug_event(struct udev_device *udev_dev);
+static void *linux_udev_event_thread_main(void *arg);
+
+int linux_udev_start_event_monitor(void)
+{
+	int r;
+
+	assert(udev_ctx == NULL);
+	udev_ctx = udev_new();
+	if (!udev_ctx) {
+		usbi_err(NULL, "could not create udev context");
+		goto err;
+	}
+
+	udev_monitor = udev_monitor_new_from_netlink(udev_ctx, "udev");
+	if (!udev_monitor) {
+		usbi_err(NULL, "could not initialize udev monitor");
+		goto err_free_ctx;
+	}
+
+	r = udev_monitor_filter_add_match_subsystem_devtype(udev_monitor, "usb", "usb_device");
+	if (r) {
+		usbi_err(NULL, "could not initialize udev monitor filter for \"usb\" subsystem");
+		goto err_free_monitor;
+	}
+
+	if (udev_monitor_enable_receiving(udev_monitor)) {
+		usbi_err(NULL, "failed to enable the udev monitor");
+		goto err_free_monitor;
+	}
+
+	udev_monitor_fd = udev_monitor_get_fd(udev_monitor);
+
+#if defined(FD_CLOEXEC)
+	/* Make sure the udev file descriptor is marked as CLOEXEC */
+	r = fcntl(udev_monitor_fd, F_GETFD);
+	if (r == -1) {
+		usbi_err(NULL, "failed to get udev monitor fd flags, errno=%d", errno);
+		goto err_free_monitor;
+	}
+	if (!(r & FD_CLOEXEC)) {
+		if (fcntl(udev_monitor_fd, F_SETFD, r | FD_CLOEXEC) == -1) {
+			usbi_err(NULL, "failed to set udev monitor fd flags, errno=%d", errno);
+			goto err_free_monitor;
+		}
+	}
+#endif
+
+	/* Some older versions of udev are not non-blocking by default,
+	 * so make sure this is set */
+	r = fcntl(udev_monitor_fd, F_GETFL);
+	if (r == -1) {
+		usbi_err(NULL, "failed to get udev monitor fd status flags, errno=%d", errno);
+		goto err_free_monitor;
+	}
+	if (!(r & O_NONBLOCK)) {
+		if (fcntl(udev_monitor_fd, F_SETFL, r | O_NONBLOCK) == -1) {
+			usbi_err(NULL, "failed to set udev monitor fd status flags, errno=%d", errno);
+			goto err_free_monitor;
+		}
+	}
+
+	r = usbi_create_event(&udev_control_event);
+	if (r) {
+		usbi_err(NULL, "failed to create udev control event");
+		goto err_free_monitor;
+	}
+
+	r = pthread_create(&linux_event_thread, NULL, linux_udev_event_thread_main, NULL);
+	if (r) {
+		usbi_err(NULL, "failed to create hotplug event thread (%d)", r);
+		goto err_destroy_event;
+	}
+
+	return LIBUSB_SUCCESS;
+
+err_destroy_event:
+	usbi_destroy_event(&udev_control_event);
+	udev_control_event = (usbi_event_t)USBI_INVALID_EVENT;
+err_free_monitor:
+	udev_monitor_unref(udev_monitor);
+	udev_monitor = NULL;
+	udev_monitor_fd = -1;
+err_free_ctx:
+	udev_unref(udev_ctx);
+err:
+	udev_ctx = NULL;
+	return LIBUSB_ERROR_OTHER;
+}
+
+int linux_udev_stop_event_monitor(void)
+{
+	int r;
+
+	assert(udev_ctx != NULL);
+	assert(udev_monitor != NULL);
+	assert(udev_monitor_fd != -1);
+
+	/* Signal the control event and wait for the thread to exit */
+	usbi_signal_event(&udev_control_event);
+
+	r = pthread_join(linux_event_thread, NULL);
+	if (r)
+		usbi_warn(NULL, "failed to join hotplug event thread (%d)", r);
+
+	usbi_destroy_event(&udev_control_event);
+	udev_control_event = (usbi_event_t)USBI_INVALID_EVENT;
+
+	/* Release the udev monitor */
+	udev_monitor_unref(udev_monitor);
+	udev_monitor = NULL;
+	udev_monitor_fd = -1;
+
+	/* Clean up the udev context */
+	udev_unref(udev_ctx);
+	udev_ctx = NULL;
+
+	return LIBUSB_SUCCESS;
+}
+
+static void *linux_udev_event_thread_main(void *arg)
+{
+	struct pollfd fds[] = {
+		{ .fd = USBI_EVENT_OS_HANDLE(&udev_control_event),
+		  .events = USBI_EVENT_POLL_EVENTS },
+		{ .fd = udev_monitor_fd,
+		  .events = POLLIN },
+	};
+	struct udev_device *udev_dev;
+	int r;
+
+	UNUSED(arg);
+
+#if defined(HAVE_PTHREAD_SETNAME_NP)
+	r = pthread_setname_np(pthread_self(), "libusb_event");
+	if (r)
+		usbi_warn(NULL, "failed to set hotplug event thread name, error=%d", r);
+#endif
+
+	usbi_dbg(NULL, "udev event thread entering");
+
+	while (1) {
+		r = poll(fds, 2, -1);
+		if (r == -1) {
+			/* check for temporary failure */
+			if (errno == EINTR)
+				continue;
+			usbi_err(NULL, "poll() failed, errno=%d", errno);
+			break;
+		}
+		if (fds[0].revents) {
+			/* activity on control event, exit */
+			break;
+		}
+		if (fds[1].revents) {
+			usbi_mutex_static_lock(&linux_hotplug_lock);
+			udev_dev = udev_monitor_receive_device(udev_monitor);
+			if (udev_dev)
+				udev_hotplug_event(udev_dev);
+			usbi_mutex_static_unlock(&linux_hotplug_lock);
+		}
+	}
+
+	usbi_dbg(NULL, "udev event thread exiting");
+
+	return NULL;
+}
+
+static int udev_device_info(struct libusb_context *ctx, int detached,
+			    struct udev_device *udev_dev, uint8_t *busnum,
+			    uint8_t *devaddr, const char **sys_name) {
+	const char *dev_node;
+
+	dev_node = udev_device_get_devnode(udev_dev);
+	if (!dev_node) {
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	*sys_name = udev_device_get_sysname(udev_dev);
+	if (!*sys_name) {
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return linux_get_device_address(ctx, detached, busnum, devaddr,
+					dev_node, *sys_name, -1);
+}
+
+static void udev_hotplug_event(struct udev_device *udev_dev)
+{
+	const char *udev_action;
+	const char *sys_name = NULL;
+	uint8_t busnum = 0, devaddr = 0;
+	int detached;
+	int r;
+
+	do {
+		udev_action = udev_device_get_action(udev_dev);
+		if (!udev_action) {
+			break;
+		}
+
+		detached = !strncmp(udev_action, "remove", 6);
+
+		r = udev_device_info(NULL, detached, udev_dev, &busnum, &devaddr, &sys_name);
+		if (LIBUSB_SUCCESS != r) {
+			break;
+		}
+
+		usbi_dbg(NULL, "udev hotplug event. action: %s.", udev_action);
+
+		if (strncmp(udev_action, "add", 3) == 0) {
+			linux_hotplug_enumerate(busnum, devaddr, sys_name);
+		} else if (detached) {
+			linux_device_disconnected(busnum, devaddr);
+		} else if (strncmp(udev_action, "bind", 4) == 0) {
+			/* silently ignore "known unhandled" action */
+		} else {
+			usbi_err(NULL, "ignoring udev action %s", udev_action);
+		}
+	} while (0);
+
+	udev_device_unref(udev_dev);
+}
+
+int linux_udev_scan_devices(struct libusb_context *ctx)
+{
+	struct udev_enumerate *enumerator;
+	struct udev_list_entry *devices, *entry;
+	struct udev_device *udev_dev;
+	const char *sys_name;
+	int r;
+
+	assert(udev_ctx != NULL);
+
+	enumerator = udev_enumerate_new(udev_ctx);
+	if (NULL == enumerator) {
+		usbi_err(ctx, "error creating udev enumerator");
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	udev_enumerate_add_match_subsystem(enumerator, "usb");
+	udev_enumerate_add_match_property(enumerator, "DEVTYPE", "usb_device");
+	udev_enumerate_scan_devices(enumerator);
+	devices = udev_enumerate_get_list_entry(enumerator);
+
+	entry = NULL;
+	udev_list_entry_foreach(entry, devices) {
+		const char *path = udev_list_entry_get_name(entry);
+		uint8_t busnum = 0, devaddr = 0;
+
+		udev_dev = udev_device_new_from_syspath(udev_ctx, path);
+
+		r = udev_device_info(ctx, 0, udev_dev, &busnum, &devaddr, &sys_name);
+		if (r) {
+			udev_device_unref(udev_dev);
+			continue;
+		}
+
+		linux_enumerate_device(ctx, busnum, devaddr, sys_name);
+		udev_device_unref(udev_dev);
+	}
+
+	udev_enumerate_unref(enumerator);
+
+	return LIBUSB_SUCCESS;
+}
+
+void linux_udev_hotplug_poll(void)
+{
+	struct udev_device *udev_dev;
+
+	usbi_mutex_static_lock(&linux_hotplug_lock);
+	do {
+		udev_dev = udev_monitor_receive_device(udev_monitor);
+		if (udev_dev) {
+			usbi_dbg(NULL, "Handling hotplug event from hotplug_poll");
+			udev_hotplug_event(udev_dev);
+		}
+	} while (udev_dev);
+	usbi_mutex_static_unlock(&linux_hotplug_lock);
+}
diff --git a/hardware/src/libusb/os/linux_usbfs.c b/hardware/src/libusb/os/linux_usbfs.c
new file mode 100644
index 0000000..c300675
--- /dev/null
+++ b/hardware/src/libusb/os/linux_usbfs.c
@@ -0,0 +1,2807 @@
+/* -*- Mode: C; c-basic-offset:8 ; indent-tabs-mode:t -*- */
+/*
+ * Linux usbfs backend for libusb
+ * Copyright © 2007-2009 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ * Copyright © 2013 Nathan Hjelm <hjelmn@mac.com>
+ * Copyright © 2012-2013 Hans de Goede <hdegoede@redhat.com>
+ * Copyright © 2020 Chris Dickens <christopher.a.dickens@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+#include "linux_usbfs.h"
+
+#include <alloca.h>
+#include <ctype.h>
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <sys/utsname.h>
+#include <sys/vfs.h>
+#include <unistd.h>
+
+/* sysfs vs usbfs:
+ * opening a usbfs node causes the device to be resumed, so we attempt to
+ * avoid this during enumeration.
+ *
+ * sysfs allows us to read the kernel's in-memory copies of device descriptors
+ * and so forth, avoiding the need to open the device:
+ *  - The binary "descriptors" file contains all config descriptors since
+ *    2.6.26, commit 217a9081d8e69026186067711131b77f0ce219ed
+ *  - The binary "descriptors" file was added in 2.6.23, commit
+ *    69d42a78f935d19384d1f6e4f94b65bb162b36df, but it only contains the
+ *    active config descriptors
+ *  - The "busnum" file was added in 2.6.22, commit
+ *    83f7d958eab2fbc6b159ee92bf1493924e1d0f72
+ *  - The "devnum" file has been present since pre-2.6.18
+ *  - the "bConfigurationValue" file has been present since pre-2.6.18
+ *
+ * If we have bConfigurationValue, busnum, and devnum, then we can determine
+ * the active configuration without having to open the usbfs node in RDWR mode.
+ * The busnum file is important as that is the only way we can relate sysfs
+ * devices to usbfs nodes.
+ *
+ * If we also have all descriptors, we can obtain the device descriptor and
+ * configuration without touching usbfs at all.
+ */
+
+/* endianness for multi-byte fields:
+ *
+ * Descriptors exposed by usbfs have the multi-byte fields in the device
+ * descriptor as host endian. Multi-byte fields in the other descriptors are
+ * bus-endian. The kernel documentation says otherwise, but it is wrong.
+ *
+ * In sysfs all descriptors are bus-endian.
+ */
+
+#define USBDEV_PATH		"/dev"
+#define USB_DEVTMPFS_PATH	"/dev/bus/usb"
+
+/* use usbdev*.* device names in /dev instead of the usbfs bus directories */
+static int usbdev_names = 0;
+
+/* Linux has changed the maximum length of an individual isochronous packet
+ * over time.  Initially this limit was 1,023 bytes, but Linux 2.6.18
+ * (commit 3612242e527eb47ee4756b5350f8bdf791aa5ede) increased this value to
+ * 8,192 bytes to support higher bandwidth devices.  Linux 3.10
+ * (commit e2e2f0ea1c935edcf53feb4c4c8fdb4f86d57dd9) further increased this
+ * value to 49,152 bytes to support super speed devices.  Linux 5.2
+ * (commit 8a1dbc8d91d3d1602282c7e6b4222c7759c916fa) even further increased
+ * this value to 98,304 bytes to support super speed plus devices.
+ */
+static unsigned int max_iso_packet_len = 0;
+
+/* is sysfs available (mounted) ? */
+static int sysfs_available = -1;
+
+/* how many times have we initted (and not exited) ? */
+static int init_count = 0;
+
+/* have no authority to operate usb device directly */
+static int no_enumeration = 0;
+
+/* Serialize scan-devices, event-thread, and poll */
+usbi_mutex_static_t linux_hotplug_lock = USBI_MUTEX_INITIALIZER;
+
+static int linux_scan_devices(struct libusb_context *ctx);
+static int detach_kernel_driver_and_claim(struct libusb_device_handle *, uint8_t);
+
+#if !defined(HAVE_LIBUDEV)
+static int linux_default_scan_devices(struct libusb_context *ctx);
+#endif
+
+struct kernel_version {
+	int major;
+	int minor;
+	int sublevel;
+};
+
+struct config_descriptor {
+	struct usbi_configuration_descriptor *desc;
+	size_t actual_len;
+};
+
+struct linux_device_priv {
+	char *sysfs_dir;
+	void *descriptors;
+	size_t descriptors_len;
+	struct config_descriptor *config_descriptors;
+	int active_config; /* cache val for !sysfs_available  */
+};
+
+struct linux_device_handle_priv {
+	int fd;
+	int fd_removed;
+	int fd_keep;
+	uint32_t caps;
+};
+
+enum reap_action {
+	NORMAL = 0,
+	/* submission failed after the first URB, so await cancellation/completion
+	 * of all the others */
+	SUBMIT_FAILED,
+
+	/* cancelled by user or timeout */
+	CANCELLED,
+
+	/* completed multi-URB transfer in non-final URB */
+	COMPLETED_EARLY,
+
+	/* one or more urbs encountered a low-level error */
+	ERROR,
+};
+
+struct linux_transfer_priv {
+	union {
+		struct usbfs_urb *urbs;
+		struct usbfs_urb **iso_urbs;
+	};
+
+	enum reap_action reap_action;
+	int num_urbs;
+	int num_retired;
+	enum libusb_transfer_status reap_status;
+
+	/* next iso packet in user-supplied transfer to be populated */
+	int iso_packet_offset;
+};
+
+static int dev_has_config0(struct libusb_device *dev)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+	struct config_descriptor *config;
+	uint8_t idx;
+
+	for (idx = 0; idx < dev->device_descriptor.bNumConfigurations; idx++) {
+		config = &priv->config_descriptors[idx];
+		if (config->desc->bConfigurationValue == 0)
+			return 1;
+	}
+
+	return 0;
+}
+
+static int get_usbfs_fd(struct libusb_device *dev, mode_t mode, int silent)
+{
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+	char path[24];
+	int fd;
+
+	if (usbdev_names)
+		sprintf(path, USBDEV_PATH "/usbdev%u.%u",
+			dev->bus_number, dev->device_address);
+	else
+		sprintf(path, USB_DEVTMPFS_PATH "/%03u/%03u",
+			dev->bus_number, dev->device_address);
+
+	fd = open(path, mode | O_CLOEXEC);
+	if (fd != -1)
+		return fd; /* Success */
+
+	if (errno == ENOENT) {
+		const long delay_ms = 10L;
+		const struct timespec delay_ts = { 0L, delay_ms * 1000L * 1000L };
+
+		if (!silent)
+			usbi_err(ctx, "File doesn't exist, wait %ld ms and try again", delay_ms);
+
+		/* Wait 10ms for USB device path creation.*/
+		nanosleep(&delay_ts, NULL);
+
+		fd = open(path, mode | O_CLOEXEC);
+		if (fd != -1)
+			return fd; /* Success */
+	}
+
+	if (!silent) {
+		usbi_err(ctx, "libusb couldn't open USB device %s, errno=%d", path, errno);
+		if (errno == EACCES && mode == O_RDWR)
+			usbi_err(ctx, "libusb requires write access to USB device nodes");
+	}
+
+	if (errno == EACCES)
+		return LIBUSB_ERROR_ACCESS;
+	if (errno == ENOENT)
+		return LIBUSB_ERROR_NO_DEVICE;
+	return LIBUSB_ERROR_IO;
+}
+
+/* check dirent for a /dev/usbdev%d.%d name
+ * optionally return bus/device on success */
+static int is_usbdev_entry(const char *name, uint8_t *bus_p, uint8_t *dev_p)
+{
+	int busnum, devnum;
+
+	if (sscanf(name, "usbdev%d.%d", &busnum, &devnum) != 2)
+		return 0;
+	if (busnum < 0 || busnum > UINT8_MAX || devnum < 0 || devnum > UINT8_MAX) {
+		usbi_dbg(NULL, "invalid usbdev format '%s'", name);
+		return 0;
+	}
+
+	usbi_dbg(NULL, "found: %s", name);
+	if (bus_p)
+		*bus_p = (uint8_t)busnum;
+	if (dev_p)
+		*dev_p = (uint8_t)devnum;
+	return 1;
+}
+
+static const char *find_usbfs_path(void)
+{
+	const char *path;
+	DIR *dir;
+	struct dirent *entry;
+
+	path = USB_DEVTMPFS_PATH;
+	dir = opendir(path);
+	if (dir) {
+		while ((entry = readdir(dir))) {
+			if (entry->d_name[0] == '.')
+				continue;
+
+			/* We assume if we find any files that it must be the right place */
+			break;
+		}
+
+		closedir(dir);
+
+		if (entry)
+			return path;
+	}
+
+	/* look for /dev/usbdev*.* if the normal place fails */
+	path = USBDEV_PATH;
+	dir = opendir(path);
+	if (dir) {
+		while ((entry = readdir(dir))) {
+			if (entry->d_name[0] == '.')
+				continue;
+
+			if (is_usbdev_entry(entry->d_name, NULL, NULL)) {
+				/* found one; that's enough */
+				break;
+			}
+		}
+
+		closedir(dir);
+
+		if (entry) {
+			usbdev_names = 1;
+			return path;
+		}
+	}
+
+/* On udev based systems without any usb-devices /dev/bus/usb will not
+ * exist. So if we've not found anything and we're using udev for hotplug
+ * simply assume /dev/bus/usb rather then making libusb_init fail.
+ * Make the same assumption for Android where SELinux policies might block us
+ * from reading /dev on newer devices. */
+#if defined(HAVE_LIBUDEV) || defined(__ANDROID__)
+	return USB_DEVTMPFS_PATH;
+#else
+	return NULL;
+#endif
+}
+
+static int get_kernel_version(struct libusb_context *ctx,
+	struct kernel_version *ver)
+{
+	struct utsname uts;
+	int atoms;
+
+	if (uname(&uts) < 0) {
+		usbi_err(ctx, "uname failed, errno=%d", errno);
+		return -1;
+	}
+
+	atoms = sscanf(uts.release, "%d.%d.%d", &ver->major, &ver->minor, &ver->sublevel);
+	if (atoms < 2) {
+		usbi_err(ctx, "failed to parse uname release '%s'", uts.release);
+		return -1;
+	}
+
+	if (atoms < 3)
+		ver->sublevel = -1;
+
+	usbi_dbg(ctx, "reported kernel version is %s", uts.release);
+
+	return 0;
+}
+
+static int kernel_version_ge(const struct kernel_version *ver,
+	int major, int minor, int sublevel)
+{
+	if (ver->major > major)
+		return 1;
+	else if (ver->major < major)
+		return 0;
+
+	/* kmajor == major */
+	if (ver->minor > minor)
+		return 1;
+	else if (ver->minor < minor)
+		return 0;
+
+	/* kminor == minor */
+	if (ver->sublevel == -1)
+		return sublevel == 0;
+
+	return ver->sublevel >= sublevel;
+}
+
+static int op_init(struct libusb_context *ctx)
+{
+	struct kernel_version kversion;
+	const char *usbfs_path;
+	int r;
+
+	if (get_kernel_version(ctx, &kversion) < 0)
+		return LIBUSB_ERROR_OTHER;
+
+	if (!kernel_version_ge(&kversion, 2, 6, 32)) {
+		usbi_err(ctx, "kernel version is too old (reported as %d.%d.%d)",
+			 kversion.major, kversion.minor,
+			 kversion.sublevel != -1 ? kversion.sublevel : 0);
+		return LIBUSB_ERROR_NOT_SUPPORTED;
+	}
+
+	usbfs_path = find_usbfs_path();
+	if (!usbfs_path) {
+		usbi_err(ctx, "could not find usbfs");
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	usbi_dbg(ctx, "found usbfs at %s", usbfs_path);
+
+	if (!max_iso_packet_len) {
+		if (kernel_version_ge(&kversion, 5, 2, 0))
+			max_iso_packet_len = 98304;
+		else if (kernel_version_ge(&kversion, 3, 10, 0))
+			max_iso_packet_len = 49152;
+		else
+			max_iso_packet_len = 8192;
+	}
+
+	usbi_dbg(ctx, "max iso packet length is (likely) %u bytes", max_iso_packet_len);
+
+	if (sysfs_available == -1) {
+		struct statfs statfsbuf;
+
+		r = statfs(SYSFS_MOUNT_PATH, &statfsbuf);
+		if (r == 0 && statfsbuf.f_type == SYSFS_MAGIC) {
+			usbi_dbg(ctx, "sysfs is available");
+			sysfs_available = 1;
+		} else {
+			usbi_warn(ctx, "sysfs not mounted");
+			sysfs_available = 0;
+		}
+	}
+
+	if (no_enumeration) {
+		return LIBUSB_SUCCESS;
+	}
+
+	r = LIBUSB_SUCCESS;
+	if (init_count == 0) {
+		/* start up hotplug event handler */
+		r = linux_start_event_monitor();
+	}
+	if (r == LIBUSB_SUCCESS) {
+		r = linux_scan_devices(ctx);
+		if (r == LIBUSB_SUCCESS)
+			init_count++;
+		else if (init_count == 0)
+			linux_stop_event_monitor();
+	} else {
+		usbi_err(ctx, "error starting hotplug event monitor");
+	}
+
+	return r;
+}
+
+static void op_exit(struct libusb_context *ctx)
+{
+	UNUSED(ctx);
+
+	if (no_enumeration) {
+		return;
+	}
+
+	assert(init_count != 0);
+	if (!--init_count) {
+		/* tear down event handler */
+		linux_stop_event_monitor();
+	}
+}
+
+static int op_set_option(struct libusb_context *ctx, enum libusb_option option, va_list ap)
+{
+	UNUSED(ctx);
+	UNUSED(ap);
+
+	if (option == LIBUSB_OPTION_NO_DEVICE_DISCOVERY) {
+		usbi_dbg(ctx, "no enumeration will be performed");
+		no_enumeration = 1;
+		return LIBUSB_SUCCESS;
+	}
+
+	return LIBUSB_ERROR_NOT_SUPPORTED;
+}
+
+static int linux_scan_devices(struct libusb_context *ctx)
+{
+	int ret;
+
+	usbi_mutex_static_lock(&linux_hotplug_lock);
+
+#if defined(HAVE_LIBUDEV)
+	ret = linux_udev_scan_devices(ctx);
+#else
+	ret = linux_default_scan_devices(ctx);
+#endif
+
+	usbi_mutex_static_unlock(&linux_hotplug_lock);
+
+	return ret;
+}
+
+static void op_hotplug_poll(void)
+{
+	linux_hotplug_poll();
+}
+
+static int open_sysfs_attr(struct libusb_context *ctx,
+	const char *sysfs_dir, const char *attr)
+{
+	char filename[256];
+	int fd;
+
+	snprintf(filename, sizeof(filename), SYSFS_DEVICE_PATH "/%s/%s", sysfs_dir, attr);
+	fd = open(filename, O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		if (errno == ENOENT) {
+			/* File doesn't exist. Assume the device has been
+			   disconnected (see trac ticket #70). */
+			return LIBUSB_ERROR_NO_DEVICE;
+		}
+		usbi_err(ctx, "open %s failed, errno=%d", filename, errno);
+		return LIBUSB_ERROR_IO;
+	}
+
+	return fd;
+}
+
+/* Note only suitable for attributes which always read >= 0, < 0 is error */
+static int read_sysfs_attr(struct libusb_context *ctx,
+	const char *sysfs_dir, const char *attr, int max_value, int *value_p)
+{
+	char buf[20], *endptr;
+	long value;
+	ssize_t r;
+	int fd;
+
+	fd = open_sysfs_attr(ctx, sysfs_dir, attr);
+	if (fd < 0)
+		return fd;
+
+	r = read(fd, buf, sizeof(buf) - 1);
+	if (r < 0) {
+		r = errno;
+		close(fd);
+		if (r == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+		usbi_err(ctx, "attribute %s read failed, errno=%zd", attr, r);
+		return LIBUSB_ERROR_IO;
+	}
+	close(fd);
+
+	if (r == 0) {
+		/* Certain attributes (e.g. bConfigurationValue) are not
+		 * populated if the device is not configured. */
+		*value_p = -1;
+		return 0;
+	}
+
+	/* The kernel does *not* NUL-terminate the string, but every attribute
+	 * should be terminated with a newline character. */
+	if (!isdigit(buf[0])) {
+		usbi_err(ctx, "attribute %s doesn't have numeric value?", attr);
+		return LIBUSB_ERROR_IO;
+	} else if (buf[r - 1] != '\n') {
+		usbi_warn(ctx, "attribute %s doesn't end with newline?", attr);
+	} else {
+		/* Remove the terminating newline character */
+		r--;
+	}
+	buf[r] = '\0';
+
+	errno = 0;
+	value = strtol(buf, &endptr, 10);
+	if (value < 0 || value > (long)max_value || errno) {
+		usbi_err(ctx, "attribute %s contains an invalid value: '%s'", attr, buf);
+		return LIBUSB_ERROR_INVALID_PARAM;
+	} else if (*endptr != '\0') {
+		/* Consider the value to be valid if the remainder is a '.'
+		 * character followed by numbers.  This occurs, for example,
+		 * when reading the "speed" attribute for a low-speed device
+		 * (e.g. "1.5") */
+		if (*endptr == '.' && isdigit(*(endptr + 1))) {
+			endptr++;
+			while (isdigit(*endptr))
+				endptr++;
+		}
+		if (*endptr != '\0') {
+			usbi_err(ctx, "attribute %s contains an invalid value: '%s'", attr, buf);
+			return LIBUSB_ERROR_INVALID_PARAM;
+		}
+	}
+
+	*value_p = (int)value;
+	return 0;
+}
+
+static int sysfs_scan_device(struct libusb_context *ctx, const char *devname)
+{
+	uint8_t busnum, devaddr;
+	int ret;
+
+	ret = linux_get_device_address(ctx, 0, &busnum, &devaddr, NULL, devname, -1);
+	if (ret != LIBUSB_SUCCESS)
+		return ret;
+
+	return linux_enumerate_device(ctx, busnum, devaddr, devname);
+}
+
+/* read the bConfigurationValue for a device */
+static int sysfs_get_active_config(struct libusb_device *dev, int *config)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+
+	return read_sysfs_attr(DEVICE_CTX(dev), priv->sysfs_dir, "bConfigurationValue",
+			UINT8_MAX, config);
+}
+
+int linux_get_device_address(struct libusb_context *ctx, int detached,
+	uint8_t *busnum, uint8_t *devaddr, const char *dev_node,
+	const char *sys_name, int fd)
+{
+	int sysfs_val;
+	int r;
+
+	usbi_dbg(ctx, "getting address for device: %s detached: %d", sys_name, detached);
+	/* can't use sysfs to read the bus and device number if the
+	 * device has been detached */
+	if (!sysfs_available || detached || !sys_name) {
+		if (!dev_node && fd >= 0) {
+			char *fd_path = alloca(PATH_MAX);
+			char proc_path[32];
+
+			/* try to retrieve the device node from fd */
+			sprintf(proc_path, "/proc/self/fd/%d", fd);
+			r = readlink(proc_path, fd_path, PATH_MAX - 1);
+			if (r > 0) {
+				fd_path[r] = '\0';
+				dev_node = fd_path;
+			}
+		}
+
+		if (!dev_node)
+			return LIBUSB_ERROR_OTHER;
+
+		/* will this work with all supported kernel versions? */
+		if (!strncmp(dev_node, "/dev/bus/usb", 12))
+			sscanf(dev_node, "/dev/bus/usb/%hhu/%hhu", busnum, devaddr);
+		else
+			return LIBUSB_ERROR_OTHER;
+
+		return LIBUSB_SUCCESS;
+	}
+
+	usbi_dbg(ctx, "scan %s", sys_name);
+
+	r = read_sysfs_attr(ctx, sys_name, "busnum", UINT8_MAX, &sysfs_val);
+	if (r < 0)
+		return r;
+	*busnum = (uint8_t)sysfs_val;
+
+	r = read_sysfs_attr(ctx, sys_name, "devnum", UINT8_MAX, &sysfs_val);
+	if (r < 0)
+		return r;
+	*devaddr = (uint8_t)sysfs_val;
+
+	usbi_dbg(ctx, "bus=%u dev=%u", *busnum, *devaddr);
+
+	return LIBUSB_SUCCESS;
+}
+
+/* Return offset of the next config descriptor */
+static int seek_to_next_config(struct libusb_context *ctx,
+	uint8_t *buffer, size_t len)
+{
+	struct usbi_descriptor_header *header;
+	int offset;
+
+	/* Start seeking past the config descriptor */
+	offset = LIBUSB_DT_CONFIG_SIZE;
+	buffer += LIBUSB_DT_CONFIG_SIZE;
+	len -= LIBUSB_DT_CONFIG_SIZE;
+
+	while (len > 0) {
+		if (len < 2) {
+			usbi_err(ctx, "short descriptor read %zu/2", len);
+			return LIBUSB_ERROR_IO;
+		}
+
+		header = (struct usbi_descriptor_header *)buffer;
+		if (header->bDescriptorType == LIBUSB_DT_CONFIG)
+			return offset;
+
+		if (len < header->bLength) {
+			usbi_err(ctx, "bLength overflow by %zu bytes",
+				 (size_t)header->bLength - len);
+			return LIBUSB_ERROR_IO;
+		}
+
+		offset += header->bLength;
+		buffer += header->bLength;
+		len -= header->bLength;
+	}
+
+	usbi_err(ctx, "config descriptor not found");
+	return LIBUSB_ERROR_IO;
+}
+
+static int parse_config_descriptors(struct libusb_device *dev)
+{
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+	struct usbi_device_descriptor *device_desc;
+	uint8_t idx, num_configs;
+	uint8_t *buffer;
+	size_t remaining;
+
+	device_desc = priv->descriptors;
+	num_configs = device_desc->bNumConfigurations;
+
+	if (num_configs == 0)
+		return 0;	/* no configurations? */
+
+	priv->config_descriptors = malloc(num_configs * sizeof(priv->config_descriptors[0]));
+	if (!priv->config_descriptors)
+		return LIBUSB_ERROR_NO_MEM;
+
+	buffer = (uint8_t *)priv->descriptors + LIBUSB_DT_DEVICE_SIZE;
+	remaining = priv->descriptors_len - LIBUSB_DT_DEVICE_SIZE;
+
+	for (idx = 0; idx < num_configs; idx++) {
+		struct usbi_configuration_descriptor *config_desc;
+		uint16_t config_len;
+
+		if (remaining < LIBUSB_DT_CONFIG_SIZE) {
+			usbi_err(ctx, "short descriptor read %zu/%d",
+				 remaining, LIBUSB_DT_CONFIG_SIZE);
+			return LIBUSB_ERROR_IO;
+		}
+
+		config_desc = (struct usbi_configuration_descriptor *)buffer;
+		if (config_desc->bDescriptorType != LIBUSB_DT_CONFIG) {
+			usbi_err(ctx, "descriptor is not a config desc (type 0x%02x)",
+				 config_desc->bDescriptorType);
+			return LIBUSB_ERROR_IO;
+		} else if (config_desc->bLength < LIBUSB_DT_CONFIG_SIZE) {
+			usbi_err(ctx, "invalid descriptor bLength %u",
+				 config_desc->bLength);
+			return LIBUSB_ERROR_IO;
+		}
+
+		config_len = libusb_le16_to_cpu(config_desc->wTotalLength);
+		if (config_len < LIBUSB_DT_CONFIG_SIZE) {
+			usbi_err(ctx, "invalid wTotalLength %u", config_len);
+			return LIBUSB_ERROR_IO;
+		}
+
+		if (priv->sysfs_dir) {
+			/*
+			 * In sysfs wTotalLength is ignored, instead the kernel returns a
+			 * config descriptor with verified bLength fields, with descriptors
+			 * with an invalid bLength removed.
+			 */
+			uint16_t sysfs_config_len;
+			int offset;
+
+			if (num_configs > 1 && idx < num_configs - 1) {
+				offset = seek_to_next_config(ctx, buffer, remaining);
+				if (offset < 0)
+					return offset;
+				sysfs_config_len = (uint16_t)offset;
+			} else {
+				sysfs_config_len = (uint16_t)remaining;
+			}
+
+			if (config_len != sysfs_config_len) {
+				usbi_warn(ctx, "config length mismatch wTotalLength %u real %u",
+					  config_len, sysfs_config_len);
+				config_len = sysfs_config_len;
+			}
+		} else {
+			/*
+			 * In usbfs the config descriptors are wTotalLength bytes apart,
+			 * with any short reads from the device appearing as holes in the file.
+			 */
+			if (config_len > remaining) {
+				usbi_warn(ctx, "short descriptor read %zu/%u", remaining, config_len);
+				config_len = (uint16_t)remaining;
+			}
+		}
+
+		if (config_desc->bConfigurationValue == 0)
+			usbi_warn(ctx, "device has configuration 0");
+
+		priv->config_descriptors[idx].desc = config_desc;
+		priv->config_descriptors[idx].actual_len = config_len;
+
+		buffer += config_len;
+		remaining -= config_len;
+	}
+
+	return LIBUSB_SUCCESS;
+}
+
+static int op_get_config_descriptor_by_value(struct libusb_device *dev,
+	uint8_t value, void **buffer)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+	struct config_descriptor *config;
+	uint8_t idx;
+
+	for (idx = 0; idx < dev->device_descriptor.bNumConfigurations; idx++) {
+		config = &priv->config_descriptors[idx];
+		if (config->desc->bConfigurationValue == value) {
+			*buffer = config->desc;
+			return (int)config->actual_len;
+		}
+	}
+
+	return LIBUSB_ERROR_NOT_FOUND;
+}
+
+static int op_get_active_config_descriptor(struct libusb_device *dev,
+	void *buffer, size_t len)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+	void *config_desc;
+	int active_config;
+	int r;
+
+	if (priv->sysfs_dir) {
+		r = sysfs_get_active_config(dev, &active_config);
+		if (r < 0)
+			return r;
+	} else {
+		/* Use cached bConfigurationValue */
+		active_config = priv->active_config;
+	}
+
+	if (active_config == -1) {
+		usbi_err(DEVICE_CTX(dev), "device unconfigured");
+		return LIBUSB_ERROR_NOT_FOUND;
+	}
+
+	r = op_get_config_descriptor_by_value(dev, (uint8_t)active_config, &config_desc);
+	if (r < 0)
+		return r;
+
+	len = MIN(len, (size_t)r);
+	memcpy(buffer, config_desc, len);
+	return len;
+}
+
+static int op_get_config_descriptor(struct libusb_device *dev,
+	uint8_t config_index, void *buffer, size_t len)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+	struct config_descriptor *config;
+
+	if (config_index >= dev->device_descriptor.bNumConfigurations)
+		return LIBUSB_ERROR_NOT_FOUND;
+
+	config = &priv->config_descriptors[config_index];
+	len = MIN(len, config->actual_len);
+	memcpy(buffer, config->desc, len);
+	return len;
+}
+
+/* send a control message to retrieve active configuration */
+static int usbfs_get_active_config(struct libusb_device *dev, int fd)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+	uint8_t active_config = 0;
+	int r;
+
+	struct usbfs_ctrltransfer ctrl = {
+		.bmRequestType = LIBUSB_ENDPOINT_IN,
+		.bRequest = LIBUSB_REQUEST_GET_CONFIGURATION,
+		.wValue = 0,
+		.wIndex = 0,
+		.wLength = 1,
+		.timeout = 1000,
+		.data = &active_config
+	};
+
+	r = ioctl(fd, IOCTL_USBFS_CONTROL, &ctrl);
+	if (r < 0) {
+		if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		/* we hit this error path frequently with buggy devices :( */
+		usbi_warn(DEVICE_CTX(dev), "get configuration failed, errno=%d", errno);
+
+		/* assume the current configuration is the first one if we have
+		 * the configuration descriptors, otherwise treat the device
+		 * as unconfigured. */
+		if (priv->config_descriptors)
+			priv->active_config = (int)priv->config_descriptors[0].desc->bConfigurationValue;
+		else
+			priv->active_config = -1;
+	} else if (active_config == 0) {
+		if (dev_has_config0(dev)) {
+			/* some buggy devices have a configuration 0, but we're
+			 * reaching into the corner of a corner case here. */
+			priv->active_config = 0;
+		} else {
+			priv->active_config = -1;
+		}
+	} else {
+		priv->active_config = (int)active_config;
+	}
+
+	return LIBUSB_SUCCESS;
+}
+
+static enum libusb_speed usbfs_get_speed(struct libusb_context *ctx, int fd)
+{
+	int r;
+
+	r = ioctl(fd, IOCTL_USBFS_GET_SPEED, NULL);
+	switch (r) {
+	case USBFS_SPEED_UNKNOWN:	return LIBUSB_SPEED_UNKNOWN;
+	case USBFS_SPEED_LOW:		return LIBUSB_SPEED_LOW;
+	case USBFS_SPEED_FULL:		return LIBUSB_SPEED_FULL;
+	case USBFS_SPEED_HIGH:		return LIBUSB_SPEED_HIGH;
+	case USBFS_SPEED_WIRELESS:	return LIBUSB_SPEED_HIGH;
+	case USBFS_SPEED_SUPER:		return LIBUSB_SPEED_SUPER;
+	case USBFS_SPEED_SUPER_PLUS:	return LIBUSB_SPEED_SUPER_PLUS;
+	default:
+		usbi_warn(ctx, "Error getting device speed: %d", r);
+	}
+
+	return LIBUSB_SPEED_UNKNOWN;
+}
+
+static int initialize_device(struct libusb_device *dev, uint8_t busnum,
+	uint8_t devaddr, const char *sysfs_dir, int wrapped_fd)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+	size_t alloc_len;
+	int fd, speed, r;
+	ssize_t nb;
+
+	dev->bus_number = busnum;
+	dev->device_address = devaddr;
+
+	if (sysfs_dir) {
+		priv->sysfs_dir = strdup(sysfs_dir);
+		if (!priv->sysfs_dir)
+			return LIBUSB_ERROR_NO_MEM;
+
+		/* Note speed can contain 1.5, in this case read_sysfs_attr()
+		   will stop parsing at the '.' and return 1 */
+		if (read_sysfs_attr(ctx, sysfs_dir, "speed", INT_MAX, &speed) == 0) {
+			switch (speed) {
+			case     1: dev->speed = LIBUSB_SPEED_LOW; break;
+			case    12: dev->speed = LIBUSB_SPEED_FULL; break;
+			case   480: dev->speed = LIBUSB_SPEED_HIGH; break;
+			case  5000: dev->speed = LIBUSB_SPEED_SUPER; break;
+			case 10000: dev->speed = LIBUSB_SPEED_SUPER_PLUS; break;
+			default:
+				usbi_warn(ctx, "unknown device speed: %d Mbps", speed);
+			}
+		}
+	} else if (wrapped_fd >= 0) {
+		dev->speed = usbfs_get_speed(ctx, wrapped_fd);
+	}
+
+	/* cache descriptors in memory */
+	if (sysfs_dir) {
+		fd = open_sysfs_attr(ctx, sysfs_dir, "descriptors");
+	} else if (wrapped_fd < 0) {
+		fd = get_usbfs_fd(dev, O_RDONLY, 0);
+	} else {
+		fd = wrapped_fd;
+		r = lseek(fd, 0, SEEK_SET);
+		if (r < 0) {
+			usbi_err(ctx, "lseek failed, errno=%d", errno);
+			return LIBUSB_ERROR_IO;
+		}
+	}
+	if (fd < 0)
+		return fd;
+
+	alloc_len = 0;
+	do {
+		const size_t desc_read_length = 256;
+		uint8_t *read_ptr;
+
+		alloc_len += desc_read_length;
+		priv->descriptors = usbi_reallocf(priv->descriptors, alloc_len);
+		if (!priv->descriptors) {
+			if (fd != wrapped_fd)
+				close(fd);
+			return LIBUSB_ERROR_NO_MEM;
+		}
+		read_ptr = (uint8_t *)priv->descriptors + priv->descriptors_len;
+		/* usbfs has holes in the file */
+		if (!sysfs_dir)
+			memset(read_ptr, 0, desc_read_length);
+		nb = read(fd, read_ptr, desc_read_length);
+		if (nb < 0) {
+			usbi_err(ctx, "read descriptor failed, errno=%d", errno);
+			if (fd != wrapped_fd)
+				close(fd);
+			return LIBUSB_ERROR_IO;
+		}
+		priv->descriptors_len += (size_t)nb;
+	} while (priv->descriptors_len == alloc_len);
+
+	if (fd != wrapped_fd)
+		close(fd);
+
+	if (priv->descriptors_len < LIBUSB_DT_DEVICE_SIZE) {
+		usbi_err(ctx, "short descriptor read (%zu)", priv->descriptors_len);
+		return LIBUSB_ERROR_IO;
+	}
+
+	r = parse_config_descriptors(dev);
+	if (r < 0)
+		return r;
+
+	memcpy(&dev->device_descriptor, priv->descriptors, LIBUSB_DT_DEVICE_SIZE);
+
+	if (sysfs_dir) {
+		/* sysfs descriptors are in bus-endian format */
+		usbi_localize_device_descriptor(&dev->device_descriptor);
+		return LIBUSB_SUCCESS;
+	}
+
+	/* cache active config */
+	if (wrapped_fd < 0)
+		fd = get_usbfs_fd(dev, O_RDWR, 1);
+	else
+		fd = wrapped_fd;
+	if (fd < 0) {
+		/* cannot send a control message to determine the active
+		 * config. just assume the first one is active. */
+		usbi_warn(ctx, "Missing rw usbfs access; cannot determine "
+			       "active configuration descriptor");
+		if (priv->config_descriptors)
+			priv->active_config = (int)priv->config_descriptors[0].desc->bConfigurationValue;
+		else
+			priv->active_config = -1; /* No config dt */
+
+		return LIBUSB_SUCCESS;
+	}
+
+	r = usbfs_get_active_config(dev, fd);
+	if (fd != wrapped_fd)
+		close(fd);
+
+	return r;
+}
+
+static int linux_get_parent_info(struct libusb_device *dev, const char *sysfs_dir)
+{
+	struct libusb_context *ctx = DEVICE_CTX(dev);
+	struct libusb_device *it;
+	char *parent_sysfs_dir, *tmp;
+	int ret, add_parent = 1;
+
+	/* XXX -- can we figure out the topology when using usbfs? */
+	if (!sysfs_dir || !strncmp(sysfs_dir, "usb", 3)) {
+		/* either using usbfs or finding the parent of a root hub */
+		return LIBUSB_SUCCESS;
+	}
+
+	parent_sysfs_dir = strdup(sysfs_dir);
+	if (!parent_sysfs_dir)
+		return LIBUSB_ERROR_NO_MEM;
+
+	if ((tmp = strrchr(parent_sysfs_dir, '.')) ||
+	    (tmp = strrchr(parent_sysfs_dir, '-'))) {
+	        dev->port_number = atoi(tmp + 1);
+		*tmp = '\0';
+	} else {
+		usbi_warn(ctx, "Can not parse sysfs_dir: %s, no parent info",
+			  parent_sysfs_dir);
+		free(parent_sysfs_dir);
+		return LIBUSB_SUCCESS;
+	}
+
+	/* is the parent a root hub? */
+	if (!strchr(parent_sysfs_dir, '-')) {
+		tmp = parent_sysfs_dir;
+		ret = asprintf(&parent_sysfs_dir, "usb%s", tmp);
+		free(tmp);
+		if (ret < 0)
+			return LIBUSB_ERROR_NO_MEM;
+	}
+
+retry:
+	/* find the parent in the context */
+	usbi_mutex_lock(&ctx->usb_devs_lock);
+	for_each_device(ctx, it) {
+		struct linux_device_priv *priv = usbi_get_device_priv(it);
+
+		if (priv->sysfs_dir) {
+			if (!strcmp(priv->sysfs_dir, parent_sysfs_dir)) {
+				dev->parent_dev = libusb_ref_device(it);
+				break;
+			}
+		}
+	}
+	usbi_mutex_unlock(&ctx->usb_devs_lock);
+
+	if (!dev->parent_dev && add_parent) {
+		usbi_dbg(ctx, "parent_dev %s not enumerated yet, enumerating now",
+			 parent_sysfs_dir);
+		sysfs_scan_device(ctx, parent_sysfs_dir);
+		add_parent = 0;
+		goto retry;
+	}
+
+	usbi_dbg(ctx, "dev %p (%s) has parent %p (%s) port %u", dev, sysfs_dir,
+		 dev->parent_dev, parent_sysfs_dir, dev->port_number);
+
+	free(parent_sysfs_dir);
+
+	return LIBUSB_SUCCESS;
+}
+
+int linux_enumerate_device(struct libusb_context *ctx,
+	uint8_t busnum, uint8_t devaddr, const char *sysfs_dir)
+{
+	unsigned long session_id;
+	struct libusb_device *dev;
+	int r;
+
+	/* FIXME: session ID is not guaranteed unique as addresses can wrap and
+	 * will be reused. instead we should add a simple sysfs attribute with
+	 * a session ID. */
+	session_id = busnum << 8 | devaddr;
+	usbi_dbg(ctx, "busnum %u devaddr %u session_id %lu", busnum, devaddr, session_id);
+
+	dev = usbi_get_device_by_session_id(ctx, session_id);
+	if (dev) {
+		/* device already exists in the context */
+		usbi_dbg(ctx, "session_id %lu already exists", session_id);
+		libusb_unref_device(dev);
+		return LIBUSB_SUCCESS;
+	}
+
+	usbi_dbg(ctx, "allocating new device for %u/%u (session %lu)",
+		 busnum, devaddr, session_id);
+	dev = usbi_alloc_device(ctx, session_id);
+	if (!dev)
+		return LIBUSB_ERROR_NO_MEM;
+
+	r = initialize_device(dev, busnum, devaddr, sysfs_dir, -1);
+	if (r < 0)
+		goto out;
+	r = usbi_sanitize_device(dev);
+	if (r < 0)
+		goto out;
+
+	r = linux_get_parent_info(dev, sysfs_dir);
+	if (r < 0)
+		goto out;
+out:
+	if (r < 0)
+		libusb_unref_device(dev);
+	else
+		usbi_connect_device(dev);
+
+	return r;
+}
+
+void linux_hotplug_enumerate(uint8_t busnum, uint8_t devaddr, const char *sys_name)
+{
+	struct libusb_context *ctx;
+
+	usbi_mutex_static_lock(&active_contexts_lock);
+	for_each_context(ctx) {
+		linux_enumerate_device(ctx, busnum, devaddr, sys_name);
+	}
+	usbi_mutex_static_unlock(&active_contexts_lock);
+}
+
+void linux_device_disconnected(uint8_t busnum, uint8_t devaddr)
+{
+	struct libusb_context *ctx;
+	struct libusb_device *dev;
+	unsigned long session_id = busnum << 8 | devaddr;
+
+	usbi_mutex_static_lock(&active_contexts_lock);
+	for_each_context(ctx) {
+		dev = usbi_get_device_by_session_id(ctx, session_id);
+		if (dev) {
+			usbi_disconnect_device(dev);
+			libusb_unref_device(dev);
+		} else {
+			usbi_dbg(ctx, "device not found for session %lx", session_id);
+		}
+	}
+	usbi_mutex_static_unlock(&active_contexts_lock);
+}
+
+#if !defined(HAVE_LIBUDEV)
+static int parse_u8(const char *str, uint8_t *val_p)
+{
+	char *endptr;
+	long num;
+
+	errno = 0;
+	num = strtol(str, &endptr, 10);
+	if (num < 0 || num > UINT8_MAX || errno)
+		return 0;
+	if (endptr == str || *endptr != '\0')
+		return 0;
+
+	*val_p = (uint8_t)num;
+	return 1;
+}
+
+/* open a bus directory and adds all discovered devices to the context */
+static int usbfs_scan_busdir(struct libusb_context *ctx, uint8_t busnum)
+{
+	DIR *dir;
+	char dirpath[20];
+	struct dirent *entry;
+	int r = LIBUSB_ERROR_IO;
+
+	sprintf(dirpath, USB_DEVTMPFS_PATH "/%03u", busnum);
+	usbi_dbg(ctx, "%s", dirpath);
+	dir = opendir(dirpath);
+	if (!dir) {
+		usbi_err(ctx, "opendir '%s' failed, errno=%d", dirpath, errno);
+		/* FIXME: should handle valid race conditions like hub unplugged
+		 * during directory iteration - this is not an error */
+		return r;
+	}
+
+	while ((entry = readdir(dir))) {
+		uint8_t devaddr;
+
+		if (entry->d_name[0] == '.')
+			continue;
+
+		if (!parse_u8(entry->d_name, &devaddr)) {
+			usbi_dbg(ctx, "unknown dir entry %s", entry->d_name);
+			continue;
+		}
+
+		if (linux_enumerate_device(ctx, busnum, devaddr, NULL)) {
+			usbi_dbg(ctx, "failed to enumerate dir entry %s", entry->d_name);
+			continue;
+		}
+
+		r = 0;
+	}
+
+	closedir(dir);
+	return r;
+}
+
+static int usbfs_get_device_list(struct libusb_context *ctx)
+{
+	struct dirent *entry;
+	DIR *buses;
+	uint8_t busnum, devaddr;
+	int r = 0;
+
+	if (usbdev_names)
+		buses = opendir(USBDEV_PATH);
+	else
+		buses = opendir(USB_DEVTMPFS_PATH);
+
+	if (!buses) {
+		usbi_err(ctx, "opendir buses failed, errno=%d", errno);
+		return LIBUSB_ERROR_IO;
+	}
+
+	while ((entry = readdir(buses))) {
+		if (entry->d_name[0] == '.')
+			continue;
+
+		if (usbdev_names) {
+			if (!is_usbdev_entry(entry->d_name, &busnum, &devaddr))
+				continue;
+
+			r = linux_enumerate_device(ctx, busnum, devaddr, NULL);
+			if (r < 0) {
+				usbi_dbg(ctx, "failed to enumerate dir entry %s", entry->d_name);
+				continue;
+			}
+		} else {
+			if (!parse_u8(entry->d_name, &busnum)) {
+				usbi_dbg(ctx, "unknown dir entry %s", entry->d_name);
+				continue;
+			}
+
+			r = usbfs_scan_busdir(ctx, busnum);
+			if (r < 0)
+				break;
+		}
+	}
+
+	closedir(buses);
+	return r;
+
+}
+
+static int sysfs_get_device_list(struct libusb_context *ctx)
+{
+	DIR *devices = opendir(SYSFS_DEVICE_PATH);
+	struct dirent *entry;
+	int num_devices = 0;
+	int num_enumerated = 0;
+
+	if (!devices) {
+		usbi_err(ctx, "opendir devices failed, errno=%d", errno);
+		return LIBUSB_ERROR_IO;
+	}
+
+	while ((entry = readdir(devices))) {
+		if ((!isdigit(entry->d_name[0]) && strncmp(entry->d_name, "usb", 3))
+		    || strchr(entry->d_name, ':'))
+			continue;
+
+		num_devices++;
+
+		if (sysfs_scan_device(ctx, entry->d_name)) {
+			usbi_dbg(ctx, "failed to enumerate dir entry %s", entry->d_name);
+			continue;
+		}
+
+		num_enumerated++;
+	}
+
+	closedir(devices);
+
+	/* successful if at least one device was enumerated or no devices were found */
+	if (num_enumerated || !num_devices)
+		return LIBUSB_SUCCESS;
+	else
+		return LIBUSB_ERROR_IO;
+}
+
+static int linux_default_scan_devices(struct libusb_context *ctx)
+{
+	/* we can retrieve device list and descriptors from sysfs or usbfs.
+	 * sysfs is preferable, because if we use usbfs we end up resuming
+	 * any autosuspended USB devices. however, sysfs is not available
+	 * everywhere, so we need a usbfs fallback too.
+	 */
+	if (sysfs_available)
+		return sysfs_get_device_list(ctx);
+	else
+		return usbfs_get_device_list(ctx);
+}
+#endif
+
+static int initialize_handle(struct libusb_device_handle *handle, int fd)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int r;
+
+	hpriv->fd = fd;
+
+	r = ioctl(fd, IOCTL_USBFS_GET_CAPABILITIES, &hpriv->caps);
+	if (r < 0) {
+		if (errno == ENOTTY)
+			usbi_dbg(HANDLE_CTX(handle), "getcap not available");
+		else
+			usbi_err(HANDLE_CTX(handle), "getcap failed, errno=%d", errno);
+		hpriv->caps = USBFS_CAP_BULK_CONTINUATION;
+	}
+
+	return usbi_add_event_source(HANDLE_CTX(handle), hpriv->fd, POLLOUT);
+}
+
+static int op_wrap_sys_device(struct libusb_context *ctx,
+	struct libusb_device_handle *handle, intptr_t sys_dev)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = (int)sys_dev;
+	uint8_t busnum, devaddr;
+	struct usbfs_connectinfo ci;
+	struct libusb_device *dev;
+	int r;
+
+	r = linux_get_device_address(ctx, 1, &busnum, &devaddr, NULL, NULL, fd);
+	if (r < 0) {
+		r = ioctl(fd, IOCTL_USBFS_CONNECTINFO, &ci);
+		if (r < 0) {
+			usbi_err(ctx, "connectinfo failed, errno=%d", errno);
+			return LIBUSB_ERROR_IO;
+		}
+		/* There is no ioctl to get the bus number. We choose 0 here
+		 * as linux starts numbering buses from 1. */
+		busnum = 0;
+		devaddr = ci.devnum;
+	}
+
+	/* Session id is unused as we do not add the device to the list of
+	 * connected devices. */
+	usbi_dbg(ctx, "allocating new device for fd %d", fd);
+	dev = usbi_alloc_device(ctx, 0);
+	if (!dev)
+		return LIBUSB_ERROR_NO_MEM;
+
+	r = initialize_device(dev, busnum, devaddr, NULL, fd);
+	if (r < 0)
+		goto out;
+	r = usbi_sanitize_device(dev);
+	if (r < 0)
+		goto out;
+	/* Consider the device as connected, but do not add it to the managed
+	 * device list. */
+	usbi_atomic_store(&dev->attached, 1);
+	handle->dev = dev;
+
+	r = initialize_handle(handle, fd);
+	hpriv->fd_keep = 1;
+
+out:
+	if (r < 0)
+		libusb_unref_device(dev);
+	return r;
+}
+
+static int op_open(struct libusb_device_handle *handle)
+{
+	int fd, r;
+
+	fd = get_usbfs_fd(handle->dev, O_RDWR, 0);
+	if (fd < 0) {
+		if (fd == LIBUSB_ERROR_NO_DEVICE) {
+			/* device will still be marked as attached if hotplug monitor thread
+			 * hasn't processed remove event yet */
+			usbi_mutex_static_lock(&linux_hotplug_lock);
+			if (usbi_atomic_load(&handle->dev->attached)) {
+				usbi_dbg(HANDLE_CTX(handle), "open failed with no device, but device still attached");
+				linux_device_disconnected(handle->dev->bus_number,
+							  handle->dev->device_address);
+			}
+			usbi_mutex_static_unlock(&linux_hotplug_lock);
+		}
+		return fd;
+	}
+
+	r = initialize_handle(handle, fd);
+	if (r < 0)
+		close(fd);
+
+	return r;
+}
+
+static void op_close(struct libusb_device_handle *dev_handle)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(dev_handle);
+
+	/* fd may have already been removed by POLLERR condition in op_handle_events() */
+	if (!hpriv->fd_removed)
+		usbi_remove_event_source(HANDLE_CTX(dev_handle), hpriv->fd);
+	if (!hpriv->fd_keep)
+		close(hpriv->fd);
+}
+
+static int op_get_configuration(struct libusb_device_handle *handle,
+	uint8_t *config)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(handle->dev);
+	int active_config;
+	int r;
+
+	if (priv->sysfs_dir) {
+		r = sysfs_get_active_config(handle->dev, &active_config);
+	} else {
+		struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+
+		r = usbfs_get_active_config(handle->dev, hpriv->fd);
+		if (r == LIBUSB_SUCCESS)
+			active_config = priv->active_config;
+	}
+	if (r < 0)
+		return r;
+
+	if (active_config == -1) {
+		usbi_warn(HANDLE_CTX(handle), "device unconfigured");
+		active_config = 0;
+	}
+
+	*config = (uint8_t)active_config;
+
+	return 0;
+}
+
+static int op_set_configuration(struct libusb_device_handle *handle, int config)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(handle->dev);
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	int r = ioctl(fd, IOCTL_USBFS_SETCONFIGURATION, &config);
+
+	if (r < 0) {
+		if (errno == EINVAL)
+			return LIBUSB_ERROR_NOT_FOUND;
+		else if (errno == EBUSY)
+			return LIBUSB_ERROR_BUSY;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "set configuration failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	/* if necessary, update our cached active config descriptor */
+	if (!priv->sysfs_dir) {
+		if (config == 0 && !dev_has_config0(handle->dev))
+			config = -1;
+
+		priv->active_config = config;
+	}
+
+	return LIBUSB_SUCCESS;
+}
+
+static int claim_interface(struct libusb_device_handle *handle, unsigned int iface)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	int r = ioctl(fd, IOCTL_USBFS_CLAIMINTERFACE, &iface);
+
+	if (r < 0) {
+		if (errno == ENOENT)
+			return LIBUSB_ERROR_NOT_FOUND;
+		else if (errno == EBUSY)
+			return LIBUSB_ERROR_BUSY;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "claim interface failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+	return 0;
+}
+
+static int release_interface(struct libusb_device_handle *handle, unsigned int iface)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	int r = ioctl(fd, IOCTL_USBFS_RELEASEINTERFACE, &iface);
+
+	if (r < 0) {
+		if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "release interface failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+	return 0;
+}
+
+static int op_set_interface(struct libusb_device_handle *handle, uint8_t interface,
+	uint8_t altsetting)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	struct usbfs_setinterface setintf;
+	int r;
+
+	setintf.interface = interface;
+	setintf.altsetting = altsetting;
+	r = ioctl(fd, IOCTL_USBFS_SETINTERFACE, &setintf);
+	if (r < 0) {
+		if (errno == EINVAL)
+			return LIBUSB_ERROR_NOT_FOUND;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "set interface failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return 0;
+}
+
+static int op_clear_halt(struct libusb_device_handle *handle,
+	unsigned char endpoint)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	unsigned int _endpoint = endpoint;
+	int r = ioctl(fd, IOCTL_USBFS_CLEAR_HALT, &_endpoint);
+
+	if (r < 0) {
+		if (errno == ENOENT)
+			return LIBUSB_ERROR_NOT_FOUND;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "clear halt failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return 0;
+}
+
+static int op_reset_device(struct libusb_device_handle *handle)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	int r, ret = 0;
+	uint8_t i;
+
+	/* Doing a device reset will cause the usbfs driver to get unbound
+	 * from any interfaces it is bound to. By voluntarily unbinding
+	 * the usbfs driver ourself, we stop the kernel from rebinding
+	 * the interface after reset (which would end up with the interface
+	 * getting bound to the in kernel driver if any). */
+	for (i = 0; i < USB_MAXINTERFACES; i++) {
+		if (handle->claimed_interfaces & (1UL << i))
+			release_interface(handle, i);
+	}
+
+	usbi_mutex_lock(&handle->lock);
+	r = ioctl(fd, IOCTL_USBFS_RESET, NULL);
+	if (r < 0) {
+		if (errno == ENODEV) {
+			ret = LIBUSB_ERROR_NOT_FOUND;
+			goto out;
+		}
+
+		usbi_err(HANDLE_CTX(handle), "reset failed, errno=%d", errno);
+		ret = LIBUSB_ERROR_OTHER;
+		goto out;
+	}
+
+	/* And re-claim any interfaces which were claimed before the reset */
+	for (i = 0; i < USB_MAXINTERFACES; i++) {
+		if (!(handle->claimed_interfaces & (1UL << i)))
+			continue;
+		/*
+		 * A driver may have completed modprobing during
+		 * IOCTL_USBFS_RESET, and bound itself as soon as
+		 * IOCTL_USBFS_RESET released the device lock
+		 */
+		r = detach_kernel_driver_and_claim(handle, i);
+		if (r) {
+			usbi_warn(HANDLE_CTX(handle), "failed to re-claim interface %u after reset: %s",
+				  i, libusb_error_name(r));
+			handle->claimed_interfaces &= ~(1UL << i);
+			ret = LIBUSB_ERROR_NOT_FOUND;
+		}
+	}
+out:
+	usbi_mutex_unlock(&handle->lock);
+	return ret;
+}
+
+static int do_streams_ioctl(struct libusb_device_handle *handle, long req,
+	uint32_t num_streams, unsigned char *endpoints, int num_endpoints)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int r, fd = hpriv->fd;
+	struct usbfs_streams *streams;
+
+	if (num_endpoints > 30) /* Max 15 in + 15 out eps */
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	streams = malloc(sizeof(*streams) + num_endpoints);
+	if (!streams)
+		return LIBUSB_ERROR_NO_MEM;
+
+	streams->num_streams = num_streams;
+	streams->num_eps = num_endpoints;
+	memcpy(streams->eps, endpoints, num_endpoints);
+
+	r = ioctl(fd, req, streams);
+
+	free(streams);
+
+	if (r < 0) {
+		if (errno == ENOTTY)
+			return LIBUSB_ERROR_NOT_SUPPORTED;
+		else if (errno == EINVAL)
+			return LIBUSB_ERROR_INVALID_PARAM;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "streams-ioctl failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+	return r;
+}
+
+static int op_alloc_streams(struct libusb_device_handle *handle,
+	uint32_t num_streams, unsigned char *endpoints, int num_endpoints)
+{
+	return do_streams_ioctl(handle, IOCTL_USBFS_ALLOC_STREAMS,
+				num_streams, endpoints, num_endpoints);
+}
+
+static int op_free_streams(struct libusb_device_handle *handle,
+		unsigned char *endpoints, int num_endpoints)
+{
+	return do_streams_ioctl(handle, IOCTL_USBFS_FREE_STREAMS, 0,
+				endpoints, num_endpoints);
+}
+
+static void *op_dev_mem_alloc(struct libusb_device_handle *handle, size_t len)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	void *buffer;
+
+	buffer = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, hpriv->fd, 0);
+	if (buffer == MAP_FAILED) {
+		usbi_err(HANDLE_CTX(handle), "alloc dev mem failed, errno=%d", errno);
+		return NULL;
+	}
+	return buffer;
+}
+
+static int op_dev_mem_free(struct libusb_device_handle *handle, void *buffer,
+	size_t len)
+{
+	if (munmap(buffer, len) != 0) {
+		usbi_err(HANDLE_CTX(handle), "free dev mem failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	} else {
+		return LIBUSB_SUCCESS;
+	}
+}
+
+static int op_kernel_driver_active(struct libusb_device_handle *handle,
+	uint8_t interface)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	struct usbfs_getdriver getdrv;
+	int r;
+
+	getdrv.interface = interface;
+	r = ioctl(fd, IOCTL_USBFS_GETDRIVER, &getdrv);
+	if (r < 0) {
+		if (errno == ENODATA)
+			return 0;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "get driver failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return strcmp(getdrv.driver, "usbfs") != 0;
+}
+
+static int op_detach_kernel_driver(struct libusb_device_handle *handle,
+	uint8_t interface)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	struct usbfs_ioctl command;
+	struct usbfs_getdriver getdrv;
+	int r;
+
+	command.ifno = interface;
+	command.ioctl_code = IOCTL_USBFS_DISCONNECT;
+	command.data = NULL;
+
+	getdrv.interface = interface;
+	r = ioctl(fd, IOCTL_USBFS_GETDRIVER, &getdrv);
+	if (r == 0 && !strcmp(getdrv.driver, "usbfs"))
+		return LIBUSB_ERROR_NOT_FOUND;
+
+	r = ioctl(fd, IOCTL_USBFS_IOCTL, &command);
+	if (r < 0) {
+		if (errno == ENODATA)
+			return LIBUSB_ERROR_NOT_FOUND;
+		else if (errno == EINVAL)
+			return LIBUSB_ERROR_INVALID_PARAM;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "detach failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	return 0;
+}
+
+static int op_attach_kernel_driver(struct libusb_device_handle *handle,
+	uint8_t interface)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int fd = hpriv->fd;
+	struct usbfs_ioctl command;
+	int r;
+
+	command.ifno = interface;
+	command.ioctl_code = IOCTL_USBFS_CONNECT;
+	command.data = NULL;
+
+	r = ioctl(fd, IOCTL_USBFS_IOCTL, &command);
+	if (r < 0) {
+		if (errno == ENODATA)
+			return LIBUSB_ERROR_NOT_FOUND;
+		else if (errno == EINVAL)
+			return LIBUSB_ERROR_INVALID_PARAM;
+		else if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+		else if (errno == EBUSY)
+			return LIBUSB_ERROR_BUSY;
+
+		usbi_err(HANDLE_CTX(handle), "attach failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	} else if (r == 0) {
+		return LIBUSB_ERROR_NOT_FOUND;
+	}
+
+	return 0;
+}
+
+static int detach_kernel_driver_and_claim(struct libusb_device_handle *handle,
+	uint8_t interface)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	struct usbfs_disconnect_claim dc;
+	int r, fd = hpriv->fd;
+
+	dc.interface = interface;
+	strcpy(dc.driver, "usbfs");
+	dc.flags = USBFS_DISCONNECT_CLAIM_EXCEPT_DRIVER;
+	r = ioctl(fd, IOCTL_USBFS_DISCONNECT_CLAIM, &dc);
+	if (r == 0)
+		return 0;
+	switch (errno) {
+	case ENOTTY:
+		break;
+	case EBUSY:
+		return LIBUSB_ERROR_BUSY;
+	case EINVAL:
+		return LIBUSB_ERROR_INVALID_PARAM;
+	case ENODEV:
+		return LIBUSB_ERROR_NO_DEVICE;
+	default:
+		usbi_err(HANDLE_CTX(handle), "disconnect-and-claim failed, errno=%d", errno);
+		return LIBUSB_ERROR_OTHER;
+	}
+
+	/* Fallback code for kernels which don't support the
+	   disconnect-and-claim ioctl */
+	r = op_detach_kernel_driver(handle, interface);
+	if (r != 0 && r != LIBUSB_ERROR_NOT_FOUND)
+		return r;
+
+	return claim_interface(handle, interface);
+}
+
+static int op_claim_interface(struct libusb_device_handle *handle, uint8_t interface)
+{
+	if (handle->auto_detach_kernel_driver)
+		return detach_kernel_driver_and_claim(handle, interface);
+	else
+		return claim_interface(handle, interface);
+}
+
+static int op_release_interface(struct libusb_device_handle *handle, uint8_t interface)
+{
+	int r;
+
+	r = release_interface(handle, interface);
+	if (r)
+		return r;
+
+	if (handle->auto_detach_kernel_driver)
+		op_attach_kernel_driver(handle, interface);
+
+	return 0;
+}
+
+static void op_destroy_device(struct libusb_device *dev)
+{
+	struct linux_device_priv *priv = usbi_get_device_priv(dev);
+
+	free(priv->config_descriptors);
+	free(priv->descriptors);
+	free(priv->sysfs_dir);
+}
+
+/* URBs are discarded in reverse order of submission to avoid races. */
+static int discard_urbs(struct usbi_transfer *itransfer, int first, int last_plus_one)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	struct linux_device_handle_priv *hpriv =
+		usbi_get_device_handle_priv(transfer->dev_handle);
+	int i, ret = 0;
+	struct usbfs_urb *urb;
+
+	for (i = last_plus_one - 1; i >= first; i--) {
+		if (transfer->type == LIBUSB_TRANSFER_TYPE_ISOCHRONOUS)
+			urb = tpriv->iso_urbs[i];
+		else
+			urb = &tpriv->urbs[i];
+
+		if (ioctl(hpriv->fd, IOCTL_USBFS_DISCARDURB, urb) == 0)
+			continue;
+
+		if (errno == EINVAL) {
+			usbi_dbg(TRANSFER_CTX(transfer), "URB not found --> assuming ready to be reaped");
+			if (i == (last_plus_one - 1))
+				ret = LIBUSB_ERROR_NOT_FOUND;
+		} else if (errno == ENODEV) {
+			usbi_dbg(TRANSFER_CTX(transfer), "Device not found for URB --> assuming ready to be reaped");
+			ret = LIBUSB_ERROR_NO_DEVICE;
+		} else {
+			usbi_warn(TRANSFER_CTX(transfer), "unrecognised discard errno %d", errno);
+			ret = LIBUSB_ERROR_OTHER;
+		}
+	}
+	return ret;
+}
+
+static void free_iso_urbs(struct linux_transfer_priv *tpriv)
+{
+	int i;
+
+	for (i = 0; i < tpriv->num_urbs; i++) {
+		struct usbfs_urb *urb = tpriv->iso_urbs[i];
+
+		if (!urb)
+			break;
+		free(urb);
+	}
+
+	free(tpriv->iso_urbs);
+	tpriv->iso_urbs = NULL;
+}
+
+static int submit_bulk_transfer(struct usbi_transfer *itransfer)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	struct linux_device_handle_priv *hpriv =
+		usbi_get_device_handle_priv(transfer->dev_handle);
+	struct usbfs_urb *urbs;
+	int is_out = IS_XFEROUT(transfer);
+	int bulk_buffer_len, use_bulk_continuation;
+	int num_urbs;
+	int last_urb_partial = 0;
+	int r;
+	int i;
+
+	/*
+	 * Older versions of usbfs place a 16kb limit on bulk URBs. We work
+	 * around this by splitting large transfers into 16k blocks, and then
+	 * submit all urbs at once. it would be simpler to submit one urb at
+	 * a time, but there is a big performance gain doing it this way.
+	 *
+	 * Newer versions lift the 16k limit (USBFS_CAP_NO_PACKET_SIZE_LIM),
+	 * using arbitrary large transfers can still be a bad idea though, as
+	 * the kernel needs to allocate physical contiguous memory for this,
+	 * which may fail for large buffers.
+	 *
+	 * The kernel solves this problem by splitting the transfer into
+	 * blocks itself when the host-controller is scatter-gather capable
+	 * (USBFS_CAP_BULK_SCATTER_GATHER), which most controllers are.
+	 *
+	 * Last, there is the issue of short-transfers when splitting, for
+	 * short split-transfers to work reliable USBFS_CAP_BULK_CONTINUATION
+	 * is needed, but this is not always available.
+	 */
+	if (hpriv->caps & USBFS_CAP_BULK_SCATTER_GATHER) {
+		/* Good! Just submit everything in one go */
+		bulk_buffer_len = transfer->length ? transfer->length : 1;
+		use_bulk_continuation = 0;
+	} else if (hpriv->caps & USBFS_CAP_BULK_CONTINUATION) {
+		/* Split the transfers and use bulk-continuation to
+		   avoid issues with short-transfers */
+		bulk_buffer_len = MAX_BULK_BUFFER_LENGTH;
+		use_bulk_continuation = 1;
+	} else if (hpriv->caps & USBFS_CAP_NO_PACKET_SIZE_LIM) {
+		/* Don't split, assume the kernel can alloc the buffer
+		   (otherwise the submit will fail with -ENOMEM) */
+		bulk_buffer_len = transfer->length ? transfer->length : 1;
+		use_bulk_continuation = 0;
+	} else {
+		/* Bad, splitting without bulk-continuation, short transfers
+		   which end before the last urb will not work reliable! */
+		/* Note we don't warn here as this is "normal" on kernels <
+		   2.6.32 and not a problem for most applications */
+		bulk_buffer_len = MAX_BULK_BUFFER_LENGTH;
+		use_bulk_continuation = 0;
+	}
+
+	num_urbs = transfer->length / bulk_buffer_len;
+
+	if (transfer->length == 0) {
+		num_urbs = 1;
+	} else if ((transfer->length % bulk_buffer_len) > 0) {
+		last_urb_partial = 1;
+		num_urbs++;
+	}
+	usbi_dbg(TRANSFER_CTX(transfer), "need %d urbs for new transfer with length %d", num_urbs, transfer->length);
+	urbs = calloc(num_urbs, sizeof(*urbs));
+	if (!urbs)
+		return LIBUSB_ERROR_NO_MEM;
+	tpriv->urbs = urbs;
+	tpriv->num_urbs = num_urbs;
+	tpriv->num_retired = 0;
+	tpriv->reap_action = NORMAL;
+	tpriv->reap_status = LIBUSB_TRANSFER_COMPLETED;
+
+	for (i = 0; i < num_urbs; i++) {
+		struct usbfs_urb *urb = &urbs[i];
+
+		urb->usercontext = itransfer;
+		switch (transfer->type) {
+		case LIBUSB_TRANSFER_TYPE_BULK:
+			urb->type = USBFS_URB_TYPE_BULK;
+			urb->stream_id = 0;
+			break;
+		case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
+			urb->type = USBFS_URB_TYPE_BULK;
+			urb->stream_id = itransfer->stream_id;
+			break;
+		case LIBUSB_TRANSFER_TYPE_INTERRUPT:
+			urb->type = USBFS_URB_TYPE_INTERRUPT;
+			break;
+		}
+		urb->endpoint = transfer->endpoint;
+		urb->buffer = transfer->buffer + (i * bulk_buffer_len);
+
+		/* don't set the short not ok flag for the last URB */
+		if (use_bulk_continuation && !is_out && (i < num_urbs - 1))
+			urb->flags = USBFS_URB_SHORT_NOT_OK;
+
+		if (i == num_urbs - 1 && last_urb_partial)
+			urb->buffer_length = transfer->length % bulk_buffer_len;
+		else if (transfer->length == 0)
+			urb->buffer_length = 0;
+		else
+			urb->buffer_length = bulk_buffer_len;
+
+		if (i > 0 && use_bulk_continuation)
+			urb->flags |= USBFS_URB_BULK_CONTINUATION;
+
+		/* we have already checked that the flag is supported */
+		if (is_out && i == num_urbs - 1 &&
+		    (transfer->flags & LIBUSB_TRANSFER_ADD_ZERO_PACKET))
+			urb->flags |= USBFS_URB_ZERO_PACKET;
+
+		r = ioctl(hpriv->fd, IOCTL_USBFS_SUBMITURB, urb);
+		if (r == 0)
+			continue;
+
+		if (errno == ENODEV) {
+			r = LIBUSB_ERROR_NO_DEVICE;
+		} else if (errno == ENOMEM) {
+			r = LIBUSB_ERROR_NO_MEM;
+		} else {
+			usbi_err(TRANSFER_CTX(transfer), "submiturb failed, errno=%d", errno);
+			r = LIBUSB_ERROR_IO;
+		}
+
+		/* if the first URB submission fails, we can simply free up and
+		 * return failure immediately. */
+		if (i == 0) {
+			usbi_dbg(TRANSFER_CTX(transfer), "first URB failed, easy peasy");
+			free(urbs);
+			tpriv->urbs = NULL;
+			return r;
+		}
+
+		/* if it's not the first URB that failed, the situation is a bit
+		 * tricky. we may need to discard all previous URBs. there are
+		 * complications:
+		 *  - discarding is asynchronous - discarded urbs will be reaped
+		 *    later. the user must not have freed the transfer when the
+		 *    discarded URBs are reaped, otherwise libusb will be using
+		 *    freed memory.
+		 *  - the earlier URBs may have completed successfully and we do
+		 *    not want to throw away any data.
+		 *  - this URB failing may be no error; EREMOTEIO means that
+		 *    this transfer simply didn't need all the URBs we submitted
+		 * so, we report that the transfer was submitted successfully and
+		 * in case of error we discard all previous URBs. later when
+		 * the final reap completes we can report error to the user,
+		 * or success if an earlier URB was completed successfully.
+		 */
+		tpriv->reap_action = errno == EREMOTEIO ? COMPLETED_EARLY : SUBMIT_FAILED;
+
+		/* The URBs we haven't submitted yet we count as already
+		 * retired. */
+		tpriv->num_retired += num_urbs - i;
+
+		/* If we completed short then don't try to discard. */
+		if (tpriv->reap_action == COMPLETED_EARLY)
+			return 0;
+
+		discard_urbs(itransfer, 0, i);
+
+		usbi_dbg(TRANSFER_CTX(transfer), "reporting successful submission but waiting for %d "
+			 "discards before reporting error", i);
+		return 0;
+	}
+
+	return 0;
+}
+
+static int submit_iso_transfer(struct usbi_transfer *itransfer)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	struct linux_device_handle_priv *hpriv =
+		usbi_get_device_handle_priv(transfer->dev_handle);
+	struct usbfs_urb **urbs;
+	int num_packets = transfer->num_iso_packets;
+	int num_packets_remaining;
+	int i, j;
+	int num_urbs;
+	unsigned int packet_len;
+	unsigned int total_len = 0;
+	unsigned char *urb_buffer = transfer->buffer;
+
+	if (num_packets < 1)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	/* usbfs places arbitrary limits on iso URBs. this limit has changed
+	 * at least three times, but we attempt to detect this limit during
+	 * init and check it here. if the kernel rejects the request due to
+	 * its size, we return an error indicating such to the user.
+	 */
+	for (i = 0; i < num_packets; i++) {
+		packet_len = transfer->iso_packet_desc[i].length;
+
+		if (packet_len > max_iso_packet_len) {
+			usbi_warn(TRANSFER_CTX(transfer),
+				  "iso packet length of %u bytes exceeds maximum of %u bytes",
+				  packet_len, max_iso_packet_len);
+			return LIBUSB_ERROR_INVALID_PARAM;
+		}
+
+		total_len += packet_len;
+	}
+
+	if (transfer->length < (int)total_len)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	/* usbfs limits the number of iso packets per URB */
+	num_urbs = (num_packets + (MAX_ISO_PACKETS_PER_URB - 1)) / MAX_ISO_PACKETS_PER_URB;
+
+	usbi_dbg(TRANSFER_CTX(transfer), "need %d urbs for new transfer with length %d", num_urbs, transfer->length);
+
+	urbs = calloc(num_urbs, sizeof(*urbs));
+	if (!urbs)
+		return LIBUSB_ERROR_NO_MEM;
+
+	tpriv->iso_urbs = urbs;
+	tpriv->num_urbs = num_urbs;
+	tpriv->num_retired = 0;
+	tpriv->reap_action = NORMAL;
+	tpriv->iso_packet_offset = 0;
+
+	/* allocate + initialize each URB with the correct number of packets */
+	num_packets_remaining = num_packets;
+	for (i = 0, j = 0; i < num_urbs; i++) {
+		int num_packets_in_urb = MIN(num_packets_remaining, MAX_ISO_PACKETS_PER_URB);
+		struct usbfs_urb *urb;
+		size_t alloc_size;
+		int k;
+
+		alloc_size = sizeof(*urb)
+			+ (num_packets_in_urb * sizeof(struct usbfs_iso_packet_desc));
+		urb = calloc(1, alloc_size);
+		if (!urb) {
+			free_iso_urbs(tpriv);
+			return LIBUSB_ERROR_NO_MEM;
+		}
+		urbs[i] = urb;
+
+		/* populate packet lengths */
+		for (k = 0; k < num_packets_in_urb; j++, k++) {
+			packet_len = transfer->iso_packet_desc[j].length;
+			urb->buffer_length += packet_len;
+			urb->iso_frame_desc[k].length = packet_len;
+		}
+
+		urb->usercontext = itransfer;
+		urb->type = USBFS_URB_TYPE_ISO;
+		/* FIXME: interface for non-ASAP data? */
+		urb->flags = USBFS_URB_ISO_ASAP;
+		urb->endpoint = transfer->endpoint;
+		urb->number_of_packets = num_packets_in_urb;
+		urb->buffer = urb_buffer;
+
+		urb_buffer += urb->buffer_length;
+		num_packets_remaining -= num_packets_in_urb;
+	}
+
+	/* submit URBs */
+	for (i = 0; i < num_urbs; i++) {
+		int r = ioctl(hpriv->fd, IOCTL_USBFS_SUBMITURB, urbs[i]);
+
+		if (r == 0)
+			continue;
+
+		if (errno == ENODEV) {
+			r = LIBUSB_ERROR_NO_DEVICE;
+		} else if (errno == EINVAL) {
+			usbi_warn(TRANSFER_CTX(transfer), "submiturb failed, transfer too large");
+			r = LIBUSB_ERROR_INVALID_PARAM;
+		} else if (errno == EMSGSIZE) {
+			usbi_warn(TRANSFER_CTX(transfer), "submiturb failed, iso packet length too large");
+			r = LIBUSB_ERROR_INVALID_PARAM;
+		} else {
+			usbi_err(TRANSFER_CTX(transfer), "submiturb failed, errno=%d", errno);
+			r = LIBUSB_ERROR_IO;
+		}
+
+		/* if the first URB submission fails, we can simply free up and
+		 * return failure immediately. */
+		if (i == 0) {
+			usbi_dbg(TRANSFER_CTX(transfer), "first URB failed, easy peasy");
+			free_iso_urbs(tpriv);
+			return r;
+		}
+
+		/* if it's not the first URB that failed, the situation is a bit
+		 * tricky. we must discard all previous URBs. there are
+		 * complications:
+		 *  - discarding is asynchronous - discarded urbs will be reaped
+		 *    later. the user must not have freed the transfer when the
+		 *    discarded URBs are reaped, otherwise libusb will be using
+		 *    freed memory.
+		 *  - the earlier URBs may have completed successfully and we do
+		 *    not want to throw away any data.
+		 * so, in this case we discard all the previous URBs BUT we report
+		 * that the transfer was submitted successfully. then later when
+		 * the final discard completes we can report error to the user.
+		 */
+		tpriv->reap_action = SUBMIT_FAILED;
+
+		/* The URBs we haven't submitted yet we count as already
+		 * retired. */
+		tpriv->num_retired = num_urbs - i;
+		discard_urbs(itransfer, 0, i);
+
+		usbi_dbg(TRANSFER_CTX(transfer), "reporting successful submission but waiting for %d "
+			 "discards before reporting error", i);
+		return 0;
+	}
+
+	return 0;
+}
+
+static int submit_control_transfer(struct usbi_transfer *itransfer)
+{
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct linux_device_handle_priv *hpriv =
+		usbi_get_device_handle_priv(transfer->dev_handle);
+	struct usbfs_urb *urb;
+	int r;
+
+	if (transfer->length - LIBUSB_CONTROL_SETUP_SIZE > MAX_CTRL_BUFFER_LENGTH)
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	urb = calloc(1, sizeof(*urb));
+	if (!urb)
+		return LIBUSB_ERROR_NO_MEM;
+	tpriv->urbs = urb;
+	tpriv->num_urbs = 1;
+	tpriv->reap_action = NORMAL;
+
+	urb->usercontext = itransfer;
+	urb->type = USBFS_URB_TYPE_CONTROL;
+	urb->endpoint = transfer->endpoint;
+	urb->buffer = transfer->buffer;
+	urb->buffer_length = transfer->length;
+
+	r = ioctl(hpriv->fd, IOCTL_USBFS_SUBMITURB, urb);
+	if (r < 0) {
+		free(urb);
+		tpriv->urbs = NULL;
+		if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(TRANSFER_CTX(transfer), "submiturb failed, errno=%d", errno);
+		return LIBUSB_ERROR_IO;
+	}
+	return 0;
+}
+
+static int op_submit_transfer(struct usbi_transfer *itransfer)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+
+	switch (transfer->type) {
+	case LIBUSB_TRANSFER_TYPE_CONTROL:
+		return submit_control_transfer(itransfer);
+	case LIBUSB_TRANSFER_TYPE_BULK:
+	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
+		return submit_bulk_transfer(itransfer);
+	case LIBUSB_TRANSFER_TYPE_INTERRUPT:
+		return submit_bulk_transfer(itransfer);
+	case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
+		return submit_iso_transfer(itransfer);
+	default:
+		usbi_err(TRANSFER_CTX(transfer), "unknown transfer type %u", transfer->type);
+		return LIBUSB_ERROR_INVALID_PARAM;
+	}
+}
+
+static int op_cancel_transfer(struct usbi_transfer *itransfer)
+{
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	int r;
+
+	if (!tpriv->urbs)
+		return LIBUSB_ERROR_NOT_FOUND;
+
+	r = discard_urbs(itransfer, 0, tpriv->num_urbs);
+	if (r != 0)
+		return r;
+
+	switch (transfer->type) {
+	case LIBUSB_TRANSFER_TYPE_BULK:
+	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
+		if (tpriv->reap_action == ERROR)
+			break;
+		/* else, fall through */
+	default:
+		tpriv->reap_action = CANCELLED;
+	}
+
+	return 0;
+}
+
+static void op_clear_transfer_priv(struct usbi_transfer *itransfer)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+
+	switch (transfer->type) {
+	case LIBUSB_TRANSFER_TYPE_CONTROL:
+	case LIBUSB_TRANSFER_TYPE_BULK:
+	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
+	case LIBUSB_TRANSFER_TYPE_INTERRUPT:
+		if (tpriv->urbs) {
+			free(tpriv->urbs);
+			tpriv->urbs = NULL;
+		}
+		break;
+	case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
+		if (tpriv->iso_urbs) {
+			free_iso_urbs(tpriv);
+			tpriv->iso_urbs = NULL;
+		}
+		break;
+	default:
+		usbi_err(TRANSFER_CTX(transfer), "unknown transfer type %u", transfer->type);
+	}
+}
+
+static int handle_bulk_completion(struct usbi_transfer *itransfer,
+	struct usbfs_urb *urb)
+{
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	struct libusb_transfer *transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	int urb_idx = urb - tpriv->urbs;
+
+	usbi_mutex_lock(&itransfer->lock);
+	usbi_dbg(TRANSFER_CTX(transfer), "handling completion status %d of bulk urb %d/%d", urb->status,
+		 urb_idx + 1, tpriv->num_urbs);
+
+	tpriv->num_retired++;
+
+	if (tpriv->reap_action != NORMAL) {
+		/* cancelled, submit_fail, or completed early */
+		usbi_dbg(TRANSFER_CTX(transfer), "abnormal reap: urb status %d", urb->status);
+
+		/* even though we're in the process of cancelling, it's possible that
+		 * we may receive some data in these URBs that we don't want to lose.
+		 * examples:
+		 * 1. while the kernel is cancelling all the packets that make up an
+		 *    URB, a few of them might complete. so we get back a successful
+		 *    cancellation *and* some data.
+		 * 2. we receive a short URB which marks the early completion condition,
+		 *    so we start cancelling the remaining URBs. however, we're too
+		 *    slow and another URB completes (or at least completes partially).
+		 *    (this can't happen since we always use BULK_CONTINUATION.)
+		 *
+		 * When this happens, our objectives are not to lose any "surplus" data,
+		 * and also to stick it at the end of the previously-received data
+		 * (closing any holes), so that libusb reports the total amount of
+		 * transferred data and presents it in a contiguous chunk.
+		 */
+		if (urb->actual_length > 0) {
+			unsigned char *target = transfer->buffer + itransfer->transferred;
+
+			usbi_dbg(TRANSFER_CTX(transfer), "received %d bytes of surplus data", urb->actual_length);
+			if (urb->buffer != target) {
+				usbi_dbg(TRANSFER_CTX(transfer), "moving surplus data from offset %zu to offset %zu",
+					 (unsigned char *)urb->buffer - transfer->buffer,
+					 target - transfer->buffer);
+				memmove(target, urb->buffer, urb->actual_length);
+			}
+			itransfer->transferred += urb->actual_length;
+		}
+
+		if (tpriv->num_retired == tpriv->num_urbs) {
+			usbi_dbg(TRANSFER_CTX(transfer), "abnormal reap: last URB handled, reporting");
+			if (tpriv->reap_action != COMPLETED_EARLY &&
+			    tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
+				tpriv->reap_status = LIBUSB_TRANSFER_ERROR;
+			goto completed;
+		}
+		goto out_unlock;
+	}
+
+	itransfer->transferred += urb->actual_length;
+
+	/* Many of these errors can occur on *any* urb of a multi-urb
+	 * transfer.  When they do, we tear down the rest of the transfer.
+	 */
+	switch (urb->status) {
+	case 0:
+		break;
+	case -EREMOTEIO: /* short transfer */
+		break;
+	case -ENOENT: /* cancelled */
+	case -ECONNRESET:
+		break;
+	case -ENODEV:
+	case -ESHUTDOWN:
+		usbi_dbg(TRANSFER_CTX(transfer), "device removed");
+		tpriv->reap_status = LIBUSB_TRANSFER_NO_DEVICE;
+		goto cancel_remaining;
+	case -EPIPE:
+		usbi_dbg(TRANSFER_CTX(transfer), "detected endpoint stall");
+		if (tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
+			tpriv->reap_status = LIBUSB_TRANSFER_STALL;
+		goto cancel_remaining;
+	case -EOVERFLOW:
+		/* overflow can only ever occur in the last urb */
+		usbi_dbg(TRANSFER_CTX(transfer), "overflow, actual_length=%d", urb->actual_length);
+		if (tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
+			tpriv->reap_status = LIBUSB_TRANSFER_OVERFLOW;
+		goto completed;
+	case -ETIME:
+	case -EPROTO:
+	case -EILSEQ:
+	case -ECOMM:
+	case -ENOSR:
+		usbi_dbg(TRANSFER_CTX(transfer), "low-level bus error %d", urb->status);
+		tpriv->reap_action = ERROR;
+		goto cancel_remaining;
+	default:
+		usbi_warn(ITRANSFER_CTX(itransfer), "unrecognised urb status %d", urb->status);
+		tpriv->reap_action = ERROR;
+		goto cancel_remaining;
+	}
+
+	/* if we've reaped all urbs or we got less data than requested then we're
+	 * done */
+	if (tpriv->num_retired == tpriv->num_urbs) {
+		usbi_dbg(TRANSFER_CTX(transfer), "all URBs in transfer reaped --> complete!");
+		goto completed;
+	} else if (urb->actual_length < urb->buffer_length) {
+		usbi_dbg(TRANSFER_CTX(transfer), "short transfer %d/%d --> complete!",
+			 urb->actual_length, urb->buffer_length);
+		if (tpriv->reap_action == NORMAL)
+			tpriv->reap_action = COMPLETED_EARLY;
+	} else {
+		goto out_unlock;
+	}
+
+cancel_remaining:
+	if (tpriv->reap_action == ERROR && tpriv->reap_status == LIBUSB_TRANSFER_COMPLETED)
+		tpriv->reap_status = LIBUSB_TRANSFER_ERROR;
+
+	if (tpriv->num_retired == tpriv->num_urbs) /* nothing to cancel */
+		goto completed;
+
+	/* cancel remaining urbs and wait for their completion before
+	 * reporting results */
+	discard_urbs(itransfer, urb_idx + 1, tpriv->num_urbs);
+
+out_unlock:
+	usbi_mutex_unlock(&itransfer->lock);
+	return 0;
+
+completed:
+	free(tpriv->urbs);
+	tpriv->urbs = NULL;
+	usbi_mutex_unlock(&itransfer->lock);
+	return tpriv->reap_action == CANCELLED ?
+		usbi_handle_transfer_cancellation(itransfer) :
+		usbi_handle_transfer_completion(itransfer, tpriv->reap_status);
+}
+
+static int handle_iso_completion(struct usbi_transfer *itransfer,
+	struct usbfs_urb *urb)
+{
+	struct libusb_transfer *transfer =
+		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	int num_urbs = tpriv->num_urbs;
+	int urb_idx = 0;
+	int i;
+	enum libusb_transfer_status status = LIBUSB_TRANSFER_COMPLETED;
+
+	usbi_mutex_lock(&itransfer->lock);
+	for (i = 0; i < num_urbs; i++) {
+		if (urb == tpriv->iso_urbs[i]) {
+			urb_idx = i + 1;
+			break;
+		}
+	}
+	if (urb_idx == 0) {
+		usbi_err(TRANSFER_CTX(transfer), "could not locate urb!");
+		usbi_mutex_unlock(&itransfer->lock);
+		return LIBUSB_ERROR_NOT_FOUND;
+	}
+
+	usbi_dbg(TRANSFER_CTX(transfer), "handling completion status %d of iso urb %d/%d", urb->status,
+		 urb_idx, num_urbs);
+
+	/* copy isochronous results back in */
+
+	for (i = 0; i < urb->number_of_packets; i++) {
+		struct usbfs_iso_packet_desc *urb_desc = &urb->iso_frame_desc[i];
+		struct libusb_iso_packet_descriptor *lib_desc =
+			&transfer->iso_packet_desc[tpriv->iso_packet_offset++];
+
+		lib_desc->status = LIBUSB_TRANSFER_COMPLETED;
+		switch (urb_desc->status) {
+		case 0:
+			break;
+		case -ENOENT: /* cancelled */
+		case -ECONNRESET:
+			break;
+		case -ENODEV:
+		case -ESHUTDOWN:
+			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - device removed", i);
+			lib_desc->status = LIBUSB_TRANSFER_NO_DEVICE;
+			break;
+		case -EPIPE:
+			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - detected endpoint stall", i);
+			lib_desc->status = LIBUSB_TRANSFER_STALL;
+			break;
+		case -EOVERFLOW:
+			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - overflow error", i);
+			lib_desc->status = LIBUSB_TRANSFER_OVERFLOW;
+			break;
+		case -ETIME:
+		case -EPROTO:
+		case -EILSEQ:
+		case -ECOMM:
+		case -ENOSR:
+		case -EXDEV:
+			usbi_dbg(TRANSFER_CTX(transfer), "packet %d - low-level USB error %d", i, urb_desc->status);
+			lib_desc->status = LIBUSB_TRANSFER_ERROR;
+			break;
+		default:
+			usbi_warn(TRANSFER_CTX(transfer), "packet %d - unrecognised urb status %d",
+				  i, urb_desc->status);
+			lib_desc->status = LIBUSB_TRANSFER_ERROR;
+			break;
+		}
+		lib_desc->actual_length = urb_desc->actual_length;
+	}
+
+	tpriv->num_retired++;
+
+	if (tpriv->reap_action != NORMAL) { /* cancelled or submit_fail */
+		usbi_dbg(TRANSFER_CTX(transfer), "CANCEL: urb status %d", urb->status);
+
+		if (tpriv->num_retired == num_urbs) {
+			usbi_dbg(TRANSFER_CTX(transfer), "CANCEL: last URB handled, reporting");
+			free_iso_urbs(tpriv);
+			if (tpriv->reap_action == CANCELLED) {
+				usbi_mutex_unlock(&itransfer->lock);
+				return usbi_handle_transfer_cancellation(itransfer);
+			} else {
+				usbi_mutex_unlock(&itransfer->lock);
+				return usbi_handle_transfer_completion(itransfer, LIBUSB_TRANSFER_ERROR);
+			}
+		}
+		goto out;
+	}
+
+	switch (urb->status) {
+	case 0:
+		break;
+	case -ENOENT: /* cancelled */
+	case -ECONNRESET:
+		break;
+	case -ESHUTDOWN:
+		usbi_dbg(TRANSFER_CTX(transfer), "device removed");
+		status = LIBUSB_TRANSFER_NO_DEVICE;
+		break;
+	default:
+		usbi_warn(TRANSFER_CTX(transfer), "unrecognised urb status %d", urb->status);
+		status = LIBUSB_TRANSFER_ERROR;
+		break;
+	}
+
+	/* if we've reaped all urbs then we're done */
+	if (tpriv->num_retired == num_urbs) {
+		usbi_dbg(TRANSFER_CTX(transfer), "all URBs in transfer reaped --> complete!");
+		free_iso_urbs(tpriv);
+		usbi_mutex_unlock(&itransfer->lock);
+		return usbi_handle_transfer_completion(itransfer, status);
+	}
+
+out:
+	usbi_mutex_unlock(&itransfer->lock);
+	return 0;
+}
+
+static int handle_control_completion(struct usbi_transfer *itransfer,
+	struct usbfs_urb *urb)
+{
+	struct linux_transfer_priv *tpriv = usbi_get_transfer_priv(itransfer);
+	int status;
+
+	usbi_mutex_lock(&itransfer->lock);
+	usbi_dbg(ITRANSFER_CTX(itransfer), "handling completion status %d", urb->status);
+
+	itransfer->transferred += urb->actual_length;
+
+	if (tpriv->reap_action == CANCELLED) {
+		if (urb->status && urb->status != -ENOENT)
+			usbi_warn(ITRANSFER_CTX(itransfer), "cancel: unrecognised urb status %d",
+				  urb->status);
+		free(tpriv->urbs);
+		tpriv->urbs = NULL;
+		usbi_mutex_unlock(&itransfer->lock);
+		return usbi_handle_transfer_cancellation(itransfer);
+	}
+
+	switch (urb->status) {
+	case 0:
+		status = LIBUSB_TRANSFER_COMPLETED;
+		break;
+	case -ENOENT: /* cancelled */
+		status = LIBUSB_TRANSFER_CANCELLED;
+		break;
+	case -ENODEV:
+	case -ESHUTDOWN:
+		usbi_dbg(ITRANSFER_CTX(itransfer), "device removed");
+		status = LIBUSB_TRANSFER_NO_DEVICE;
+		break;
+	case -EPIPE:
+		usbi_dbg(ITRANSFER_CTX(itransfer), "unsupported control request");
+		status = LIBUSB_TRANSFER_STALL;
+		break;
+	case -EOVERFLOW:
+		usbi_dbg(ITRANSFER_CTX(itransfer), "overflow, actual_length=%d", urb->actual_length);
+		status = LIBUSB_TRANSFER_OVERFLOW;
+		break;
+	case -ETIME:
+	case -EPROTO:
+	case -EILSEQ:
+	case -ECOMM:
+	case -ENOSR:
+		usbi_dbg(ITRANSFER_CTX(itransfer), "low-level bus error %d", urb->status);
+		status = LIBUSB_TRANSFER_ERROR;
+		break;
+	default:
+		usbi_warn(ITRANSFER_CTX(itransfer), "unrecognised urb status %d", urb->status);
+		status = LIBUSB_TRANSFER_ERROR;
+		break;
+	}
+
+	free(tpriv->urbs);
+	tpriv->urbs = NULL;
+	usbi_mutex_unlock(&itransfer->lock);
+	return usbi_handle_transfer_completion(itransfer, status);
+}
+
+static int reap_for_handle(struct libusb_device_handle *handle)
+{
+	struct linux_device_handle_priv *hpriv = usbi_get_device_handle_priv(handle);
+	int r;
+	struct usbfs_urb *urb = NULL;
+	struct usbi_transfer *itransfer;
+	struct libusb_transfer *transfer;
+
+	r = ioctl(hpriv->fd, IOCTL_USBFS_REAPURBNDELAY, &urb);
+	if (r < 0) {
+		if (errno == EAGAIN)
+			return 1;
+		if (errno == ENODEV)
+			return LIBUSB_ERROR_NO_DEVICE;
+
+		usbi_err(HANDLE_CTX(handle), "reap failed, errno=%d", errno);
+		return LIBUSB_ERROR_IO;
+	}
+
+	itransfer = urb->usercontext;
+	transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
+
+	usbi_dbg(HANDLE_CTX(handle), "urb type=%u status=%d transferred=%d", urb->type, urb->status, urb->actual_length);
+
+	switch (transfer->type) {
+	case LIBUSB_TRANSFER_TYPE_ISOCHRONOUS:
+		return handle_iso_completion(itransfer, urb);
+	case LIBUSB_TRANSFER_TYPE_BULK:
+	case LIBUSB_TRANSFER_TYPE_BULK_STREAM:
+	case LIBUSB_TRANSFER_TYPE_INTERRUPT:
+		return handle_bulk_completion(itransfer, urb);
+	case LIBUSB_TRANSFER_TYPE_CONTROL:
+		return handle_control_completion(itransfer, urb);
+	default:
+		usbi_err(HANDLE_CTX(handle), "unrecognised transfer type %u", transfer->type);
+		return LIBUSB_ERROR_OTHER;
+	}
+}
+
+static int op_handle_events(struct libusb_context *ctx,
+	void *event_data, unsigned int count, unsigned int num_ready)
+{
+	struct pollfd *fds = event_data;
+	unsigned int n;
+	int r;
+
+	usbi_mutex_lock(&ctx->open_devs_lock);
+	for (n = 0; n < count && num_ready > 0; n++) {
+		struct pollfd *pollfd = &fds[n];
+		struct libusb_device_handle *handle;
+		struct linux_device_handle_priv *hpriv = NULL;
+		int reap_count;
+
+		if (!pollfd->revents)
+			continue;
+
+		num_ready--;
+		for_each_open_device(ctx, handle) {
+			hpriv = usbi_get_device_handle_priv(handle);
+			if (hpriv->fd == pollfd->fd)
+				break;
+		}
+
+		if (!hpriv || hpriv->fd != pollfd->fd) {
+			usbi_err(ctx, "cannot find handle for fd %d",
+				 pollfd->fd);
+			continue;
+		}
+
+		if (pollfd->revents & POLLERR) {
+			/* remove the fd from the pollfd set so that it doesn't continuously
+			 * trigger an event, and flag that it has been removed so op_close()
+			 * doesn't try to remove it a second time */
+			usbi_remove_event_source(HANDLE_CTX(handle), hpriv->fd);
+			hpriv->fd_removed = 1;
+
+			/* device will still be marked as attached if hotplug monitor thread
+			 * hasn't processed remove event yet */
+			usbi_mutex_static_lock(&linux_hotplug_lock);
+			if (usbi_atomic_load(&handle->dev->attached))
+				linux_device_disconnected(handle->dev->bus_number,
+							  handle->dev->device_address);
+			usbi_mutex_static_unlock(&linux_hotplug_lock);
+
+			if (hpriv->caps & USBFS_CAP_REAP_AFTER_DISCONNECT) {
+				do {
+					r = reap_for_handle(handle);
+				} while (r == 0);
+			}
+
+			usbi_handle_disconnect(handle);
+			continue;
+		}
+
+		reap_count = 0;
+		do {
+			r = reap_for_handle(handle);
+		} while (r == 0 && ++reap_count <= 25);
+
+		if (r == 1 || r == LIBUSB_ERROR_NO_DEVICE)
+			continue;
+		else if (r < 0)
+			goto out;
+	}
+
+	r = 0;
+out:
+	usbi_mutex_unlock(&ctx->open_devs_lock);
+	return r;
+}
+
+const struct usbi_os_backend usbi_backend = {
+	.name = "Linux usbfs",
+	.caps = USBI_CAP_HAS_HID_ACCESS|USBI_CAP_SUPPORTS_DETACH_KERNEL_DRIVER,
+	.init = op_init,
+	.exit = op_exit,
+	.set_option = op_set_option,
+	.hotplug_poll = op_hotplug_poll,
+	.get_active_config_descriptor = op_get_active_config_descriptor,
+	.get_config_descriptor = op_get_config_descriptor,
+	.get_config_descriptor_by_value = op_get_config_descriptor_by_value,
+
+	.wrap_sys_device = op_wrap_sys_device,
+	.open = op_open,
+	.close = op_close,
+	.get_configuration = op_get_configuration,
+	.set_configuration = op_set_configuration,
+	.claim_interface = op_claim_interface,
+	.release_interface = op_release_interface,
+
+	.set_interface_altsetting = op_set_interface,
+	.clear_halt = op_clear_halt,
+	.reset_device = op_reset_device,
+
+	.alloc_streams = op_alloc_streams,
+	.free_streams = op_free_streams,
+
+	.dev_mem_alloc = op_dev_mem_alloc,
+	.dev_mem_free = op_dev_mem_free,
+
+	.kernel_driver_active = op_kernel_driver_active,
+	.detach_kernel_driver = op_detach_kernel_driver,
+	.attach_kernel_driver = op_attach_kernel_driver,
+
+	.destroy_device = op_destroy_device,
+
+	.submit_transfer = op_submit_transfer,
+	.cancel_transfer = op_cancel_transfer,
+	.clear_transfer_priv = op_clear_transfer_priv,
+
+	.handle_events = op_handle_events,
+
+	.device_priv_size = sizeof(struct linux_device_priv),
+	.device_handle_priv_size = sizeof(struct linux_device_handle_priv),
+	.transfer_priv_size = sizeof(struct linux_transfer_priv),
+};
diff --git a/hardware/src/libusb/os/linux_usbfs.h b/hardware/src/libusb/os/linux_usbfs.h
new file mode 100644
index 0000000..1238ffa
--- /dev/null
+++ b/hardware/src/libusb/os/linux_usbfs.h
@@ -0,0 +1,211 @@
+/*
+ * usbfs header structures
+ * Copyright © 2007 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef LIBUSB_USBFS_H
+#define LIBUSB_USBFS_H
+
+#include <linux/magic.h>
+#include <linux/types.h>
+
+#define SYSFS_MOUNT_PATH	"/sys"
+#define SYSFS_DEVICE_PATH	SYSFS_MOUNT_PATH "/bus/usb/devices"
+
+struct usbfs_ctrltransfer {
+	/* keep in sync with usbdevice_fs.h:usbdevfs_ctrltransfer */
+	__u8 bmRequestType;
+	__u8 bRequest;
+	__u16 wValue;
+	__u16 wIndex;
+	__u16 wLength;
+	__u32 timeout;	/* in milliseconds */
+
+	/* pointer to data */
+	void *data;
+};
+
+struct usbfs_setinterface {
+	/* keep in sync with usbdevice_fs.h:usbdevfs_setinterface */
+	unsigned int interface;
+	unsigned int altsetting;
+};
+
+#define USBFS_MAXDRIVERNAME		255
+
+struct usbfs_getdriver {
+	unsigned int interface;
+	char driver[USBFS_MAXDRIVERNAME + 1];
+};
+
+#define USBFS_URB_SHORT_NOT_OK		0x01
+#define USBFS_URB_ISO_ASAP		0x02
+#define USBFS_URB_BULK_CONTINUATION	0x04
+#define USBFS_URB_QUEUE_BULK		0x10
+#define USBFS_URB_ZERO_PACKET		0x40
+
+#define USBFS_URB_TYPE_ISO		0
+#define USBFS_URB_TYPE_INTERRUPT	1
+#define USBFS_URB_TYPE_CONTROL		2
+#define USBFS_URB_TYPE_BULK		3
+
+struct usbfs_iso_packet_desc {
+	unsigned int length;
+	unsigned int actual_length;
+	unsigned int status;
+};
+
+#define MAX_BULK_BUFFER_LENGTH		16384
+#define MAX_CTRL_BUFFER_LENGTH		4096
+
+#define MAX_ISO_PACKETS_PER_URB		128
+
+struct usbfs_urb {
+	unsigned char type;
+	unsigned char endpoint;
+	int status;
+	unsigned int flags;
+	void *buffer;
+	int buffer_length;
+	int actual_length;
+	int start_frame;
+	union {
+		int number_of_packets;	/* Only used for isoc urbs */
+		unsigned int stream_id;	/* Only used with bulk streams */
+	};
+	int error_count;
+	unsigned int signr;
+	void *usercontext;
+	struct usbfs_iso_packet_desc iso_frame_desc[0];
+};
+
+struct usbfs_connectinfo {
+	unsigned int devnum;
+	unsigned char slow;
+};
+
+struct usbfs_ioctl {
+	int ifno;	/* interface 0..N ; negative numbers reserved */
+	int ioctl_code;	/* MUST encode size + direction of data so the
+			 * macros in <asm/ioctl.h> give correct values */
+	void *data;	/* param buffer (in, or out) */
+};
+
+#define USBFS_CAP_ZERO_PACKET			0x01
+#define USBFS_CAP_BULK_CONTINUATION		0x02
+#define USBFS_CAP_NO_PACKET_SIZE_LIM		0x04
+#define USBFS_CAP_BULK_SCATTER_GATHER		0x08
+#define USBFS_CAP_REAP_AFTER_DISCONNECT		0x10
+
+#define USBFS_DISCONNECT_CLAIM_IF_DRIVER	0x01
+#define USBFS_DISCONNECT_CLAIM_EXCEPT_DRIVER	0x02
+
+struct usbfs_disconnect_claim {
+	unsigned int interface;
+	unsigned int flags;
+	char driver[USBFS_MAXDRIVERNAME + 1];
+};
+
+struct usbfs_streams {
+	unsigned int num_streams; /* Not used by USBDEVFS_FREE_STREAMS */
+	unsigned int num_eps;
+	unsigned char eps[0];
+};
+
+#define USBFS_SPEED_UNKNOWN			0
+#define USBFS_SPEED_LOW				1
+#define USBFS_SPEED_FULL			2
+#define USBFS_SPEED_HIGH			3
+#define USBFS_SPEED_WIRELESS			4
+#define USBFS_SPEED_SUPER			5
+#define USBFS_SPEED_SUPER_PLUS			6
+
+#define IOCTL_USBFS_CONTROL		_IOWR('U', 0, struct usbfs_ctrltransfer)
+#define IOCTL_USBFS_SETINTERFACE	_IOR('U', 4, struct usbfs_setinterface)
+#define IOCTL_USBFS_SETCONFIGURATION	_IOR('U', 5, unsigned int)
+#define IOCTL_USBFS_GETDRIVER		_IOW('U', 8, struct usbfs_getdriver)
+#define IOCTL_USBFS_SUBMITURB		_IOR('U', 10, struct usbfs_urb)
+#define IOCTL_USBFS_DISCARDURB		_IO('U', 11)
+#define IOCTL_USBFS_REAPURBNDELAY	_IOW('U', 13, void *)
+#define IOCTL_USBFS_CLAIMINTERFACE	_IOR('U', 15, unsigned int)
+#define IOCTL_USBFS_RELEASEINTERFACE	_IOR('U', 16, unsigned int)
+#define IOCTL_USBFS_CONNECTINFO		_IOW('U', 17, struct usbfs_connectinfo)
+#define IOCTL_USBFS_IOCTL		_IOWR('U', 18, struct usbfs_ioctl)
+#define IOCTL_USBFS_RESET		_IO('U', 20)
+#define IOCTL_USBFS_CLEAR_HALT		_IOR('U', 21, unsigned int)
+#define IOCTL_USBFS_DISCONNECT		_IO('U', 22)
+#define IOCTL_USBFS_CONNECT		_IO('U', 23)
+#define IOCTL_USBFS_GET_CAPABILITIES	_IOR('U', 26, __u32)
+#define IOCTL_USBFS_DISCONNECT_CLAIM	_IOR('U', 27, struct usbfs_disconnect_claim)
+#define IOCTL_USBFS_ALLOC_STREAMS	_IOR('U', 28, struct usbfs_streams)
+#define IOCTL_USBFS_FREE_STREAMS	_IOR('U', 29, struct usbfs_streams)
+#define IOCTL_USBFS_DROP_PRIVILEGES	_IOW('U', 30, __u32)
+#define IOCTL_USBFS_GET_SPEED		_IO('U', 31)
+
+extern usbi_mutex_static_t linux_hotplug_lock;
+
+#ifdef HAVE_LIBUDEV
+int linux_udev_start_event_monitor(void);
+int linux_udev_stop_event_monitor(void);
+int linux_udev_scan_devices(struct libusb_context *ctx);
+void linux_udev_hotplug_poll(void);
+#else
+int linux_netlink_start_event_monitor(void);
+int linux_netlink_stop_event_monitor(void);
+void linux_netlink_hotplug_poll(void);
+#endif
+
+static inline int linux_start_event_monitor(void)
+{
+#if defined(HAVE_LIBUDEV)
+	return linux_udev_start_event_monitor();
+#elif !defined(__ANDROID__)
+	return linux_netlink_start_event_monitor();
+#else
+	return LIBUSB_SUCCESS;
+#endif
+}
+
+static inline void linux_stop_event_monitor(void)
+{
+#if defined(HAVE_LIBUDEV)
+	linux_udev_stop_event_monitor();
+#elif !defined(__ANDROID__)
+	linux_netlink_stop_event_monitor();
+#endif
+}
+
+static inline void linux_hotplug_poll(void)
+{
+#if defined(HAVE_LIBUDEV)
+	linux_udev_hotplug_poll();
+#elif !defined(__ANDROID__)
+	linux_netlink_hotplug_poll();
+#endif
+}
+
+void linux_hotplug_enumerate(uint8_t busnum, uint8_t devaddr, const char *sys_name);
+void linux_device_disconnected(uint8_t busnum, uint8_t devaddr);
+
+int linux_get_device_address(struct libusb_context *ctx, int detached,
+	uint8_t *busnum, uint8_t *devaddr, const char *dev_node,
+	const char *sys_name, int fd);
+int linux_enumerate_device(struct libusb_context *ctx,
+	uint8_t busnum, uint8_t devaddr, const char *sysfs_dir);
+
+#endif
diff --git a/hardware/src/libusb/os/threads_posix.c b/hardware/src/libusb/os/threads_posix.c
new file mode 100644
index 0000000..0e0e221
--- /dev/null
+++ b/hardware/src/libusb/os/threads_posix.c
@@ -0,0 +1,129 @@
+/*
+ * libusb synchronization using POSIX Threads
+ *
+ * Copyright © 2011 Vitali Lovich <vlovich@aliph.com>
+ * Copyright © 2011 Peter Stuge <peter@stuge.se>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+
+#include <errno.h>
+#if defined(__ANDROID__)
+# include <unistd.h>
+#elif defined(__HAIKU__)
+# include <os/kernel/OS.h>
+#elif defined(__linux__)
+# include <sys/syscall.h>
+# include <unistd.h>
+#elif defined(__NetBSD__)
+# include <lwp.h>
+#elif defined(__OpenBSD__)
+# define _BSD_SOURCE
+# include <sys/syscall.h>
+# include <unistd.h>
+#elif defined(__sun__)
+# include <sys/lwp.h>
+#endif
+
+void usbi_cond_init(pthread_cond_t *cond)
+{
+#ifdef HAVE_PTHREAD_CONDATTR_SETCLOCK
+	pthread_condattr_t condattr;
+
+	PTHREAD_CHECK(pthread_condattr_init(&condattr));
+	PTHREAD_CHECK(pthread_condattr_setclock(&condattr, CLOCK_MONOTONIC));
+	PTHREAD_CHECK(pthread_cond_init(cond, &condattr));
+	PTHREAD_CHECK(pthread_condattr_destroy(&condattr));
+#else
+	PTHREAD_CHECK(pthread_cond_init(cond, NULL));
+#endif
+}
+
+int usbi_cond_timedwait(pthread_cond_t *cond,
+	pthread_mutex_t *mutex, const struct timeval *tv)
+{
+	struct timespec timeout;
+	int r;
+
+#ifdef HAVE_PTHREAD_CONDATTR_SETCLOCK
+	usbi_get_monotonic_time(&timeout);
+#else
+	usbi_get_real_time(&timeout);
+#endif
+
+	timeout.tv_sec += tv->tv_sec;
+	timeout.tv_nsec += tv->tv_usec * 1000L;
+	if (timeout.tv_nsec >= NSEC_PER_SEC) {
+		timeout.tv_nsec -= NSEC_PER_SEC;
+		timeout.tv_sec++;
+	}
+
+	r = pthread_cond_timedwait(cond, mutex, &timeout);
+	if (r == 0)
+		return 0;
+	else if (r == ETIMEDOUT)
+		return LIBUSB_ERROR_TIMEOUT;
+	else
+		return LIBUSB_ERROR_OTHER;
+}
+
+unsigned int usbi_get_tid(void)
+{
+	static _Thread_local unsigned int tl_tid;
+	int tid;
+
+	if (tl_tid)
+		return tl_tid;
+
+#if defined(__ANDROID__)
+	tid = gettid();
+#elif defined(__APPLE__)
+#ifdef HAVE_PTHREAD_THREADID_NP
+	uint64_t thread_id;
+
+	if (pthread_threadid_np(NULL, &thread_id) == 0)
+		tid = (int)thread_id;
+	else
+		tid = -1;
+#else
+	tid = (int)pthread_mach_thread_np(pthread_self());
+#endif
+#elif defined(__HAIKU__)
+	tid = get_pthread_thread_id(pthread_self());
+#elif defined(__linux__)
+	tid = (int)syscall(SYS_gettid);
+#elif defined(__NetBSD__)
+	tid = _lwp_self();
+#elif defined(__OpenBSD__)
+	/* The following only works with OpenBSD > 5.1 as it requires
+	 * real thread support. For 5.1 and earlier, -1 is returned. */
+	tid = syscall(SYS_getthrid);
+#elif defined(__sun__)
+	tid = _lwp_self();
+#else
+	tid = -1;
+#endif
+
+	if (tid == -1) {
+		/* If we don't have a thread ID, at least return a unique
+		 * value that can be used to distinguish individual
+		 * threads. */
+		tid = (int)(intptr_t)pthread_self();
+	}
+
+	return tl_tid = (unsigned int)tid;
+}
diff --git a/hardware/src/libusb/os/threads_posix.h b/hardware/src/libusb/os/threads_posix.h
new file mode 100644
index 0000000..9322834
--- /dev/null
+++ b/hardware/src/libusb/os/threads_posix.h
@@ -0,0 +1,98 @@
+/*
+ * libusb synchronization using POSIX Threads
+ *
+ * Copyright © 2010 Peter Stuge <peter@stuge.se>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef LIBUSB_THREADS_POSIX_H
+#define LIBUSB_THREADS_POSIX_H
+
+#include <pthread.h>
+
+#define PTHREAD_CHECK(expression)	ASSERT_EQ(expression, 0)
+
+#define USBI_MUTEX_INITIALIZER	PTHREAD_MUTEX_INITIALIZER
+typedef pthread_mutex_t usbi_mutex_static_t;
+static inline void usbi_mutex_static_lock(usbi_mutex_static_t *mutex)
+{
+	PTHREAD_CHECK(pthread_mutex_lock(mutex));
+}
+static inline void usbi_mutex_static_unlock(usbi_mutex_static_t *mutex)
+{
+	PTHREAD_CHECK(pthread_mutex_unlock(mutex));
+}
+
+typedef pthread_mutex_t usbi_mutex_t;
+static inline void usbi_mutex_init(usbi_mutex_t *mutex)
+{
+	PTHREAD_CHECK(pthread_mutex_init(mutex, NULL));
+}
+static inline void usbi_mutex_lock(usbi_mutex_t *mutex)
+{
+	PTHREAD_CHECK(pthread_mutex_lock(mutex));
+}
+static inline void usbi_mutex_unlock(usbi_mutex_t *mutex)
+{
+	PTHREAD_CHECK(pthread_mutex_unlock(mutex));
+}
+static inline int usbi_mutex_trylock(usbi_mutex_t *mutex)
+{
+	return pthread_mutex_trylock(mutex) == 0;
+}
+static inline void usbi_mutex_destroy(usbi_mutex_t *mutex)
+{
+	PTHREAD_CHECK(pthread_mutex_destroy(mutex));
+}
+
+typedef pthread_cond_t usbi_cond_t;
+void usbi_cond_init(pthread_cond_t *cond);
+static inline void usbi_cond_wait(usbi_cond_t *cond, usbi_mutex_t *mutex)
+{
+	PTHREAD_CHECK(pthread_cond_wait(cond, mutex));
+}
+int usbi_cond_timedwait(usbi_cond_t *cond,
+	usbi_mutex_t *mutex, const struct timeval *tv);
+static inline void usbi_cond_broadcast(usbi_cond_t *cond)
+{
+	PTHREAD_CHECK(pthread_cond_broadcast(cond));
+}
+static inline void usbi_cond_destroy(usbi_cond_t *cond)
+{
+	PTHREAD_CHECK(pthread_cond_destroy(cond));
+}
+
+typedef pthread_key_t usbi_tls_key_t;
+static inline void usbi_tls_key_create(usbi_tls_key_t *key)
+{
+	PTHREAD_CHECK(pthread_key_create(key, NULL));
+}
+static inline void *usbi_tls_key_get(usbi_tls_key_t key)
+{
+	return pthread_getspecific(key);
+}
+static inline void usbi_tls_key_set(usbi_tls_key_t key, void *ptr)
+{
+	PTHREAD_CHECK(pthread_setspecific(key, ptr));
+}
+static inline void usbi_tls_key_delete(usbi_tls_key_t key)
+{
+	PTHREAD_CHECK(pthread_key_delete(key));
+}
+
+unsigned int usbi_get_tid(void);
+
+#endif /* LIBUSB_THREADS_POSIX_H */
diff --git a/hardware/src/libusb/strerror.c b/hardware/src/libusb/strerror.c
new file mode 100644
index 0000000..9445fa9
--- /dev/null
+++ b/hardware/src/libusb/strerror.c
@@ -0,0 +1,223 @@
+/*
+ * libusb strerror code
+ * Copyright © 2013 Hans de Goede <hdegoede@redhat.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+
+#include <ctype.h>
+#include <string.h>
+
+/** \ingroup libusb_misc
+ * How to add a new \ref libusb_strerror() translation:
+ * <ol>
+ * <li> Download the latest \c strerror.c from:<br>
+ *      https://raw.github.com/libusb/libusb/master/libusb/strerror.c </li>
+ * <li> Open the file in an UTF-8 capable editor </li>
+ * <li> Add the 2 letter <a href="http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes">ISO 639-1</a>
+ *      code for your locale at the end of \c usbi_locale_supported[]<br>
+ *    Eg. for Chinese, you would add "zh" so that:
+ *    \code... usbi_locale_supported[] = { "en", "nl", "fr" };\endcode
+ *    becomes:
+ *    \code... usbi_locale_supported[] = { "en", "nl", "fr", "zh" };\endcode </li>
+ * <li> Copy the <tt>{ / * English (en) * / ... }</tt> section and add it at the end of \c usbi_localized_errors<br>
+ *    Eg. for Chinese, the last section of \c usbi_localized_errors could look like:
+ *    \code
+ *     }, { / * Chinese (zh) * /
+ *         "Success",
+ *         ...
+ *         "Other error",
+ *     },
+ * };\endcode </li>
+ * <li> Translate each of the English messages from the section you copied into your language </li>
+ * <li> Save the file (in UTF-8 format) and send it to \c libusb-devel\@lists.sourceforge.net </li>
+ * </ol>
+ */
+
+static const char * const usbi_locale_supported[] = { "en", "nl", "fr", "ru", "de", "hu" };
+static const char * const usbi_localized_errors[ARRAYSIZE(usbi_locale_supported)][LIBUSB_ERROR_COUNT] = {
+	{ /* English (en) */
+		"Success",
+		"Input/Output Error",
+		"Invalid parameter",
+		"Access denied (insufficient permissions)",
+		"No such device (it may have been disconnected)",
+		"Entity not found",
+		"Resource busy",
+		"Operation timed out",
+		"Overflow",
+		"Pipe error",
+		"System call interrupted (perhaps due to signal)",
+		"Insufficient memory",
+		"Operation not supported or unimplemented on this platform",
+		"Other error",
+	}, { /* Dutch (nl) */
+		"Gelukt",
+		"Invoer-/uitvoerfout",
+		"Ongeldig argument",
+		"Toegang geweigerd (onvoldoende toegangsrechten)",
+		"Apparaat bestaat niet (verbinding met apparaat verbroken?)",
+		"Niet gevonden",
+		"Apparaat of hulpbron is bezig",
+		"Bewerking verlopen",
+		"Waarde is te groot",
+		"Gebroken pijp",
+		"Onderbroken systeemaanroep",
+		"Onvoldoende geheugen beschikbaar",
+		"Bewerking wordt niet ondersteund",
+		"Andere fout",
+	}, { /* French (fr) */
+		"Succès",
+		"Erreur d'entrée/sortie",
+		"Paramètre invalide",
+		"Accès refusé (permissions insuffisantes)",
+		"Périphérique introuvable (peut-être déconnecté)",
+		"Elément introuvable",
+		"Resource déjà occupée",
+		"Operation expirée",
+		"Débordement",
+		"Erreur de pipe",
+		"Appel système abandonné (peut-être à cause d’un signal)",
+		"Mémoire insuffisante",
+		"Opération non supportée or non implémentée sur cette plateforme",
+		"Autre erreur",
+	}, { /* Russian (ru) */
+		"Успех",
+		"Ошибка ввода/вывода",
+		"Неверный параметр",
+		"Доступ запрещён (не хватает прав)",
+		"Устройство отсутствует (возможно, оно было отсоединено)",
+		"Элемент не найден",
+		"Ресурс занят",
+		"Истекло время ожидания операции",
+		"Переполнение",
+		"Ошибка канала",
+		"Системный вызов прерван (возможно, сигналом)",
+		"Память исчерпана",
+		"Операция не поддерживается данной платформой",
+		"Неизвестная ошибка"
+	}, { /* German (de) */
+		"Erfolgreich",
+		"Eingabe-/Ausgabefehler",
+		"Ungültiger Parameter",
+		"Keine Berechtigung (Zugriffsrechte fehlen)",
+		"Kein passendes Gerät gefunden (es könnte entfernt worden sein)",
+		"Entität nicht gefunden",
+		"Die Ressource ist belegt",
+		"Die Wartezeit für die Operation ist abgelaufen",
+		"Mehr Daten empfangen als erwartet",
+		"Datenübergabe unterbrochen (broken pipe)",
+		"Unterbrechung während des Betriebssystemaufrufs",
+		"Nicht genügend Hauptspeicher verfügbar",
+		"Die Operation wird nicht unterstützt oder ist auf dieser Platform nicht implementiert",
+		"Allgemeiner Fehler",
+	}, { /* Hungarian (hu) */
+		"Sikeres",
+		"Be-/kimeneti hiba",
+		"Érvénytelen paraméter",
+		"Hozzáférés megtagadva",
+		"Az eszköz nem található (eltávolították?)",
+		"Nem található",
+		"Az erőforrás foglalt",
+		"Időtúllépés",
+		"Túlcsordulás",
+		"Törött adatcsatorna",
+		"Rendszerhívás megszakítva",
+		"Nincs elég memória",
+		"A művelet nem támogatott ezen a rendszeren",
+		"Általános hiba",
+	},
+};
+
+static const char * const (*usbi_error_strings)[LIBUSB_ERROR_COUNT] = &usbi_localized_errors[0];
+
+/** \ingroup libusb_misc
+ * Set the language, and only the language, not the encoding! used for
+ * translatable libusb messages.
+ *
+ * This takes a locale string in the default setlocale format: lang[-region]
+ * or lang[_country_region][.codeset]. Only the lang part of the string is
+ * used, and only 2 letter ISO 639-1 codes are accepted for it, such as "de".
+ * The optional region, country_region or codeset parts are ignored. This
+ * means that functions which return translatable strings will NOT honor the
+ * specified encoding.
+ * All strings returned are encoded as UTF-8 strings.
+ *
+ * If libusb_setlocale() is not called, all messages will be in English.
+ *
+ * The following functions return translatable strings: libusb_strerror().
+ * Note that the libusb log messages controlled through libusb_set_debug()
+ * are not translated, they are always in English.
+ *
+ * For POSIX UTF-8 environments if you want libusb to follow the standard
+ * locale settings, call libusb_setlocale(setlocale(LC_MESSAGES, NULL)),
+ * after your app has done its locale setup.
+ *
+ * \param locale locale-string in the form of lang[_country_region][.codeset]
+ * or lang[-region], where lang is a 2 letter ISO 639-1 code
+ * \returns LIBUSB_SUCCESS on success
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the locale doesn't meet the requirements
+ * \returns LIBUSB_ERROR_NOT_FOUND if the requested language is not supported
+ * \returns a LIBUSB_ERROR code on other errors
+ */
+
+int API_EXPORTED libusb_setlocale(const char *locale)
+{
+	size_t i;
+
+	if (!locale || strlen(locale) < 2
+	    || (locale[2] != '\0' && locale[2] != '-' && locale[2] != '_' && locale[2] != '.'))
+		return LIBUSB_ERROR_INVALID_PARAM;
+
+	for (i = 0; i < ARRAYSIZE(usbi_locale_supported); i++) {
+		if (usbi_locale_supported[i][0] == tolower((unsigned char)locale[0])
+		    && usbi_locale_supported[i][1] == tolower((unsigned char)locale[1]))
+			break;
+	}
+
+	if (i == ARRAYSIZE(usbi_locale_supported))
+		return LIBUSB_ERROR_NOT_FOUND;
+
+	usbi_error_strings = &usbi_localized_errors[i];
+
+	return LIBUSB_SUCCESS;
+}
+
+/** \ingroup libusb_misc
+ * Returns a constant string with a short description of the given error code,
+ * this description is intended for displaying to the end user and will be in
+ * the language set by libusb_setlocale().
+ *
+ * The returned string is encoded in UTF-8.
+ *
+ * The messages always start with a capital letter and end without any dot.
+ * The caller must not free() the returned string.
+ *
+ * \param errcode the error code whose description is desired
+ * \returns a short description of the error code in UTF-8 encoding
+ */
+DEFAULT_VISIBILITY const char * LIBUSB_CALL libusb_strerror(int errcode)
+{
+	int errcode_index = -errcode;
+
+	if (errcode_index < 0 || errcode_index >= LIBUSB_ERROR_COUNT) {
+		/* "Other Error", which should always be our last message, is returned */
+		errcode_index = LIBUSB_ERROR_COUNT - 1;
+	}
+
+	return (*usbi_error_strings)[errcode_index];
+}
diff --git a/hardware/src/libusb/sync.c b/hardware/src/libusb/sync.c
new file mode 100644
index 0000000..1fa1f0b
--- /dev/null
+++ b/hardware/src/libusb/sync.c
@@ -0,0 +1,334 @@
+/* -*- Mode: C; indent-tabs-mode:t ; c-basic-offset:8 -*- */
+/*
+ * Synchronous I/O functions for libusb
+ * Copyright © 2007-2008 Daniel Drake <dsd@gentoo.org>
+ * Copyright © 2019 Nathan Hjelm <hjelmn@cs.unm.edu>
+ * Copyright © 2019 Google LLC. All rights reserved.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libusbi.h"
+
+#include <string.h>
+
+/**
+ * @defgroup libusb_syncio Synchronous device I/O
+ *
+ * This page documents libusb's synchronous (blocking) API for USB device I/O.
+ * This interface is easy to use but has some limitations. More advanced users
+ * may wish to consider using the \ref libusb_asyncio "asynchronous I/O API" instead.
+ */
+
+static void LIBUSB_CALL sync_transfer_cb(struct libusb_transfer *transfer)
+{
+	int *completed = transfer->user_data;
+	*completed = 1;
+	usbi_dbg(TRANSFER_CTX(transfer), "actual_length=%d", transfer->actual_length);
+	/* caller interprets result and frees transfer */
+}
+
+static void sync_transfer_wait_for_completion(struct libusb_transfer *transfer)
+{
+	int r, *completed = transfer->user_data;
+	struct libusb_context *ctx = HANDLE_CTX(transfer->dev_handle);
+
+	while (!*completed) {
+		r = libusb_handle_events_completed(ctx, completed);
+		if (r < 0) {
+			if (r == LIBUSB_ERROR_INTERRUPTED)
+				continue;
+			usbi_err(ctx, "libusb_handle_events failed: %s, cancelling transfer and retrying",
+				 libusb_error_name(r));
+			libusb_cancel_transfer(transfer);
+			continue;
+		}
+		if (NULL == transfer->dev_handle) {
+			/* transfer completion after libusb_close() */
+			transfer->status = LIBUSB_TRANSFER_NO_DEVICE;
+			*completed = 1;
+		}
+	}
+}
+
+/** \ingroup libusb_syncio
+ * Perform a USB control transfer.
+ *
+ * The direction of the transfer is inferred from the bmRequestType field of
+ * the setup packet.
+ *
+ * The wValue, wIndex and wLength fields values should be given in host-endian
+ * byte order.
+ *
+ * \param dev_handle a handle for the device to communicate with
+ * \param bmRequestType the request type field for the setup packet
+ * \param bRequest the request field for the setup packet
+ * \param wValue the value field for the setup packet
+ * \param wIndex the index field for the setup packet
+ * \param data a suitably-sized data buffer for either input or output
+ * (depending on direction bits within bmRequestType)
+ * \param wLength the length field for the setup packet. The data buffer should
+ * be at least this size.
+ * \param timeout timeout (in milliseconds) that this function should wait
+ * before giving up due to no response being received. For an unlimited
+ * timeout, use value 0.
+ * \returns on success, the number of bytes actually transferred
+ * \returns LIBUSB_ERROR_TIMEOUT if the transfer timed out
+ * \returns LIBUSB_ERROR_PIPE if the control request was not supported by the
+ * device
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_BUSY if called from event handling context
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the transfer size is larger than
+ * the operating system and/or hardware can support (see \ref asynclimits)
+ * \returns another LIBUSB_ERROR code on other failures
+ */
+int API_EXPORTED libusb_control_transfer(libusb_device_handle *dev_handle,
+	uint8_t bmRequestType, uint8_t bRequest, uint16_t wValue, uint16_t wIndex,
+	unsigned char *data, uint16_t wLength, unsigned int timeout)
+{
+	struct libusb_transfer *transfer;
+	unsigned char *buffer;
+	int completed = 0;
+	int r;
+
+	if (usbi_handling_events(HANDLE_CTX(dev_handle)))
+		return LIBUSB_ERROR_BUSY;
+
+	transfer = libusb_alloc_transfer(0);
+	if (!transfer)
+		return LIBUSB_ERROR_NO_MEM;
+
+	buffer = malloc(LIBUSB_CONTROL_SETUP_SIZE + wLength);
+	if (!buffer) {
+		libusb_free_transfer(transfer);
+		return LIBUSB_ERROR_NO_MEM;
+	}
+
+	libusb_fill_control_setup(buffer, bmRequestType, bRequest, wValue, wIndex,
+		wLength);
+	if ((bmRequestType & LIBUSB_ENDPOINT_DIR_MASK) == LIBUSB_ENDPOINT_OUT)
+		memcpy(buffer + LIBUSB_CONTROL_SETUP_SIZE, data, wLength);
+
+	libusb_fill_control_transfer(transfer, dev_handle, buffer,
+		sync_transfer_cb, &completed, timeout);
+	transfer->flags = LIBUSB_TRANSFER_FREE_BUFFER;
+	r = libusb_submit_transfer(transfer);
+	if (r < 0) {
+		libusb_free_transfer(transfer);
+		return r;
+	}
+
+	sync_transfer_wait_for_completion(transfer);
+
+	if ((bmRequestType & LIBUSB_ENDPOINT_DIR_MASK) == LIBUSB_ENDPOINT_IN)
+		memcpy(data, libusb_control_transfer_get_data(transfer),
+			transfer->actual_length);
+
+	switch (transfer->status) {
+	case LIBUSB_TRANSFER_COMPLETED:
+		r = transfer->actual_length;
+		break;
+	case LIBUSB_TRANSFER_TIMED_OUT:
+		r = LIBUSB_ERROR_TIMEOUT;
+		break;
+	case LIBUSB_TRANSFER_STALL:
+		r = LIBUSB_ERROR_PIPE;
+		break;
+	case LIBUSB_TRANSFER_NO_DEVICE:
+		r = LIBUSB_ERROR_NO_DEVICE;
+		break;
+	case LIBUSB_TRANSFER_OVERFLOW:
+		r = LIBUSB_ERROR_OVERFLOW;
+		break;
+	case LIBUSB_TRANSFER_ERROR:
+	case LIBUSB_TRANSFER_CANCELLED:
+		r = LIBUSB_ERROR_IO;
+		break;
+	default:
+		usbi_warn(HANDLE_CTX(dev_handle),
+			"unrecognised status code %d", transfer->status);
+		r = LIBUSB_ERROR_OTHER;
+	}
+
+	libusb_free_transfer(transfer);
+	return r;
+}
+
+static int do_sync_bulk_transfer(struct libusb_device_handle *dev_handle,
+	unsigned char endpoint, unsigned char *buffer, int length,
+	int *transferred, unsigned int timeout, unsigned char type)
+{
+	struct libusb_transfer *transfer;
+	int completed = 0;
+	int r;
+
+	if (usbi_handling_events(HANDLE_CTX(dev_handle)))
+		return LIBUSB_ERROR_BUSY;
+
+	transfer = libusb_alloc_transfer(0);
+	if (!transfer)
+		return LIBUSB_ERROR_NO_MEM;
+
+	libusb_fill_bulk_transfer(transfer, dev_handle, endpoint, buffer, length,
+		sync_transfer_cb, &completed, timeout);
+	transfer->type = type;
+
+	r = libusb_submit_transfer(transfer);
+	if (r < 0) {
+		libusb_free_transfer(transfer);
+		return r;
+	}
+
+	sync_transfer_wait_for_completion(transfer);
+
+	if (transferred)
+		*transferred = transfer->actual_length;
+
+	switch (transfer->status) {
+	case LIBUSB_TRANSFER_COMPLETED:
+		r = 0;
+		break;
+	case LIBUSB_TRANSFER_TIMED_OUT:
+		r = LIBUSB_ERROR_TIMEOUT;
+		break;
+	case LIBUSB_TRANSFER_STALL:
+		r = LIBUSB_ERROR_PIPE;
+		break;
+	case LIBUSB_TRANSFER_OVERFLOW:
+		r = LIBUSB_ERROR_OVERFLOW;
+		break;
+	case LIBUSB_TRANSFER_NO_DEVICE:
+		r = LIBUSB_ERROR_NO_DEVICE;
+		break;
+	case LIBUSB_TRANSFER_ERROR:
+	case LIBUSB_TRANSFER_CANCELLED:
+		r = LIBUSB_ERROR_IO;
+		break;
+	default:
+		usbi_warn(HANDLE_CTX(dev_handle),
+			"unrecognised status code %d", transfer->status);
+		r = LIBUSB_ERROR_OTHER;
+	}
+
+	libusb_free_transfer(transfer);
+	return r;
+}
+
+/** \ingroup libusb_syncio
+ * Perform a USB bulk transfer. The direction of the transfer is inferred from
+ * the direction bits of the endpoint address.
+ *
+ * For bulk reads, the <tt>length</tt> field indicates the maximum length of
+ * data you are expecting to receive. If less data arrives than expected,
+ * this function will return that data, so be sure to check the
+ * <tt>transferred</tt> output parameter.
+ *
+ * You should also check the <tt>transferred</tt> parameter for bulk writes.
+ * Not all of the data may have been written.
+ *
+ * Also check <tt>transferred</tt> when dealing with a timeout error code.
+ * libusb may have to split your transfer into a number of chunks to satisfy
+ * underlying O/S requirements, meaning that the timeout may expire after
+ * the first few chunks have completed. libusb is careful not to lose any data
+ * that may have been transferred; do not assume that timeout conditions
+ * indicate a complete lack of I/O. See \ref asynctimeout for more details.
+ *
+ * \param dev_handle a handle for the device to communicate with
+ * \param endpoint the address of a valid endpoint to communicate with
+ * \param data a suitably-sized data buffer for either input or output
+ * (depending on endpoint)
+ * \param length for bulk writes, the number of bytes from data to be sent. for
+ * bulk reads, the maximum number of bytes to receive into the data buffer.
+ * \param transferred output location for the number of bytes actually
+ * transferred. Since version 1.0.21 (\ref LIBUSB_API_VERSION >= 0x01000105),
+ * it is legal to pass a NULL pointer if you do not wish to receive this
+ * information.
+ * \param timeout timeout (in milliseconds) that this function should wait
+ * before giving up due to no response being received. For an unlimited
+ * timeout, use value 0.
+ *
+ * \returns 0 on success (and populates <tt>transferred</tt>)
+ * \returns LIBUSB_ERROR_TIMEOUT if the transfer timed out (and populates
+ * <tt>transferred</tt>)
+ * \returns LIBUSB_ERROR_PIPE if the endpoint halted
+ * \returns LIBUSB_ERROR_OVERFLOW if the device offered more data, see
+ * \ref libusb_packetoverflow
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_BUSY if called from event handling context
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the transfer size is larger than
+ * the operating system and/or hardware can support (see \ref asynclimits)
+ * \returns another LIBUSB_ERROR code on other failures
+ */
+int API_EXPORTED libusb_bulk_transfer(libusb_device_handle *dev_handle,
+	unsigned char endpoint, unsigned char *data, int length,
+	int *transferred, unsigned int timeout)
+{
+	return do_sync_bulk_transfer(dev_handle, endpoint, data, length,
+		transferred, timeout, LIBUSB_TRANSFER_TYPE_BULK);
+}
+
+/** \ingroup libusb_syncio
+ * Perform a USB interrupt transfer. The direction of the transfer is inferred
+ * from the direction bits of the endpoint address.
+ *
+ * For interrupt reads, the <tt>length</tt> field indicates the maximum length
+ * of data you are expecting to receive. If less data arrives than expected,
+ * this function will return that data, so be sure to check the
+ * <tt>transferred</tt> output parameter.
+ *
+ * You should also check the <tt>transferred</tt> parameter for interrupt
+ * writes. Not all of the data may have been written.
+ *
+ * Also check <tt>transferred</tt> when dealing with a timeout error code.
+ * libusb may have to split your transfer into a number of chunks to satisfy
+ * underlying O/S requirements, meaning that the timeout may expire after
+ * the first few chunks have completed. libusb is careful not to lose any data
+ * that may have been transferred; do not assume that timeout conditions
+ * indicate a complete lack of I/O. See \ref asynctimeout for more details.
+ *
+ * The default endpoint bInterval value is used as the polling interval.
+ *
+ * \param dev_handle a handle for the device to communicate with
+ * \param endpoint the address of a valid endpoint to communicate with
+ * \param data a suitably-sized data buffer for either input or output
+ * (depending on endpoint)
+ * \param length for bulk writes, the number of bytes from data to be sent. for
+ * bulk reads, the maximum number of bytes to receive into the data buffer.
+ * \param transferred output location for the number of bytes actually
+ * transferred. Since version 1.0.21 (\ref LIBUSB_API_VERSION >= 0x01000105),
+ * it is legal to pass a NULL pointer if you do not wish to receive this
+ * information.
+ * \param timeout timeout (in milliseconds) that this function should wait
+ * before giving up due to no response being received. For an unlimited
+ * timeout, use value 0.
+ *
+ * \returns 0 on success (and populates <tt>transferred</tt>)
+ * \returns LIBUSB_ERROR_TIMEOUT if the transfer timed out
+ * \returns LIBUSB_ERROR_PIPE if the endpoint halted
+ * \returns LIBUSB_ERROR_OVERFLOW if the device offered more data, see
+ * \ref libusb_packetoverflow
+ * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
+ * \returns LIBUSB_ERROR_BUSY if called from event handling context
+ * \returns LIBUSB_ERROR_INVALID_PARAM if the transfer size is larger than
+ * the operating system and/or hardware can support (see \ref asynclimits)
+ * \returns another LIBUSB_ERROR code on other error
+ */
+int API_EXPORTED libusb_interrupt_transfer(libusb_device_handle *dev_handle,
+	unsigned char endpoint, unsigned char *data, int length,
+	int *transferred, unsigned int timeout)
+{
+	return do_sync_bulk_transfer(dev_handle, endpoint, data, length,
+		transferred, timeout, LIBUSB_TRANSFER_TYPE_INTERRUPT);
+}
diff --git a/hardware/src/libusb/version.h b/hardware/src/libusb/version.h
new file mode 100644
index 0000000..1fe9946
--- /dev/null
+++ b/hardware/src/libusb/version.h
@@ -0,0 +1,18 @@
+/* This file is parsed by m4 and windres and RC.EXE so please keep it simple. */
+#include "version_nano.h"
+#ifndef LIBUSB_MAJOR
+#define LIBUSB_MAJOR 1
+#endif
+#ifndef LIBUSB_MINOR
+#define LIBUSB_MINOR 0
+#endif
+#ifndef LIBUSB_MICRO
+#define LIBUSB_MICRO 25
+#endif
+#ifndef LIBUSB_NANO
+#define LIBUSB_NANO 0
+#endif
+/* LIBUSB_RC is the release candidate suffix. Should normally be empty. */
+#ifndef LIBUSB_RC
+#define LIBUSB_RC ""
+#endif
diff --git a/hardware/src/libusb/version_nano.h b/hardware/src/libusb/version_nano.h
new file mode 100644
index 0000000..03df9b4
--- /dev/null
+++ b/hardware/src/libusb/version_nano.h
@@ -0,0 +1 @@
+#define LIBUSB_NANO 11692