.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SEM_POST" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" sem_post 
.SH 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.
.SH NAME
sem_post \- unlock a semaphore (\fBREALTIME\fP)
.SH SYNOPSIS
.LP
\fB#include <semaphore.h>
.br
.sp
int sem_post(sem_t *\fP\fIsem\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIsem_post\fP() function shall unlock the semaphore referenced
by \fIsem\fP by performing a semaphore unlock operation on
that semaphore.
.LP
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.
.LP
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 \fIsem_wait\fP().
\ 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. 
.LP
If
the Process Sporadic Server option is supported, and the scheduling
policy is SCHED_SPORADIC, the semantics are as per SCHED_FIFO
above. 
.LP
The \fIsem_post\fP() function shall be reentrant with respect to signals
and may be invoked from a signal-catching
function.
.SH RETURN VALUE
.LP
If successful, the \fIsem_post\fP() function shall return zero; otherwise,
the function shall return -1 and set \fIerrno\fP to
indicate the error.
.SH ERRORS
.LP
The \fIsem_post\fP() function shall fail if:
.TP 7
.B EINVAL
The \fIsem\fP argument does not refer to a valid semaphore.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
The \fIsem_post\fP() function is part of the Semaphores option and
need not be available on all implementations.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIsemctl\fP(), \fIsemget\fP(), \fIsemop\fP(),
\fIsem_timedwait\fP(), \fIsem_trywait\fP(), \fIsem_wait\fP(),
the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<semaphore.h>\fP
.SH COPYRIGHT
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 .