SLEEP(9) FreeBSD Kernel Developer's Manual SLEEP(9)
NAME
msleep, msleep_sbt, msleep_spin, msleep_spin_sbt, pause, pause_sig,
pause_sbt, tsleep, tsleep_sbt, wakeup, wakeup_one, wakeup_any - wait for
events
SYNOPSIS
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/proc.h>
int
msleep(const void *chan, struct mtx *mtx, int priority,
const char *wmesg, int timo);
int
msleep_sbt(const void *chan, struct mtx *mtx, int priority,
const char *wmesg, sbintime_t sbt, sbintime_t pr, int flags);
int
msleep_spin(const void *chan, struct mtx *mtx, const char *wmesg,
int timo);
int
msleep_spin_sbt(const 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(const void *chan, int priority, const char *wmesg, int timo);
int
tsleep_sbt(const void *chan, int priority, const char *wmesg,
sbintime_t sbt, sbintime_t pr, int flags);
void
wakeup(const void *chan);
void
wakeup_one(const void *chan);
void
wakeup_any(const void *chan);
DESCRIPTION
The functions tsleep(), msleep(), msleep_spin(), pause(), pause_sig(),
pause_sbt(), wakeup(), wakeup_one(), and wakeup_any() 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(), wakeup_one() or
wakeup_any(). The pause_sig() function is a variant of pause() which can
be awakened early by signals.
The wakeup_one() function makes the first highest priority 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.
The wakeup_any() function is similar to wakeup_one(), except that it
makes runnable last thread on the queue (sleeping less), ignoring
fairness. It can be used when threads sleeping on the chan are known to
be identical and there is no reason to be fair.
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 J"rg Wunsch <joerg@FreeBSD.org>.
FreeBSD 13.1-RELEASE-p6 June 19, 2019 FreeBSD 13.1-RELEASE-p6
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