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

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 sema-
       phore 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 incre-
       mented.

       If the value of the semaphore resulting from this operation  is	zero,  then  one  of  the
       threads blocked waiting for the semaphore shall be allowed to return successfully from its
       call to sem_wait().   If the Process Scheduling option is  supported,  the  thread  to  be
       unblocked  shall  be chosen in a manner appropriate to the scheduling policies and parame-
       ters 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  pri-
       ority thread that has been waiting the longest shall be unblocked. If the Process Schedul-
       ing 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 indicate 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, Copyright (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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				      SEM_POST(P)
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