SLEEP(9) MidnightBSD Kernel Developer’s Manual SLEEP(9)

NAME

msleep, msleep_spin, pause, tsleep, 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_spin(void *chan, struct mtx *mtx, const char *wmesg, int timo);

void

pause(const char *wmesg, int timo);

int

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

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. If priority includes the PCATCH flag, signals are checked before and after sleeping, otherwise signals are not checked. If PCATCH is set and a signal needs to be delivered, 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.

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.

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

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)

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.

AUTHORS

This manual page was written by Jörg Wunsch 〈joerg@FreeBSD.org〉.

MidnightBSD 0.3 February 27, 2007 MidnightBSD 0.3