SEM_POST(P) POSIX Programmer's Manual SEM_POST(P)
sem_post - unlock a semaphore (REALTIME)
int sem_post(sem_t *sem);
The sem_post() function shall unlock the semaphore referenced by sem by performing a sema-
phore 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 incre-
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 parame-
ters 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 pri-
ority thread that has been waiting the longest shall be unblocked. If the Process Schedul-
ing 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.
If successful, the sem_post() function shall return zero; otherwise, the function shall
return -1 and set errno to indicate the error.
The sem_post() function shall fail if:
EINVAL The sem argument does not refer to a valid semaphore.
The following sections are informative.
The sem_post() function is part of the Semaphores option and need not be available on all
semctl() , semget() , semop() , sem_timedwait() , sem_trywait() , sem_wait() , the Base
Definitions volume of IEEE Std 1003.1-2001, <semaphore.h>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SEM_POST(P)