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.\" $FreeBSD$ 29.\" 30.Dd April 16, 2013 31.Dt SEM_WAIT 3 32.Os 33.Sh NAME 34.Nm sem_wait , 35.Nm sem_trywait 36.Nd decrement (lock) a semaphore 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In semaphore.h 41.Ft int 42.Fn sem_wait "sem_t *sem" 43.Ft int 44.Fn sem_trywait "sem_t *sem" 45.Sh DESCRIPTION 46The 47.Fn sem_wait 48function decrements (locks) the semaphore pointed to by 49.Fa sem , 50but blocks if the value of 51.Fa sem 52is zero, until the value is non-zero and the value can be decremented. 53.Pp 54The 55.Fn sem_trywait 56function decrements (locks) the semaphore pointed to by 57.Fa sem 58only if the value is non-zero. 59Otherwise, the semaphore is not decremented and 60an error is returned. 61.Sh RETURN VALUES 62.Rv -std 63.Sh ERRORS 64The 65.Fn sem_wait 66and 67.Fn sem_trywait 68functions will fail if: 69.Bl -tag -width Er 70.It Bq Er EINVAL 71The 72.Fa sem 73argument 74points to an invalid semaphore. 75.El 76.Pp 77Additionally, 78.Fn sem_wait 79will fail if: 80.Bl -tag -width Er 81.It Bq Er EINTR 82A signal interrupted this function. 83.El 84.Pp 85Additionally, 86.Fn sem_trywait 87will fail if: 88.Bl -tag -width Er 89.It Bq Er EAGAIN 90The semaphore value was zero, and thus could not be decremented. 91.El 92.Sh SEE ALSO 93.Xr sem_getvalue 3 , 94.Xr sem_post 3 , 95.Xr sem_timedwait 3 , 96.Xr sem 4 97.Sh STANDARDS 98The 99.Fn sem_wait 100and 101.Fn sem_trywait 102functions conform to 103.St -p1003.1-96 . 104