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.
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
The sem_post() function shall be async-signal-safe and may be invoked
from a signal-catching function.
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 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
IEEE/The Open Group 2013 SEM_POST(3P)