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Revision 1.2, Fri Dec 20 13:48:45 2013 UTC (5 years, 10 months ago) by wiz
.\" $NetBSD: shm_open.3,v 1.2 2013/12/20 13:48:45 wiz Exp $ .\" .\" Copyright 2000 Massachusetts Institute of Technology .\" .\" Permission to use, copy, modify, and distribute this software and .\" its documentation for any purpose and without fee is hereby .\" granted, provided that both the above copyright notice and this .\" permission notice appear in all copies, that both the above .\" copyright notice and this permission notice appear in all .\" supporting documentation, and that the name of M.I.T. not be used .\" in advertising or publicity pertaining to distribution of the .\" software without specific, written prior permission. M.I.T. makes .\" no representations about the suitability of this software for any .\" purpose. It is provided "as is" without express or implied .\" warranty. .\" .\" THIS SOFTWARE IS PROVIDED BY M.I.T. ``AS IS''. M.I.T. DISCLAIMS .\" ALL EXPRESS OR IMPLIED WARRANTIES WITH REGARD TO THIS SOFTWARE, .\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT .\" SHALL M.I.T. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT .\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF .\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND .\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, .\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT .\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD$ .\" .Dd December 19, 2013 .Dt SHM_OPEN 3 .Os .Sh NAME .Nm shm_open , shm_unlink .Nd "shared memory object operations" .Sh LIBRARY .Lb librt .Sh SYNOPSIS .In sys/types.h .In sys/mman.h .In fcntl.h .Ft int .Fn shm_open "const char *name" "int flags" "mode_t mode" .Ft int .Fn shm_unlink "const char *name" .Sh DESCRIPTION The .Fn shm_open function creates or opens a .Tn POSIX shared memory object named .Fa name . The .Fa flags argument contains a subset of the flags used by .Xr open 2 . An access mode of either .Dv O_RDONLY or .Dv O_RDWR must be included in .Fa flags . The optional flags .Dv O_CREAT , .Dv O_EXCL , and .Dv O_TRUNC may also be specified. .Pp If .Dv O_CREAT is specified, then a new shared memory object named .Fa name will be created if it does not exist. In this case, the shared memory object is created with mode .Fa mode subject to the process' umask value. If both the .Dv O_CREAT and .Dv O_EXCL flags are specified and a shared memory object named .Fa path already exists, then .Fn shm_open will fail with .Er EEXIST . .Pp Newly created objects start off with a size of zero. If an existing shared memory object is opened with .Dv O_RDWR and the .Dv O_TRUNC flag is specified, then the shared memory object will be truncated to a size of zero. The size of the object can be adjusted via .Xr ftruncate 2 and queried via .Xr fstat 2 . .Pp The new descriptor is set to close during .Xr execve 2 system calls; see .Xr close 2 and .Xr fcntl 2 . .Pp The .Fn shm_unlink system call removes a shared memory object named .Fa path . .Sh RETURN VALUES If successful, .Fn shm_open returns a non-negative integer, and .Fn shm_unlink returns zero. Both functions return -1 on failure, and set .Va errno to indicate the error. .Sh COMPATIBILITY The .Fa path argument does not necessarily represent a pathname (although it does in most other implementations). Two processes opening the same .Fa path are guaranteed to access the same shared memory object if and only if .Fa path begins with a slash .Pq Ql \&/ character. .Pp Only the .Dv O_RDONLY , .Dv O_RDWR , .Dv O_CREAT , .Dv O_EXCL , and .Dv O_TRUNC flags may be used in portable programs. .Pp The result of using .Xr open 2 , .Xr read 2 , or .Xr write 2 on a shared memory object, or on the descriptor returned by .Fn shm_open , is undefined. It is also undefined whether the shared memory object itself, or its contents, persist across reboots. .Sh ERRORS The following errors are defined for .Fn shm_open : .Bl -tag -width Er .It Bq Er EACCES The required permissions (for reading or reading and writing) are denied. .It Bq Er EEXIST .Dv O_CREAT and .Dv O_EXCL are specified and the named shared memory object does exist. .It Bq Er EFAULT The .Fa path argument points outside the process' allocated address space. .It Bq Er EINVAL A flag other than .Dv O_RDONLY , .Dv O_RDWR , .Dv O_CREAT , .Dv O_EXCL , or .Dv O_TRUNC was included in .Fa flags ; or the .Fa path does not begin with a slash .Pq Ql \&/ character. .It Bq Er EMFILE The process has already reached its limit for open file descriptors. .It Bq Er ENAMETOOLONG The entire pathname exceeded .Brq Dv PATH_MAX characters. .It Bq Er ENFILE The system file table is full. .It Bq Er ENOENT .Dv O_CREAT is specified and the named shared memory object does not exist. .It Bq Er ENOTSUP Not supported, most likely due to missing or incorrect .Pa /var/shm mount. .El .Pp The following errors are defined for .Fn shm_unlink : .Bl -tag -width Er .It Bq Er EACCES The required permissions are denied. .Fn shm_unlink requires write permission to the shared memory object. .It Bq Er EFAULT The .Fa path argument points outside the process' allocated address space. .It Bq Er ENAMETOOLONG The entire pathname exceeded .Brq Dv PATH_MAX characters. .It Bq Er ENOENT The named shared memory object does not exist. .El .Sh SEE ALSO .Xr close 2 , .Xr fstat 2 , .Xr ftruncate 2 , .Xr mmap 2 , .Xr munmap 2 .Sh STANDARDS The .Fn shm_open and .Fn shm_unlink functions are expected to conform to .St -p1003.1b-93 . .Sh HISTORY These functions first appeared in .Nx 7.0 .