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	1	.\" $Darwin$
	2	.\"
	3	.\" Copyright (c) 2000-2002 Apple Computer, Inc. All rights reserved.
	4	.\"
	5	.\" @APPLE_LICENSE_HEADER_START@
	6	.\"
	7	.\" The contents of this file constitute Original Code as defined in and
	8	.\" are subject to the Apple Public Source License Version 1.1 (the
	9	.\" "License"). You may not use this file except in compliance with the
	10	.\" License. Please obtain a copy of the License at
	11	.\" http://www.apple.com/publicsource and read it before using this file.
	12	.\"
	13	.\" This Original Code and all software distributed under the License are
	14	.\" distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
	15	.\" EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
	16	.\" INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
	17	.\" FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT. Please see the
	18	.\" License for the specific language governing rights and limitations
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	21	.\" @APPLE_LICENSE_HEADER_END@
	22	.\"
	23	.Dd June 8, 2000
	24	.Dt SEM_WAIT 2
	25	.Os Darwin
	26	.Sh NAME
	27	.Nm sem_trywait, sem_wait
	28	.Nd lock a semaphore
	29	.Sh SYNOPSIS
	30	.Fd #include <semaphore.h>
	31	.Ft int
	32	.Fn sem_trywait "sem_t *sem"
	33	.Ft int
	34	.Fn sem_wait "sem_t *sem"
	35	.Sh DESCRIPTION
	36	The semaphore referenced by
	37	.Fa sem
	38	is locked. When calling
	39	.Fn sem_wait ,
	40	if the semaphore's value is zero, the calling thread will block until
	41	the lock is acquired or until the call is interrupted by a
	42	signal. Alternatively, the
	43	.Fn sem_trywait
	44	function will fail if the semaphore is already locked, rather than
	45	blocking on the semaphore.
	46	.Pp
	47	If successful (the lock was acquired),
	48	.Fn sem_wait
	49	and
	50	.Fn sem_trywait
	51	will return 0. Otherwise, -1 is returned and
	52	.Va errno
	53	is set, and the state of the semaphore is unchanged.
	54	.Sh ERRORS
	55	.Fn sem_wait
	56	and
	57	.Fn sem_trywait
	58	succeed unless:
	59	.Bl -tag -width Er
	60	.It Bq Er EAGAIN
	61	The semaphore is already locked.
	62	.It Bq Er EDEADLK
	63	A deadlock was detected.
	64	.It Bq Er EINTR
	65	The call was interrupted by a signal.
	66	.It Bq Er EINVAL
	67	.Fa sem
	68	is not a valid semaphore descriptor.
	69	.El
	70	.Sh NOTES
	71	Applications may encounter a priority inversion while using
	72	semaphores. When a thread is waiting on a semaphore which is about to
	73	be posted by a lower-priority thread and the lower-priority thread is
	74	preempted by another thread (of medium priority), a priority inversion
	75	has occured, and the higher-priority thread will be blocked for an
	76	unlimited time period. Programmers using the realtime functionality
	77	of the system should take care to avoid priority inversions.
	78	.Sh SEE ALSO
	79	.Xr sem_open 2 ,
	80	.Xr sem_post 2 ,
	81	.Xr semctl 2 ,
	82	.Xr semget 2 ,
	83	.Xr semop 2
	84	.Sh HISTORY
	85	.Fn sem_wait
	86	and
	87	.Fn sem_trywait
	88	are specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).