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 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  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.1‐2017,  Section  4.12,  Memory Synchronization,
       <semaphore.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  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 .

       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.kernel.org/doc/man-pages/reporting_bugs.html .