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SLEEP(9)               FreeBSD Kernel Developer's Manual              SLEEP(9)

     sleep, msleep, tsleep, wakeup -- wait for events

     #include <sys/param.h>
     #include <sys/systm.h>
     #include <sys/proc.h>

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

     msleep(void *ident, struct mtx *mtx, int priority, const char *wmesg,
         int timo);

     wakeup(void *ident);

     wakeup_one(void *ident);

     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 arbi-
     trary 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 the Giant lock is not
     held and mtx is NULL, then timo must be non-zero.  If priority includes
     the PCATCH flag, signals are checked before and after sleeping, else sig-
     nals 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 priority 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 associ-
     ated wakeup.

     See above.

     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 manual page was written by Jorg Wunsch <>.

FreeBSD 6.2                    December 17, 1998                   FreeBSD 6.2


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