January 27, 2009 KTHREAD 9

### NAME

kthread_create, kthread_destroy, kthread_exit - kernel threads

### SYNOPSIS

```int` kthread_create(```pri_t pri``, int flags``, struct cpu_info *ci``, void (*func)(void *)``, void *arg``, lwp_t **newlp``, const char *fmt``, ...`)
```void` kthread_destroy(```lwp_t *l`)
```void` kthread_exit(```int ecode`)

### DESCRIPTION

Kernel threads are light-weight processes which execute entirely within the kernel.

Any process can request the creation of a new kernel thread. Kernel threads are not swapped out during memory congestion. The VM space and limits are shared with proc0 (usually swapper).

### FUNCTIONS

kthread_create(```pri``, flags``, ci``, func``, arg``, newlp``, fmt``, ...`)
  Create a kernel thread. The arguments are as follows.

  ```pri`
    Priority level for the thread. If no priority level is desired specify ```PRI_NONE`, causing kthread_create(``) to select the default priority level.

  ```flags`
    Flags that can be logically ORed together to alter the thread's behaviour.

    ```KTHREAD_IDLE`: causes the thread to be created in the ```LSIDL` (idle) state. By default, the threads are created in the ```LSRUN` (runnable) state, meaning they will begin execution shortly after creation.

    ```KTHREAD_MPSAFE`: Specifies that the thread does its own locking and so is multiprocessor safe. If not specified, the global kernel lock will be held whenever the thread is running (unless explicitly dropped by the thread).

    ```KTHREAD_INTR`: Specifies that the thread services device interrupts. This flag is intended for kernel internal use and should not normally be specified.

    ```KTHREAD_TS`: Causes the kthread to be created in the SCHED_OTHER class (timeshared). The threads' priority will be dynamically adjusted by the scheduler. Increased activity by the kthread will cause its priority to fall; decreased activity will cause its priority to rise. By default, kthreads are created in the SCHED_RR class, with a fixed priority specified by pri. Threads in the SCHED_RR class do not have their priority dynamically adjusted by the scheduler.

  ```ci`
    If non-NULL, the thread will be created bound to the CPU specified by ```ci`, meaning that it will only ever execute on that CPU. By default, the threads are free to execute on any CPU in the system.

  ```func`
    A function to be called when the thread begins executing. This function must not return. If the thread runs to completion, it must call kthread_exit(``) to properly terminate itself.

  ```arg`
    An argument to be passed to func(``). May be NULL if not required.

  ```newpl`
    A pointer to receive the new lwp structure for the kernel thread. May be NULL if not required.

  ```fmt`
    A string containing format information used to display the kernel thread name. Must not be NULL.

kthread_destroy(```l`)
  From another thread executing in the kernel, cause a kthread to exit. The kthread must be in the ```LSIDL` (idle) state.

kthread_exit(```ecode`)
  Exit from a kernel thread. Must only be called by a kernel thread.

### RETURN VALUES

Upon successful completion, kthread_create(``) returns 0. Otherwise, the following error values are returned:

[```EAGAIN`]
  The limit on the total number of system processes would be exceeded.

[```EAGAIN`]
  The limit ```RLIMIT_NPROC` on the total number of processes under execution by this user id would be exceeded.

### CODE REFERENCES

This section describes places within the NetBSD source tree where actual code implementing or using the kthread framework can be found. All pathnames are relative to ```/usr/src`.

The kthread framework itself is implemented within the file ```sys/kern/kern_kthread.c`. Data structures and function prototypes for the framework are located in ```sys/sys/kthread.h`.

### SEE ALSO

driver(9)

### HISTORY

The kthread framework appeared in NetBSD1.4.