[apple/libc.git] / sys / shm_open.2

.\"	$Darwin$
.\"
.\" Copyright (c) 1999-2002 Apple Computer, Inc. All rights reserved.
.\"
.\" @APPLE_LICENSE_HEADER_START@
.\" 
.\" The contents of this file constitute Original Code as defined in and
.\" are subject to the Apple Public Source License Version 1.1 (the
.\" "License").  You may not use this file except in compliance with the
.\" License.  Please obtain a copy of the License at
.\" http://www.apple.com/publicsource and read it before using this file.
.\" 
.\" This Original Code and all software distributed under the License are
.\" distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
.\" EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
.\" INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
.\" FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
.\" License for the specific language governing rights and limitations
.\" under the License.
.\" 
.\" @APPLE_LICENSE_HEADER_END@
.\"
.Dd September 20, 1999
.Dt SHM_OPEN 2
.Os Darwin
.Sh NAME
.Nm shm_open
.Nd open a shared memory object
.Sh SYNOPSIS
.Fd #include <sys/mman.h>
.Ft int
.Fn shm_open "const char *name" "int flags" "mode_t mode"
.Sh DESCRIPTION
The shared memory object referenced by
.Fa name
is opened for reading and/or writing as specified by the argument
.Fa flags
and the file descriptor returned to the calling process.
The returned file descriptor will be the lowest non-open file
descriptor for the calling process, and is not shared with any
other processes, as it is a new file descriptor. The new file
descriptor will have the
.Dv FD_CLOEXEC
flag set.
Repeated calls
to
.Nm shm_open
with the same string value for
.Fn name
will return a file descriptor referring to the same shared memory
object, provided that the object has not been unlinked by a call to
.Fn shm_unlink .
The
.Fa flags
argument may indicate the file is to be
created if it does not exist (by specifying the
.Dv O_CREAT
flag), in which case the file is created with mode
.Fa mode
as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
.Pp
The flags specified are formed by
.Em or Ns 'ing
the following values:
.Pp
.Bd -literal -offset indent -compact
O_RDONLY	open for reading only
O_RDWR		open for reading and writing
O_CREAT		create object if it does not exist
O_EXCL		error if create and object exists
O_TRUNC		truncate size to 0
.Ed
.Pp
Exactly one of
.Dv O_RDONLY
or
.Dv O_RDWR
must be specified.
.Pp
If
.Dv O_TRUNC
is specified and the
file exists, the file is truncated to zero length.
If
.Dv O_EXCL
is set with
.Dv O_CREAT
and the file already
exists,
.Fn shm_open
returns an error.  This may be used to
implement a simple exclusive access locking mechanism.
.Pp
If successful,
.Fn shm_open
returns a non-negative integer, termed a file descriptor.
It returns -1 and sets
.Va errno
on failure.
The file pointer used to mark the current position within the
memory object is set to the beginning of the object.
.Pp
When a new shared memory object is created it is given the
owner and group corresponding to the effective user and
group of the calling process. There is no visible entry in the
file system for the created object in this implementation.
.Pp
When a shared memory object is created, it persists until it
it unlinked and all other references are gone. Objects do
not persist across a system reboot.
.Pp
The new descriptor is set to remain open across
.Xr execve
system calls; see
.Xr close 2
and
.Xr fcntl 2 .
.Pp
The system imposes a limit on the number of file descriptors
open simultaneously by one process.
.Xr Getdtablesize 2
returns the current system limit.
.Sh ERRORS
The named object is opened unless:
.Bl -tag -width Er
.It Bq Er EACCES
The required permissions (for reading and/or writing)
are denied for the given flags.
.It Bq Er EACCES
.Dv O_CREAT
is specified, the object does not exist, and permission to
create the object is denied.
.It Bq Er EEXIST
.Dv O_CREAT
and
.Dv O_EXCL
were specified and the object exists.
.It Bq Er EINTR
The
.Fn shm_open
operation was interrupted by a signal.
.It Bq Er EINVAL
The
.Fn shm_open
operation is not supported.
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.It Bq Er ENAMETOOLONG
.Fa name
exceeded
.Dv SHM_NAME_MAX
characters.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named object does not exist.
.It Bq Er ENOSPC
.Dv O_CREAT
is specified, the file does not exist, and there is insufficient
space available to create the object.
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr close 2 ,
.Xr getdtablesize 2 ,
.Xr mmap 2 ,
.Xr shm_unlink 2 ,
.Xr umask 2
.Sh HISTORY
.Fn shm_open
is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).
Commit	Line	Data
e9ce8d39 A	1	.\" $Darwin$
e9ce8d39 A	2	.\"
5b2abdfb	3	.\" Copyright (c) 1999-2002 Apple Computer, Inc. All rights reserved.
e9ce8d39 A	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
	19	.\" under the License.
	20	.\"
	21	.\" @APPLE_LICENSE_HEADER_END@
	22	.\"
	23	.Dd September 20, 1999
	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	.Fn shm_open "const char *name" "int flags" "mode_t mode"
	33	.Sh DESCRIPTION
	34	The shared memory object referenced by
	35	.Fa name
	36	is opened for reading and/or writing as specified by the argument
	37	.Fa flags
	38	and the file descriptor returned to the calling process.
	39	The returned file descriptor will be the lowest non-open file
	40	descriptor for the calling process, and is not shared with any
	41	other processes, as it is a new file descriptor. The new file
	42	descriptor will have the
	43	.Dv FD_CLOEXEC
	44	flag set.
	45	Repeated calls
	46	to
	47	.Nm shm_open
	48	with the same string value for
	49	.Fn name
	50	will return a file descriptor referring to the same shared memory
	51	object, provided that the object has not been unlinked by a call to
	52	.Fn shm_unlink .
	53	The
	54	.Fa flags
	55	argument may indicate the file is to be
	56	created if it does not exist (by specifying the
	57	.Dv O_CREAT
	58	flag), in which case the file is created with mode
	59	.Fa mode
	60	as described in
	61	.Xr chmod 2
	62	and modified by the process' umask value (see
	63	.Xr umask 2 ) .
	64	.Pp
	65	The flags specified are formed by
	66	.Em or Ns 'ing
	67	the following values:
68	.Pp
69	.Bd -literal -offset indent -compact
70	O_RDONLY open for reading only
71	O_RDWR open for reading and writing
72	O_CREAT create object if it does not exist
73	O_EXCL error if create and object exists
74	O_TRUNC truncate size to 0
75	.Ed
76	.Pp
77	Exactly one of
78	.Dv O_RDONLY
79	or
80	.Dv O_RDWR
81	must be specified.
82	.Pp
83	If
84	.Dv O_TRUNC
85	is specified and the
86	file exists, the file is truncated to zero length.
87	If
88	.Dv O_EXCL
89	is set with
90	.Dv O_CREAT
91	and the file already
92	exists,
93	.Fn shm_open
94	returns an error. This may be used to
95	implement a simple exclusive access locking mechanism.
96	.Pp
97	If successful,
98	.Fn shm_open
99	returns a non-negative integer, termed a file descriptor.
100	It returns -1 and sets
101	.Va errno
102	on failure.
103	The file pointer used to mark the current position within the
104	memory object is set to the beginning of the object.
105	.Pp
106	When a new shared memory object is created it is given the
107	owner and group corresponding to the effective user and
108	group of the calling process. There is no visible entry in the
109	file system for the created object in this implementation.
110	.Pp
111	When a shared memory object is created, it persists until it
112	it unlinked and all other references are gone. Objects do
113	not persist across a system reboot.
114	.Pp
115	The new descriptor is set to remain open across
116	.Xr execve
117	system calls; see
118	.Xr close 2
119	and
120	.Xr fcntl 2 .
121	.Pp
122	The system imposes a limit on the number of file descriptors
123	open simultaneously by one process.
124	.Xr Getdtablesize 2
125	returns the current system limit.
126	.Sh ERRORS
127	The named object is opened unless:
128	.Bl -tag -width Er
129	.It Bq Er EACCES
130	The required permissions (for reading and/or writing)
131	are denied for the given flags.
132	.It Bq Er EACCES
133	.Dv O_CREAT
134	is specified, the object does not exist, and permission to
135	create the object is denied.
136	.It Bq Er EEXIST
137	.Dv O_CREAT
138	and
139	.Dv O_EXCL
140	were specified and the object exists.
141	.It Bq Er EINTR
142	The
143	.Fn shm_open
144	operation was interrupted by a signal.
145	.It Bq Er EINVAL
146	The
147	.Fn shm_open
148	operation is not supported.
149	.It Bq Er EMFILE
150	The process has already reached its limit for open file descriptors.
151	.It Bq Er ENAMETOOLONG
152	.Fa name
153	exceeded
154	.Dv SHM_NAME_MAX
155	characters.
156	.It Bq Er ENFILE
157	The system file table is full.
158	.It Bq Er ENOENT
159	.Dv O_CREAT
160	is not set and the named object does not exist.
161	.It Bq Er ENOSPC
162	.Dv O_CREAT
163	is specified, the file does not exist, and there is insufficient
164	space available to create the object.
165	.El
166	.Sh SEE ALSO
167	.Xr chmod 2 ,
168	.Xr close 2 ,
169	.Xr getdtablesize 2 ,
170	.Xr mmap 2 ,
171	.Xr shm_unlink 2 ,
172	.Xr umask 2
173	.Sh HISTORY
174	.Fn shm_open
175	is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).