1.\" Copyright (C) 2000 Jason Evans <jasone@FreeBSD.org>. 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice(s), this list of conditions and the following disclaimer as 9.\" the first lines of this file unmodified other than the possible 10.\" addition of one or more copyright notices. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice(s), this list of conditions and the following disclaimer in 13.\" the documentation and/or other materials provided with the 14.\" distribution. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY 17.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 19.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE 20.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 21.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 22.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR 23.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 24.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE 25.\" OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, 26.\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27.\" 28.\" $FreeBSD: src/lib/libc_r/man/sem_wait.3,v 1.1.2.7 2001/12/17 10:08:26 ru Exp $ 29.Dd April 15, 2009 30.Dt SEM_WAIT 3 31.Os 32.Sh NAME 33.Nm sem_wait , 34.Nm sem_trywait 35.Nd decrement (lock) a semaphore 36.Sh LIBRARY 37.Lb libpthread 38.Sh SYNOPSIS 39.In 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. Otherwise, the semaphore is not decremented and 58an error is returned. 59.Sh RETURN VALUES 60.Rv -std 61.Sh ERRORS 62.Fn sem_wait 63and 64.Fn sem_trywait 65will fail if: 66.Bl -tag -width Er 67.It Bq Er EINVAL 68.Fa sem 69points to an invalid semaphore. 70.El 71.Pp 72Additionally, 73.Fn sem_trywait 74will fail if: 75.Bl -tag -width Er 76.It Bq Er EAGAIN 77The semaphore value was zero, and thus could not be decremented. 78.El 79.Sh SEE ALSO 80.Xr sem_post 3 81.Sh STANDARDS 82.Fn sem_wait 83and 84.Fn sem_trywait 85conform to 86.St -p1003.1-96 . 87