kthread_start man page on FreeBSD

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KTHREAD(9)		 BSD Kernel Developer's Manual		    KTHREAD(9)

     kthread_start, kthread_shutdown, kthread_add, kthread_exit,
     kthread_resume, kthread_suspend, kthread_suspend_check — kernel threads

     #include <sys/kthread.h>

     kthread_start(const void *udata);

     kthread_shutdown(void *arg, int howto);


     kthread_resume(struct thread *td);

     kthread_suspend(struct thread *td, int timo);

     kthread_suspend_check(struct thread *td);

     #include <sys/unistd.h>

     kthread_add(void (*func)(void *), void *arg, struct proc *procp,
	 struct thread **newtdpp, int flags, int pages, const char *fmt, ...);

     kproc_kthread_add(void (*func)(void *), void *arg, struct proc **procptr,
	 struct thread **tdptr, int flags, int pages, char * procname,
	 const char *fmt, ...);

     In FreeBSD 8.0, the older family of kthread_*(9) functions was renamed to
     be the kproc_*(9) family of functions, as they were previously misnamed
     and actually produced kernel processes.  This new family of kthread_*(9)
     functions was added to produce real kernel threads.  See the kproc(9) man
     page for more information on the renamed calls.  Also note that the
     kproc_kthread_add(9) function appears in both pages as its functionality
     is split.

     The function kthread_start() is used to start “internal” daemons such as
     bufdaemon, pagedaemon, vmdaemon, and the syncer and is intended to be
     called from SYSINIT(9).  The udata argument is actually a pointer to a
     struct kthread_desc which describes the kernel thread that should be cre‐

	   struct kthread_desc {
		   char		   *arg0;
		   void		   (*func)(void);
		   struct thread   **global_threadpp;

     The structure members are used by kthread_start() as follows:

	   arg0		    String to be used for the name of the thread.
			    This string will be copied into the td_name member
			    of the new threads' struct thread.

	   func		    The main function for this kernel thread to run.

	   global_threadpp  A pointer to a struct thread pointer that should
			    be updated to point to the newly created thread's
			    thread structure.  If this variable is NULL, then
			    it is ignored.  The thread will be a subthread of
			    proc0 (PID 0).

     The kthread_add() function is used to create a kernel thread.  The new
     thread runs in kernel mode only.  It is added to the process specified by
     the procp argument, or if that is NULL, to proc0.	The func argument
     specifies the function that the thread should execute.  The arg argument
     is an arbitrary pointer that is passed in as the only argument to func
     when it is called by the new thread.  The newtdpp pointer points to a
     struct thread pointer that is to be updated to point to the newly created
     thread.  If this argument is NULL, then it is ignored.  The flags argu‐
     ment may be set to RFSTOPPED to leave the thread in a stopped state.  The
     caller must call sched_add() to start the thread.	The pages argument
     specifies the size of the new kernel thread's stack in pages.  If 0 is
     used, the default kernel stack size is allocated.	The rest of the argu‐
     ments form a printf(9) argument list that is used to build the name of
     the new thread and is stored in the td_name member of the new thread's
     struct thread.

     The kproc_kthread_add() function is much like the kthread_add() function
     above except that if the kproc does not already exist, it is created.
     This function is better documented in the kproc(9) manual page.

     The kthread_exit() function is used to terminate kernel threads.  It
     should be called by the main function of the kernel thread rather than
     letting the main function return to its caller.

     The kthread_resume(), kthread_suspend(), and kthread_suspend_check()
     functions are used to suspend and resume a kernel thread.	During the
     main loop of its execution, a kernel thread that wishes to allow itself
     to be suspended should call kthread_suspend_check() passing in curthread
     as the only argument.  This function checks to see if the kernel thread
     has been asked to suspend.	 If it has, it will tsleep(9) until it is told
     to resume.	 Once it has been told to resume it will return allowing exe‐
     cution of the kernel thread to continue.  The other two functions are
     used to notify a kernel thread of a suspend or resume request.  The td
     argument points to the struct thread of the kernel thread to suspend or
     resume.  For kthread_suspend(), the timo argument specifies a timeout to
     wait for the kernel thread to acknowledge the suspend request and suspend

     The kthread_shutdown() function is meant to be registered as a shutdown
     event for kernel threads that need to be suspended voluntarily during
     system shutdown so as not to interfere with system shutdown activities.
     The actual suspension of the kernel thread is done with

     The kthread_add(), kthread_resume(), and kthread_suspend() functions
     return zero on success and non-zero on failure.

     This example demonstrates the use of a struct kthread_desc and the func‐
     tions kthread_start(), kthread_shutdown(), and kthread_suspend_check() to
     run the bufdaemon process.

	   static struct thread *bufdaemonthread;

	   static struct kthread_desc buf_kp = {
	   SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kthread_start,

	   static void
		    * This process needs to be suspended prior to shutdown sync.
		   EVENTHANDLER_REGISTER(shutdown_pre_sync, kthread_shutdown,
		       bufdaemonthread, SHUTDOWN_PRI_LAST);
		   for (;;) {

     The kthread_resume() and kthread_suspend() functions will fail if:

     [EINVAL]		The td argument does not reference a kernel thread.

     The kthread_add() function will fail if:

     [ENOMEM]		Memory for a thread's stack could not be allocated.

     kproc(9), SYSINIT(9), wakeup(9)

     The kthread_start() function first appeared in FreeBSD 2.2 where it cre‐
     ated a whole process.  It was converted to create threads in FreeBSD 8.0.
     The kthread_shutdown(), kthread_exit(), kthread_resume(),
     kthread_suspend(), and kthread_suspend_check() functions were introduced
     in FreeBSD 4.0 and were converted to threads in FreeBSD 8.0.  The
     kthread_create() call was renamed to kthread_add() in FreeBSD 8.0.	 The
     old functionality of creating a kernel process was renamed to
     kproc_create(9).  Prior to FreeBSD 5.0, the kthread_shutdown(),
     kthread_resume(), kthread_suspend(), and kthread_suspend_check() func‐
     tions were named shutdown_kproc(), resume_kproc(), shutdown_kproc(), and
     kproc_suspend_loop(), respectively.

BSD			       January 26, 2009				   BSD

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