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. 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, 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). If PBDRY flag is specified in addition to PCATCH, then the sleeping thread is not stopped while sleeping upon delivery of SIGSTOP or other stop action. Instead, it is waken up, assuming that stop occurs on reaching a stop point when returning to usermode. The flag should be used when sleeping thread owns resources, for instance vnode locks, that should be freed timely.

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. 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)

HISTORY
The functions sleep() and wakeup() were present in 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
-nosplit This manual page was written by Jörg Wunsch <joerg@FreeBSD.org>.