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	1	.\" $Darwin$
	2	.\"
	3	.\" Copyright (c) 2000-2002 Apple Computer, Inc. All rights reserved.
	4	.\"
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	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
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	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_OPEN 2
	25	.Os Darwin
	26	.Sh NAME
	27	.Nm sem_open
	28	.Nd initialize and open a named semaphore
	29	.Sh SYNOPSIS
	30	.Fd #include <semaphore.h>
	31	.Ft sem_t *
	32	.Fo sem_open
	33	.Fa "const char *name"
	34	.Fa "int oflag"
	35	.Fa "..."
	36	.Fc
	37	.Pp
	38	The parameters "mode_t mode" and "unsigned int value"
	39	are optional.
	40	.Sh DESCRIPTION
	41	The named semaphore named
	42	.Fa name
	43	is initialized and opened as specified by the argument
	44	.Fa oflag
	45	and a semaphore descriptor is returned to the calling process.
	46	.Pp
	47	The value of
	48	.Fa oflag
	49	is formed by
	50	.Em or Ns 'ing
	51	the following values:
	52	.Pp
	53	.Bd -literal -offset indent -compact
	54	O_CREAT create the semaphore if it does not exist
	55	O_EXCL error if create and semaphore exists
	56	.Ed
	57	.Pp
	58	If
	59	.Dv O_CREAT
	60	is specified,
	61	.Fn sem_open
	62	requires an additional two arguments.
	63	.Fa mode
	64	specifies the permissions for the semaphore as described in
	65	.Xr chmod 2
	66	and modified by the process' umask value (see
	67	.Xr umask 2 ) .
	68	The semaphore is created with an initial
	69	.Fa value ,
	70	which must be less than or equal to
	71	.Dv SEM_VALUE_MAX .
	72	.Pp
	73	If
	74	.Dv O_EXCL
	75	is specified and the semaphore exists,
	76	.Fn sem_open
	77	fails. The check for the existence of the semaphore and the creation
	78	of the semaphore are atomic with respect to all processes calling
	79	.Fn sem_open
	80	with
	81	.Dv O_CREAT
	82	and
	83	.Dv O_EXCL
	84	set.
	85	.Pp
	86	When a new semaphore is created, it is given the user ID and group ID
	87	which correspond to the effective user and group IDs of the calling
	88	process. There is no visible entry in the file system for the created
	89	object in this implementation.
	90	.Pp
	91	The returned semaphore descriptor is available to the calling process
	92	until it is closed with
	93	.Fn sem_close ,
	94	or until the caller exits or execs.
	95	.Pp
	96	If a process makes repeated calls to
	97	.Fn sem_open ,
	98	with the same
	99	.Fa name
	100	argument, the same descriptor is returned for each successful call,
	101	unless
	102	.Fn sem_unlink
	103	has been called on the semaphore in the interim.
	104	.Pp
	105	If
	106	.Fn sem_open
	107	fails for any reason, it will return a value of
	108	.Dv SEM_FAILED
	109	and sets
	110	.Va errno .
	111	On success, it returns a semaphore descriptor.
	112	.Sh ERRORS
	113	The named semaphore is opened unless:
	114	.Bl -tag -width Er
	115	.It Bq Er EACCES
	116	The required permissions (for reading and/or writing)
	117	are denied for the given flags; or
	118	.Dv O_CREAT
	119	is specified, the object does not exist, and permission to
	120	create the semaphore is denied.
	121	.It Bq Er EEXIST
	122	.Dv O_CREAT
	123	and
	124	.Dv O_EXCL
	125	were specified and the semaphore exists.
	126	.It Bq Er EINTR
	127	The
	128	.Fn sem_open
	129	operation was interrupted by a signal.
	130	.It Bq Er EINVAL
	131	The
	132	.Fn shm_open
	133	operation is not supported; or
	134	.Dv O_CREAT
	135	is specified and
	136	.Fa value
	137	exceeds
	138	.Dv SEM_VALUE_MAX .
	139	.It Bq Er EMFILE
	140	The process has already reached its limit for semaphores or file
	141	descriptors in use.
	142	.It Bq Er ENAMETOOLONG
	143	.Fa name
	144	exceeded
	145	.Dv SEM_NAME_LEN
	146	characters.
	147	.It Bq Er ENFILE
	148	Too many semaphores or file descriptors are open on the system.
	149	.It Bq Er ENOENT
	150	.Dv O_CREAT
	151	is not set and the named semaphore does not exist.
	152	.It Bq Er ENOSPC
	153	.Dv O_CREAT
	154	is specified, the file does not exist, and there is insufficient
	155	space available to create the semaphore.
	156	.El
	157	.Sh SEE ALSO
	158	.Xr sem_close 2 ,
	159	.Xr sem_post 2 ,
	160	.Xr sem_trywait 2 ,
	161	.Xr sem_unlink 2 ,
	162	.Xr sem_wait 2 ,
	163	.Xr semctl 2 ,
	164	.Xr semget 2 ,
	165	.Xr semop 2 ,
	166	.Xr umask 2
	167	.Sh HISTORY
	168	.Fn sem_open
	169	is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).