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

NAME
     kthread_start, kthread_shutdown, kthread_add, kthread_exit,
     kthread_resume, kthread_suspend, kthread_suspend_check -- kernel threads

SYNOPSIS
     #include <sys/kthread.h>

     void
     kthread_start(const void *udata);

     void
     kthread_shutdown(void *arg, int howto);

     void
     kthread_exit(void);

     int
     kthread_resume(struct thread *td);

     int
     kthread_suspend(struct thread *td,	int timo);

     void
     kthread_suspend_check(void);

     #include <sys/unistd.h>

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

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

DESCRIPTION
     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-
     ated:

	   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() in order to check if
     the it has	been asked to suspend.	If it has, it will msleep(9) until it
     is	told to	resume.	 Once it has been told to resume it will return	allow-
     ing execution 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 sus-
     pend itself.

     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
     kthread_suspend().

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

EXAMPLES
     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 = {
		   "bufdaemon",
		   buf_daemon,
		   &bufdaemonthread
	   };
	   SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kthread_start,
	       &buf_kp)

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

ERRORS
     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.

SEE ALSO
     kproc(9), SYSINIT(9), wakeup(9)

HISTORY
     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.

FreeBSD	10.1		       January 24, 2010			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | ERRORS | SEE ALSO | HISTORY

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