[apple/xnu.git] / bsd / man / man2 / 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 August 29, 2008
.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
.Fo shm_open
.Fa "const char *name"
.Fa "int oflag"
.Fa "..."
.Fc
.Pp
The parameter "mode_t mode" is optional.
.Sh DESCRIPTION
The shared memory object referenced by
.Fa name
is opened for reading and/or writing as specified by the argument
.Fa oflag
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 oflag
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 value of
.Fa oflag
is 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 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 the name size limit.
This is currently
.Dv PSHMNAMLEN
characters (defined in
.In sys/posix_shm.h ) ,
but this may change in the future.
.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
6d2010ae A	1	.\" $Darwin$
	2	.\"
	3	.\" Copyright (c) 1999-2002 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
	19	.\" under the License.
	20	.\"
	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).