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NAME

     kproc_start, kproc_shutdown, kthread_create, kthread_exit,
     kthread_resume, kthread_suspend, kthread_suspend_check - kernel threads

SYNOPSIS

     #include <sys/kthread.h>

     void
     kproc_start(const void *udata);

     void
     kproc_shutdown(void *arg, int howto);

     int
     kthread_create(void (*func)(void *), void *arg, struct proc **newpp,
             int flags, int pages, const char *fmt, ...);

     void
     kthread_exit(int ecode);

     int
     kthread_resume(struct proc *p);

     int
     kthread_suspend(struct proc *p, int timo);

     void
     kthread_suspend_check(struct proc *p);

DESCRIPTION

     The function kproc_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 kproc_desc which describes the kernel thread that should be
     created:

           struct kproc_desc {
                   char            *arg0;
                   void            (*func)(void);
                   struct proc     **global_procpp;
           };

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

           arg0           String to be used for the name of the process.  This
                          string will be copied into the p_comm member of the
                          new process’ struct proc.

           func           The main function for this kernel process to run.

           global_procpp  A pointer to a struct proc pointer that should be
                          updated to point to the newly created process’
                          process structure.  If this variable is NULL, then
                          it is ignored.

     The kthread_create() function is used to create a kernel thread.  The new
     thread shares its address space with process 0, the swapper process, and
     runs in kernel mode only.  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
     process.  The newpp pointer points to a struct proc pointer that is to be
     updated to point to the newly created process.  If this argument is NULL,
     then it is ignored.  The flags argument specifies a set of flags as
     described in rfork(2).  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 arguments form a printf(9) argument
     list that is used to build the name of the new thread and is stored in
     the p_comm member of the new thread’s struct proc.

     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 ecode argument
     specifies the exit status of the thread.  While exiting, the function
     exit1(9) will initiate a call to wakeup(9) on the thread handle.

     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 curproc 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
     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 p
     argument points to the struct proc 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
     itself.

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

EXAMPLES

     This example demonstrates the use of a struct kproc_desc and the
     functions kproc_start(), kproc_shutdown(), and kthread_suspend_check() to
     run the “bufdaemon” process.

           static struct proc *bufdaemonproc;

           static struct kproc_desc buf_kp = {
                   "bufdaemon",
                   buf_daemon,
                   &bufdaemonproc
           };
           SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kproc_start,
               &buf_kp)

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

ERRORS

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

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

     The kthread_create() function will fail if:

     [EAGAIN]           The system-imposed limit on the total number of
                        processes under execution would be exceeded.  The
                        limit is given by the sysctl(3) MIB variable
                        KERN_MAXPROC.

     [EINVAL]           The RFCFDG flag was specified in the flags parameter.

SEE ALSO

     rfork(2), exit1(9), SYSINIT(9), wakeup(9)

HISTORY

     The kproc_start() function first appeared in FreeBSD 2.2.  The
     kproc_shutdown(), kthread_create(), kthread_exit(), kthread_resume(),
     kthread_suspend(), and kthread_suspend_check() functions were introduced
     in FreeBSD 4.0.  Prior to FreeBSD 5.0, the kproc_shutdown(),
     kthread_resume(), kthread_suspend(), and kthread_suspend_check()
     functions were named shutdown_kproc(), resume_kproc(), shutdown_kproc(),
     and kproc_suspend_loop(), respectively.