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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
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 async-signal-safe 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 may fail if:
       EINVAL The sem argument does not refer to a valid semaphore.
       The following sections are informative.
EXAMPLES
       See sem_timedwait().
APPLICATION USAGE
       None.
RATIONALE
       None.
FUTURE DIRECTIONS
       None.
SEE ALSO
       semctl(), semget(), semop(), sem_timedwait(), sem_trywait()
       The  Base Definitions volume of POSIX.1-2008, Section 4.11, Memory Syn-
       chronization, <semaphore.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                         SEM_POST(3P)