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



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

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 .



IEEE/The Open Group                  2017                          SEM_POST(3)
