SEM_POST(3P)                                        POSIX Programmer's Manual                                       SEM_POST(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       sem_post - unlock a semaphore (REALTIME)

SYNOPSIS
       #include <semaphore.h>

       int sem_post(sem_t *sem);


DESCRIPTION
       The  sem_post()  function shall unlock the semaphore referenced by sem by performing a semaphore unlock operation on that
       semaphore.

       If the semaphore value resulting from this operation is positive, then no threads were blocked waiting for the  semaphore
       to become unlocked; the semaphore value is simply incremented.

       If the value of the semaphore resulting from this operation is zero, then one of the threads blocked waiting for the sem-
       aphore shall be allowed to return successfully from its call to sem_wait().   If the Process Scheduling  option  is  sup-
       ported,  the  thread to be unblocked shall be chosen in a manner appropriate to the scheduling policies and parameters in
       effect for the blocked threads. In the case of the schedulers SCHED_FIFO  and  SCHED_RR,  the  highest  priority  waiting
       thread  shall be unblocked, and if there is more than one highest priority thread blocked waiting for the semaphore, then
       the highest priority thread that has been waiting the longest shall be unblocked. If the Process Scheduling option is not
       defined, the choice of a thread to unblock is unspecified.

       If the Process Sporadic Server option is supported, and the scheduling policy is SCHED_SPORADIC, the semantics are as per
       SCHED_FIFO above.

       The sem_post() function shall be reentrant with respect to signals and may be invoked from a signal-catching function.

RETURN VALUE
       If successful, the sem_post() function shall return zero; otherwise, the function shall return -1 and set errno to  indi-
       cate the error.

ERRORS
       The sem_post() function shall fail if:

       EINVAL The sem argument does not refer to a valid semaphore.


       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       The sem_post() function is part of the Semaphores option and need not be available on all implementations.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       semctl(),   semget(),   semop(),   sem_timedwait(),   sem_trywait(),   sem_wait(),   the   Base   Definitions  volume  of
       IEEE Std 1003.1-2001, <semaphore.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
       discrepancy  between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
       Standard  is  the  referee   document.   The   original   Standard   can   be   obtained   online   at   http://www.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                  SEM_POST(3P)