SEM_POST
Section: POSIX Programmer's Manual (3P)
Updated: 2017
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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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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