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.\" $OpenBSD: sem_wait.3,v 1.6 2007/05/31 19:19:37 jmc Exp $
.\"
.\" Copyright (C) 2000 Jason Evans <jasone@FreeBSD.org>.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice(s), this list of conditions and the following disclaimer as
.\" the first lines of this file unmodified other than the possible
.\" addition of one or more copyright notices.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice(s), this list of conditions and the following disclaimer in
.\" the documentation and/or other materials provided with the
.\" distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE
.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
.\" OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
.\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $FreeBSD: src/lib/libc_r/man/sem_wait.3,v 1.8 2001/10/01 16:09:09 ru Exp $
.Dd $Mdocdate: May 31 2007 $
.Dt SEM_WAIT 3
.Os
.Sh NAME
.Nm sem_wait ,
.Nm sem_trywait
.Nd decrement (lock) a semaphore
.Sh SYNOPSIS
.Fd #include <semaphore.h>
.Ft int
.Fn sem_wait "sem_t *sem"
.Ft int
.Fn sem_trywait "sem_t *sem"
.Sh DESCRIPTION
The
.Fn sem_wait
function decrements (locks) the semaphore pointed to by
.Fa sem ,
but blocks if the value of
.Fa sem
is zero, until the value is non-zero and the value can be decremented.
.Pp
The
.Fn sem_trywait
function decrements (locks) the semaphore pointed to by
.Fa sem
only if the value is non-zero.
Otherwise, the semaphore is not decremented and
an error is returned.
.Sh RETURN VALUES
.Rv -std sem_wait
.Sh ERRORS
.Fn sem_wait
and
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa sem
points to an invalid semaphore.
.El
.Pp
Additionally,
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The semaphore value was zero, and thus could not be decremented.
.El
.Sh SEE ALSO
.Xr sem_destroy 3 ,
.Xr sem_getvalue 3 ,
.Xr sem_init 3 ,
.Xr sem_open 3 ,
.Xr sem_post 3
.Sh STANDARDS
.Fn sem_wait
and
.Fn sem_trywait
conform to
.St -p1003.1-96 .
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