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.
NAMEsem_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 ALSOsemctl(), 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)
