sem_post(3p) — Linux manual page


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

       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),

       The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory
       Synchronization, semaphore.h(0p)

COPYRIGHT         top

       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 .

       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               2017                      SEM_POST(3P)

Pages that refer to this page: semaphore.h(0p)semctl(3p)semget(3p)sem_getvalue(3p)sem_init(3p)semop(3p)sem_open(3p)sem_timedwait(3p)sem_trywait(3p)