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

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

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

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

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

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

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

     pause(const char *wmesg, int timo);

     pause_sig(const char *wmesg, int timo);

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

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

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

     wakeup(void *chan);

     wakeup_one(void *chan);

     The functions tsleep(), msleep(), msleep_spin(), pause(), pause_sig(),
     pause_sbt(), wakeup(), and	wakeup_one() handle event-based	thread block-
     ing.  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

     If	priority includes the PCATCH flag, pending signals are allowed to in-
     terrupt the sleep,	otherwise pending signals are ignored during the
     sleep.  If	PCATCH is set and a signal becomes pending, ERESTART is	re-
     turned 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	arbi-
     trary 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 rel-
     ative or absolute wakeup time with	higher resolution in form of
     sbintime_t.  The parameter	pr allows the caller to	specify	wanted abso-
     lute event	precision.  The	parameter flags	allows the caller to pass ad-
     ditional 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	pro-
     vides 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 re-
     lated threads sleep on a specific chan address.  It is the	programmer's
     responsibility to choose a	unique chan value.  The	older wakeup() func-
     tion 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

     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.

     When awakened by a	call to	wakeup() or wakeup_one(), if a signal is pend-
     ing 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.

     msleep(), msleep_spin(), tsleep(),	and the	locking	primitive sleep	func-
     tions 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 ex-

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

     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 ap-
     peared 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.

     This manual page was written by Jorg Wunsch <>.

FreeBSD	13.0			 March 4, 2018			  FreeBSD 13.0


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