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 reentrant with respect to signals 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, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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