sleep, msleep, tsleep, wakeup -- wait for events
tsleep(void *ident, int priority, const char *wmesg, int timo);
msleep(void *ident, struct mtx *mtx, int priority, const char *wmesg,
The functions tsleep() and wakeup() handle event-based process blocking.
If a process must wait for an external event, it is put on sleep by
tsleep(). The parameter ident is an arbitrary address that uniquely
identifies the event on which the process is being asleep. All processes
sleeping on a single ident are woken up later by wakeup(), often called
from inside an interrupt routine, to indicate that the resource the
process was blocking on is available now.
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 wakeup_one() function is used to make the first process in the queue
that is sleeping on the parameter ident runnable. This can prevent the
system from becoming saturated when a large number of processes are
sleeping on the same address, but only one of them can actually do any
useful work when made runnable.
The tsleep() function is the general sleep call. Suspends the current
process until a wakeup is performed on the specified identifier. The
process will then be made runnable with the specified priority. Sleeps
at most timo / hz seconds (0 means no timeout). If pri includes the
PCATCH flag, signals are checked before and after sleeping, else signals
are not checked. Returns 0 if awakened, EWOULDBLOCK if the timeout
expires. 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 msleep() function is a variation on tsleep. The parameter mtx is a
mutex which will be released before sleeping and reacquired before
msleep() returns. If pri includes the PDROP flag, the mtx parameter will
not be reacquired before returning. The mutex is used to ensure that a
condition can be checked atomically, and that the current process can be
suspended without missing a change to the condition, or an associated
ps(1), malloc(9), mi_switch(9)
The sleep/wakeup process synchronization mechanism is very old. It
appeared in a very early version of UNIX.
The tsleep() function appeared in 4.4BSD.
The sleep() function used to be the traditional form. It did not let you
specify a timeout or a wmesg, hence it was discontinued.
This man page was written by Jorg Wunsch <joerg@FreeBSD.org>.
FreeBSD 5.2.1 December 17, 1998 FreeBSD 5.2.1 [ Back ]