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

PROLOG         top

       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         top

       sem_post — unlock a semaphore

SYNOPSIS         top

       #include <semaphore.h>

       int sem_post(sem_t *sem);

DESCRIPTION         top

       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         top

       If successful, the sem_post() function shall return zero; otherwise,
       the function shall return −1 and set errno to indicate the error.

ERRORS         top

       The sem_post() function may fail if:

       EINVAL The sem argument does not refer to a valid semaphore.

       The following sections are informative.

EXAMPLES         top

       See sem_timedwait(3p).



RATIONALE         top




SEE ALSO         top

       semctl(3p), semget(3p), semop(3p), sem_timedwait(3p), sem_trywait(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
       Synchronization, semaphore.h(0p)

COPYRIGHT         top

       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 Electrical 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 .

       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 .

IEEE/The Open Group                 2013                        SEM_POST(3P)