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+/* -*- 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;
+}