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SLEEP(9) | Kernel Developer's Manual | SLEEP(9) |
NAME¶
msleep, msleep_spin, pause, tsleep, wakeup — wait for eventsSYNOPSIS¶
#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¶
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⟩.December 12, 2009 | Debian |