Command Section
KTHREAD(9)             FreeBSD 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);


     #include <sys/unistd.h>

     kthread_add(void (*func)(void *), void *arg, struct proc *procp,)(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,)(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

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

           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
     argument 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
     arguments 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
     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 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 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

     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
     functions 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
     created 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()
     functions were named shutdown_kproc(), resume_kproc(), shutdown_kproc(),
     and kproc_suspend_loop(), respectively.

FreeBSD 11.1-RELEASE-p4          July 15, 2014         FreeBSD 11.1-RELEASE-p4
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