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.TH SEM_POST 3C "Feb 5, 2008"
.SH NAME
sem_post \- increment the count of a semaphore
.SH SYNOPSIS
.LP
.nf
#include <semaphore.h>

\fBint\fR \fBsem_post\fR(\fBsem_t *\fR\fIsem\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBsem_post()\fR function unlocks the semaphore referenced by \fIsem\fR by
performing a semaphore unlock operation on that semaphore.
.sp
.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.
.sp
.LP
If the value of the semaphore resulting from this operation is 0, then one of
the threads  blocked waiting for the semaphore will be allowed to return
successfully from its call to \fBsem_wait\fR(3C). If the symbol
\fB_POSIX_PRIORITY_SCHEDULING\fR is defined, the thread  to be unblocked will
be chosen in a manner appropriate to the scheduling policies and parameters in
effect for the blocked threads. In the case of the schedulers \fBSCHED_FIFO\fR
and \fBSCHED_RR\fR, the highest priority waiting thread  will 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
will be unblocked. If the symbol \fB_POSIX_PRIORITY_SCHEDULING\fR is not
defined, the choice of a thread  to unblock is unspecified.
.SH RETURN VALUES
.sp
.LP
If successful, \fBsem_post()\fR returns \fB0\fR; otherwise it returns
\fB\(mi1\fR and sets \fBerrno\fR to indicate the error.
.SH ERRORS
.sp
.LP
The \fBsem_post()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR \fR
.ad
.RS 14n
The \fIsem\fR argument does not refer to a valid semaphore.
.RE

.sp
.ne 2
.na
\fB\fBENOSYS\fR \fR
.ad
.RS 14n
The \fBsem_post()\fR function is not supported by the system.
.RE

.sp
.ne 2
.na
\fB\fBEOVERFLOW\fR \fR
.ad
.RS 14n
The semaphore value exceeds \fBSEM_VALUE_MAX\fR.
.RE

.SH USAGE
.sp
.LP
The \fBsem_post()\fR function is reentrant with respect to signals and may be
invoked from a signal-catching function. The semaphore functionality described
on this manual page is for the POSIX (see \fBstandards\fR(5)) threads
implementation. For the documentation of the Solaris threads interface, see
\fBsemaphore\fR(3C)).
.SH EXAMPLES
.sp
.LP
See \fBsem_wait\fR(3C).
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	Async-Signal-Safe
_
Standard	See \fBstandards\fR(5).
.TE

.SH SEE ALSO
.sp
.LP
\fBsched_setscheduler\fR(3C), \fBsem_wait\fR(3C), \fBsemaphore\fR(3C),
\fBattributes\fR(5), \fBstandards\fR(5)