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	1	.\" $Darwin$
	2	.\"
	3	.\" Copyright (c) 1999-2002 Apple Computer, Inc. All rights reserved.
	4	.\"
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	7	.\" The contents of this file constitute Original Code as defined in and
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	9	.\" "License"). You may not use this file except in compliance with the
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	15	.\" EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
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	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 August 29, 2008
	24	.Dt SHM_OPEN 2
	25	.Os Darwin
	26	.Sh NAME
	27	.Nm shm_open
	28	.Nd open a shared memory object
	29	.Sh SYNOPSIS
	30	.Fd #include <sys/mman.h>
	31	.Ft int
	32	.Fo shm_open
	33	.Fa "const char *name"
	34	.Fa "int oflag"
	35	.Fa "..."
	36	.Fc
	37	.Pp
	38	The parameter "mode_t mode" is optional.
	39	.Sh DESCRIPTION
	40	The shared memory object referenced by
	41	.Fa name
	42	is opened for reading and/or writing as specified by the argument
	43	.Fa oflag
	44	and the file descriptor returned to the calling process.
	45	The returned file descriptor will be the lowest non-open file
	46	descriptor for the calling process, and is not shared with any
	47	other processes, as it is a new file descriptor. The new file
	48	descriptor will have the
	49	.Dv FD_CLOEXEC
	50	flag set.
	51	Repeated calls
	52	to
	53	.Nm shm_open
	54	with the same string value for
	55	.Fn name
	56	will return a file descriptor referring to the same shared memory
	57	object, provided that the object has not been unlinked by a call to
	58	.Fn shm_unlink .
	59	The
	60	.Fa oflag
	61	argument may indicate the file is to be
	62	created if it does not exist (by specifying the
	63	.Dv O_CREAT
	64	flag), in which case the file is created with mode
	65	.Fa mode
	66	as described in
	67	.Xr chmod 2
	68	and modified by the process' umask value (see
	69	.Xr umask 2 ) .
	70	.Pp
	71	The value of
	72	.Fa oflag
	73	is formed by
	74	.Em or Ns 'ing
	75	the following values:
	76	.Pp
	77	.Bd -literal -offset indent -compact
	78	O_RDONLY open for reading only
	79	O_RDWR open for reading and writing
	80	O_CREAT create object if it does not exist
	81	O_EXCL error if create and object exists
	82	O_TRUNC truncate size to 0
	83	.Ed
	84	.Pp
	85	Exactly one of
	86	.Dv O_RDONLY
	87	or
	88	.Dv O_RDWR
	89	must be specified.
	90	.Pp
	91	If
	92	.Dv O_TRUNC
	93	is specified and the
	94	file exists, the file is truncated to zero length.
	95	If
	96	.Dv O_EXCL
	97	is set with
	98	.Dv O_CREAT
	99	and the file already
	100	exists,
	101	.Fn shm_open
	102	returns an error. This may be used to
	103	implement a simple exclusive access locking mechanism.
	104	.Pp
	105	If successful,
	106	.Fn shm_open
	107	returns a non-negative integer, termed a file descriptor.
	108	It returns -1 and sets
	109	.Va errno
	110	on failure.
	111	The file pointer used to mark the current position within the
	112	memory object is set to the beginning of the object.
	113	.Pp
	114	When a new shared memory object is created it is given the
	115	owner and group corresponding to the effective user and
	116	group of the calling process. There is no visible entry in the
	117	file system for the created object in this implementation.
	118	.Pp
	119	When a shared memory object is created, it persists until it
	120	it unlinked and all other references are gone. Objects do
	121	not persist across a system reboot.
	122	.Pp
	123	The system imposes a limit on the number of file descriptors
	124	open simultaneously by one process.
	125	.Xr Getdtablesize 2
	126	returns the current system limit.
	127	.Sh ERRORS
	128	The named object is opened unless:
	129	.Bl -tag -width Er
	130	.It Bq Er EACCES
	131	The required permissions (for reading and/or writing)
	132	are denied for the given flags.
	133	.It Bq Er EACCES
	134	.Dv O_CREAT
	135	is specified, the object does not exist, and permission to
	136	create the object is denied.
	137	.It Bq Er EEXIST
	138	.Dv O_CREAT
	139	and
	140	.Dv O_EXCL
	141	were specified and the object exists.
	142	.It Bq Er EINTR
	143	The
	144	.Fn shm_open
	145	operation was interrupted by a signal.
	146	.It Bq Er EINVAL
	147	The
	148	.Fn shm_open
	149	operation is not supported.
	150	.It Bq Er EMFILE
	151	The process has already reached its limit for open file descriptors.
	152	.It Bq Er ENAMETOOLONG
	153	.Fa name
	154	exceeded the name size limit.
	155	This is currently
	156	.Dv PSHMNAMLEN
	157	characters (defined in
	158	.In sys/posix_shm.h ) ,
	159	but this may change in the future.
	160	.It Bq Er ENFILE
	161	The system file table is full.
	162	.It Bq Er ENOENT
	163	.Dv O_CREAT
	164	is not set and the named object does not exist.
	165	.It Bq Er ENOSPC
	166	.Dv O_CREAT
	167	is specified, the file does not exist, and there is insufficient
	168	space available to create the object.
	169	.El
	170	.Sh SEE ALSO
	171	.Xr chmod 2 ,
	172	.Xr close 2 ,
	173	.Xr getdtablesize 2 ,
	174	.Xr mmap 2 ,
	175	.Xr shm_unlink 2 ,
	176	.Xr umask 2
	177	.Sh HISTORY
	178	.Fn shm_open
	179	is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).