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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

     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>.