pause

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

NAME
     msleep, msleep_sbt, msleep_spin, msleep_spin_sbt, pause, pause_sig,
     pause_sbt, tsleep, tsleep_sbt, wakeup — wait for events

SYNOPSIS
     #include <sys/param.h>
     #include <sys/systm.h>
     #include <sys/proc.h>

     int
     msleep(void *chan, struct mtx *mtx, int priority, const char *wmesg,
         int timo);

     int
     msleep_sbt(void *chan, struct mtx *mtx, int priority, const char *wmesg,
         sbintime_t sbt, sbintime_t pr, int flags);

     int
     msleep_spin(void *chan, struct mtx *mtx, const char *wmesg, int timo);

     int
     msleep_spin_sbt(void *chan, struct mtx *mtx, const char *wmesg,
         sbintime_t sbt, sbintime_t pr, int flags);

     int
     pause(const char *wmesg, int timo);

     int
     pause_sig(const char *wmesg, int timo);

     int
     pause_sbt(const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags);

     int
     tsleep(void *chan, int priority, const char *wmesg, int timo);

     int
     tsleep_sbt(void *chan, int priority, const char *wmesg, sbintime_t sbt,
         sbintime_t pr, int flags);

     void
     wakeup(void *chan);

     void
     wakeup_one(void *chan);

DESCRIPTION
     The functions tsleep(), msleep(), msleep_spin(), pause(), pause_sig(),
     pause_sbt(), wakeup(), and wakeup_one() handle event-based thread blocking.
     If a thread must wait for an external event, it is put to sleep by
     tsleep(), msleep(), msleep_spin(), pause(), pause_sig(), or pause_sbt().
     Threads may also wait using one of the locking primitive sleep routines
     mtx_sleep(9), rw_sleep(9), or sx_sleep(9).

     The parameter chan is an arbitrary address that uniquely identifies the
     event on which the thread is being put to sleep.  All threads sleeping on a
     single chan are woken up later by wakeup(), often called from inside an
     interrupt routine, to indicate that the resource the thread was blocking on
     is available now.

     The parameter priority specifies a new priority for the thread as well as
     some optional flags.  If the new priority is not 0, then the thread will be
     made runnable with the specified priority when it resumes.  PZERO should
     never be used, as it is for compatibility only.  A new priority of 0 means
     to use the thread's current priority when it is made runnable again.

     If priority includes the PCATCH flag, pending signals are allowed to
     interrupt the sleep, otherwise pending signals are ignored during the
     sleep.  If PCATCH is set and a signal becomes pending, ERESTART is returned
     if the current system call should be restarted if possible, and EINTR is
     returned if the system call should be interrupted by the signal (return
     EINTR).

     The parameter wmesg is a string describing the sleep condition for tools
     like ps(1).  Due to the limited space of those programs to display
     arbitrary strings, this message should not be longer than 6 characters.

     The parameter timo specifies a timeout for the sleep.  If timo is not 0,
     then the thread will sleep for at most timo / hz seconds.  If the timeout
     expires, then the sleep function will return EWOULDBLOCK.

     msleep_sbt(), msleep_spin_sbt(), pause_sbt() and tsleep_sbt() functions
     take sbt parameter instead of timo.  It allows the caller to specify
     relative or absolute wakeup time with higher resolution in form of
     sbintime_t.  The parameter pr allows the caller to specify wanted absolute
     event precision.  The parameter flags allows the caller to pass additional
     callout_reset_sbt() flags.

     Several of the sleep functions including msleep(), msleep_spin(), and the
     locking primitive sleep routines specify an additional lock parameter.  The
     lock will be released before sleeping and reacquired before the sleep
     routine returns.  If priority includes the PDROP flag, then the lock will
     not be reacquired before returning.  The lock is used to ensure that a
     condition can be checked atomically, and that the current thread can be
     suspended without missing a change to the condition, or an associated
     wakeup.  In addition, all of the sleep routines will fully drop the Giant
     mutex (even if recursed) while the thread is suspended and will reacquire
     the Giant mutex before the function returns.  Note that the Giant mutex may
     be specified as the lock to drop.  In that case, however, the PDROP flag is
     not allowed.

     To avoid lost wakeups, either a lock should be used to protect against
     races, or a timeout should be specified to place an upper bound on the
     delay due to a lost wakeup.  As a result, the tsleep() function should only
     be invoked with a timeout of 0 when the Giant mutex is held.

     The msleep() function requires that mtx reference a default, i.e. non-spin,
     mutex.  Its use is deprecated in favor of mtx_sleep(9) which provides
     identical behavior.

     The msleep_spin() function requires that mtx reference a spin mutex.  The
     msleep_spin() function does not accept a priority parameter and thus does
     not support changing the current thread's priority, the PDROP flag, or
     catching signals via the PCATCH flag.

     The pause() function is a wrapper around tsleep() that suspends execution
     of the current thread for the indicated timeout.  The thread can not be
     awakened early by signals or calls to wakeup() or wakeup_one().  The
     pause_sig() function is a variant of pause() which can be awakened early by
     signals.

     The wakeup_one() function makes the first thread in the queue that is
     sleeping on the parameter chan runnable.  This reduces the load when a
     large number of threads are sleeping on the same address, but only one of
     them can actually do any useful work when made runnable.

     Due to the way it works, the wakeup_one() function requires that only
     related threads sleep on a specific chan address.  It is the programmer's
     responsibility to choose a unique chan value.  The older wakeup() function
     did not require this, though it was never good practice for threads to
     share a chan value.  When converting from wakeup() to wakeup_one(), pay
     particular attention to ensure that no other threads wait on the same chan.

     If the timeout given by timo or sbt is based on an absolute real-time clock
     value, then the thread should copy the global rtc_generation into its
     td_rtcgen member before reading the RTC.  If the real-time clock is
     adjusted, these functions will set td_rtcgen to zero and return zero.  The
     caller should reconsider its orientation with the new RTC value.

RETURN VALUES
     When awakened by a call to wakeup() or wakeup_one(), if a signal is pending
     and PCATCH is specified, a non-zero error code is returned.  If the thread
     is awakened by a call to wakeup() or wakeup_one(), the msleep(),
     msleep_spin(), tsleep(), and locking primitive sleep functions return 0.
     Zero can also be returned when the real-time clock is adjusted; see above
     regarding td_rtcgen.  Otherwise, a non-zero error code is returned.

ERRORS
     msleep(), msleep_spin(), tsleep(), and the locking primitive sleep
     functions will fail if:

     [EINTR]            The PCATCH flag was specified, a signal was caught, and
                        the system call should be interrupted.

     [ERESTART]         The PCATCH flag was specified, a signal was caught, and
                        the system call should be restarted.

     [EWOULDBLOCK]      A non-zero timeout was specified and the timeout
                        expired.

SEE ALSO
     ps(1), locking(9), malloc(9), mi_switch(9), mtx_sleep(9), rw_sleep(9),
     sx_sleep(9), timeout(9)

HISTORY
     The functions sleep() and wakeup() were present in Version 1 AT&T UNIX.
     They were probably also present in the preceding PDP-7 version of UNIX.
     They were the basic process synchronization model.

     The tsleep() function appeared in 4.4BSD and added the parameters wmesg and
     timo.  The sleep() function was removed in FreeBSD 2.2.  The wakeup_one()
     function appeared in FreeBSD 2.2.  The msleep() function appeared in
     FreeBSD 5.0, and the msleep_spin() function appeared in FreeBSD 6.2.  The
     pause() function appeared in FreeBSD 7.0.  The pause_sig() function
     appeared in FreeBSD 12.0.

AUTHORS
     This manual page was written by Jörg Wunsch <joerg@FreeBSD.org>.

BSD                               March 4, 2018                              BSD