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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 (consult 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  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
       parameters in effect for the blocked threads. In the case of the sched-
       ulers  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 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, <sema-
       phore.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 original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                         SEM_POST(3P)