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Man Pages


Manual Reference Pages  -  MSLEEP (9)

NAME

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

CONTENTS

Synopsis
Description
Return Values
Errors
See Also
History
Authors

SYNOPSIS


.In sys/param.h
.In sys/systm.h
.In 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" void pause const char *wmesg int timo void 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, 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, or pause. 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). If the PBDRY flag is specified in addition to PCATCH, then the sleeping thread is not stopped when SIGSTOP becomes pending or some other stop action occurs while it is sleeping. Instead, it is woken up, with the assumption that the stop will occur on reaching a stop point when returning to usermode. The flag should be used when the sleeping thread owns resources, for instance vnode locks, that should be released in a timely fashion.

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
.Vt 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 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.

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. 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 AT&T v1 . 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 BSD 4.4 and added the parameters wmesg and timo. The sleep function was removed in
.Fx 2.2 . The wakeup_one function appeared in
.Fx 2.2 . The msleep function appeared in
.Fx 5.0 , and the msleep_spin function appeared in
.Fx 6.2 . The pause function appeared in
.Fx 7.0 .

AUTHORS


.An -nosplit This manual page was written by
.An J\(:org Wunsch Aq joerg@FreeBSD.org .
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