title:Create threads
keywords:kernel,linux,threads

# Create threads

## Starting point

__Makefile__
```Makefile
obj-m += create_thread.o

KDIR ?= /lib/modules/$(shell uname -r)/build

all:
	make -C $(KDIR) M=$(PWD) modules

clean:
	make -C $(KDIR) M=$(PWD) clean
```

__create_thread.c__
```c
#include <linux/module.h>  /* Needed by all modules */
#include <linux/kernel.h>

int create_thread_init( void )
{
	printk(KERN_DEBUG "Hello Thread!\n");
	return 0;
}

void create_thread_exit( void )
{
	printk(KERN_DEBUG "Exit Thread!\n");
}

module_init( create_thread_init );
module_exit( create_thread_exit );

MODULE_LICENSE("GPL");
```

## Create empty thread

Lets create kernel thread that will start and exit straight away


```c
#include <linux/delay.h>

static struct task_struct *task;

static int run_task(void *data) {
	int i=0;
	while(!kthread_should_stop())
	{
		pr_info("run task%d\n", i++);
		msleep(1000);
	} 
	return 0; 
}

int create_thread_init( void ) {
	...
	task = kthread_create(run_task, NULL, "special task");
	if(task) 
	{
		wake_up_process(task);
	} else {
		printk(KERN_ERR "Cannot create kthread\n"); 
	}
	...
}

```

## Kernel threads states
State of kernel thread  - include/linux/sched.h

```c
/* Used in tsk->state: */
#define TASK_RUNNING			0x00000000
#define TASK_INTERRUPTIBLE		0x00000001
#define TASK_UNINTERRUPTIBLE		0x00000002
#define __TASK_STOPPED			0x00000004
#define __TASK_TRACED			0x00000008
/* Used in tsk->exit_state: */
#define EXIT_DEAD			0x00000010
#define EXIT_ZOMBIE			0x00000020
#define EXIT_TRACE			(EXIT_ZOMBIE | EXIT_DEAD)
/* Used in tsk->state again: */
#define TASK_PARKED			0x00000040
#define TASK_DEAD			0x00000080
#define TASK_WAKEKILL			0x00000100
#define TASK_WAKING			0x00000200
#define TASK_NOLOAD			0x00000400
#define TASK_NEW			0x00000800
#define TASK_RTLOCK_WAIT		0x00001000
#define TASK_FREEZABLE			0x00002000
#define __TASK_FREEZABLE_UNSAFE	       (0x00004000 * IS_ENABLED(CONFIG_LOCKDEP))
#define TASK_FROZEN			0x00008000
#define TASK_STATE_MAX			0x00010000

#define TASK_ANY			(TASK_STATE_MAX-1)
```


## List of kernel thread functions used

| Function name | Note |
|---|---|
|  |  |
| wake_up_proces |  |
| kthread_create |  |



## Final result

__create_thread.c__
```c
#include <linux/module.h>  /* Needed by all modules */
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/kthread.h>
#include <linux/err.h>
#include <linux/uaccess.h>
#include <linux/cdev.h>
#include <linux/delay.h>


static struct task_struct *task;

static int run_task(void *data) {
	int i=0;
	while(!kthread_should_stop())
	{
		pr_info("run task %d\n", i++);
		msleep(1000);
	} 
	return 0; 
}

int create_thread_init( void )
{
	printk(KERN_DEBUG "Hello Thread!\n");

	task = kthread_create(run_task, NULL, "special task");
	if(task) 
	{
		wake_up_process(task);
	} else {
		printk(KERN_ERR "Cannot create kthread\n"); 
	}

	return 0;
}

void create_thread_exit( void )
{
	printk(KERN_DEBUG "Exit Thread!\n");
}

module_init( create_thread_init );
module_exit( create_thread_exit );

MODULE_LICENSE("GPL");
```

## Links


https://www.linuxtoday.com/blog/kernel-threads/  
http://www.embeddedlinux.org.cn/EssentialLinuxDeviceDrivers/final/ch03lev1sec1.html  
https://embetronicx.com/tutorials/linux/device-drivers/linux-device-drivers-tutorial-kernel-thread  
https://elixir.bootlin.com/linux/v6.1.21/source/include/linux/sched.h  
https://elixir.bootlin.com/linux/v6.1.21/source/kernel/sysctl.c  
https://girishjoshi.io/post/creating-linux-kernel-threads/  
https://tuxthink.blogspot.com/2011/02/kernel-thread-creation-1.html  
https://elixir.bootlin.com/linux/v6.1.21/source/include/linux/kthread.h#L27