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.Dd February 15, 2000 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 libc_r 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. Otherwise, the semaphore is not decremented and 58an error is returned. 59.Sh RETURN VALUES 60If successful, 61.Fn sem_wait 62and 63.Fn sem_trywait 64return 0. Otherwise, -1 is returned and 65.Va errno 66is set to indicate the type of 67error. 68.Sh ERRORS 69.Fn sem_wait 70and 71.Fn sem_trywait 72will fail if: 73.Bl -tag -width Er 74.It Bq Er EINVAL 75.Fa sem 76points to an invalid semaphore. 77.El 78.Pp 79Additionally, 80.Fn sem_trywait 81will fail if: 82.Bl -tag -width Er 83.It Bq Er EAGAIN 84The semaphore value was zero, and thus could not be decremented. 85.El 86.Sh SEE ALSO 87.Xr sem_post 3 88.Sh STANDARDS 89.Fn sem_wait 90and 91.Fn sem_trywait 92conform to ISO/IEC 9945-1 ANSI/IEEE 93.Pq Dq Tn POSIX 94Std 1003.1 1996 Edition. 95