1.\" $OpenBSD: sem_wait.3,v 1.6 2007/05/31 19:19:37 jmc Exp $ 2.\" 3.\" Copyright (C) 2000 Jason Evans <jasone@FreeBSD.org>. 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice(s), this list of conditions and the following disclaimer as 11.\" the first lines of this file unmodified other than the possible 12.\" addition of one or more copyright notices. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice(s), this list of conditions and the following disclaimer in 15.\" the documentation and/or other materials provided with the 16.\" distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY 19.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE 22.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR 25.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 26.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE 27.\" OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, 28.\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 29.\" 30.\" $FreeBSD: src/lib/libc_r/man/sem_wait.3,v 1.8 2001/10/01 16:09:09 ru Exp $ 31.Dd $Mdocdate: May 31 2007 $ 32.Dt SEM_WAIT 3 33.Os 34.Sh NAME 35.Nm sem_wait , 36.Nm sem_trywait 37.Nd decrement (lock) a semaphore 38.Sh SYNOPSIS 39.Fd #include <semaphore.h> 40.Ft int 41.Fn sem_wait "sem_t *sem" 42.Ft int 43.Fn sem_trywait "sem_t *sem" 44.Sh DESCRIPTION 45The 46.Fn sem_wait 47function decrements (locks) the semaphore pointed to by 48.Fa sem , 49but blocks if the value of 50.Fa sem 51is zero, until the value is non-zero and the value can be decremented. 52.Pp 53The 54.Fn sem_trywait 55function decrements (locks) the semaphore pointed to by 56.Fa sem 57only if the value is non-zero. 58Otherwise, the semaphore is not decremented and 59an error is returned. 60.Sh RETURN VALUES 61.Rv -std sem_wait 62.Sh ERRORS 63.Fn sem_wait 64and 65.Fn sem_trywait 66will fail if: 67.Bl -tag -width Er 68.It Bq Er EINVAL 69.Fa sem 70points to an invalid semaphore. 71.El 72.Pp 73Additionally, 74.Fn sem_trywait 75will fail if: 76.Bl -tag -width Er 77.It Bq Er EAGAIN 78The semaphore value was zero, and thus could not be decremented. 79.El 80.Sh SEE ALSO 81.Xr sem_destroy 3 , 82.Xr sem_getvalue 3 , 83.Xr sem_init 3 , 84.Xr sem_open 3 , 85.Xr sem_post 3 86.Sh STANDARDS 87.Fn sem_wait 88and 89.Fn sem_trywait 90conform to 91.St -p1003.1-96 . 92