shm_open, shm_unlink — Create/open or unlink POSIX shared memory objects
#include <sys/mman.h> #include <sys/stat.h> /* For mode constants */ #include <fcntl.h> /* For O_* constants */
int
shm_open( |
const char * | name, |
| int | oflag, | |
| mode_t | mode); |
int
shm_unlink( |
const char * | name); |
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shm_open() creates and opens
a new, or opens an existing, POSIX shared memory object. A
POSIX shared memory object is in effect a handle which can be
used by unrelated processes to mmap(2) the same region of
shared memory. The shm_unlink()
function performs the converse operation, removing an object
previously created by shm_open().
The operation of shm_open()
is analogous to that of open(2). name specifies the shared
memory object to be created or opened. For portable use,
name should have an
initial slash (/) and contain no embedded slashes.
oflag is a bit
mask created by ORing together exactly one of O_RDONLY or O_RDWR and any of the other flags listed
here:
O_RDONLYOpen the object for read access. A shared memory
object opened in this way can only be mmap(2)ed for read
(PROT_READ) access.
O_RDWROpen the object for read-write access.
O_CREATCreate the shared memory object if it does not
exist. The user and group ownership of the object are
taken from the corresponding effective IDs of the
calling process, and the object's permission bits are
set according to the low-order 9 bits of mode, except that those
bits set in the process file mode creation mask (see
umask(2)) are cleared
for the new object. A set of macro constants which can
be used to define mode is listed in
open(2). (Symbolic
definitions of these constants can be obtained by
including <sys/stat.h>
A new shared memory object initially has zero length — the size of the object can be set using ftruncate(2). The newly allocated bytes of a shared memory object are automatically initialized to 0.
O_EXCLIf O_CREAT was also
specified, and a shared memory object with the given
name already
exists, return an error. The check for the existence of
the object, and its creation if it does not exist, are
performed atomically.
O_TRUNCIf the shared memory object already exists, truncate it to zero bytes.
Definitions of these flag values can be obtained by
including <fcntl.h>
On successful completion shm_open() returns a new file descriptor
referring to the shared memory object. This file descriptor
is guaranteed to be the lowest-numbered file descriptor not
previously opened within the process. The FD_CLOEXEC flag (see fcntl(2)) is set for the
file descriptor.
The file descriptor is normally used in subsequent calls to ftruncate(2) (for a newly created object) and mmap(2). After a call to mmap(2) the file descriptor may be closed without affecting the memory mapping.
The operation of shm_unlink() is analogous to unlink(2): it removes a
shared memory object name, and, once all processes have
unmapped the object, de-allocates and destroys the contents
of the associated memory region. After a successful
shm_unlink(), attempts to
shm_open() an object with the
same name will fail
(unless O_CREAT was specified,
in which case a new, distinct object is created).
On success, shm_open()
returns a non-negative file descriptor. On failure,
shm_open() returns −1.
shm_unlink() returns 0 on
success, or −1 on error.
On failure, errno is set to
indicate the cause of the error. Values which may appear in
errno include the following:
Permission to shm_unlink() the shared memory object
was denied.
Permission was denied to shm_open() name in the specified
mode, or
O_TRUNC was specified and
the caller does not have write permission on the
object.
Both O_CREAT and
O_EXCL were specified to
shm_open() and the shared
memory object specified by name already exists.
The name
argument to shm_open()
was invalid.
The process already has the maximum number of files open.
The length of name exceeds PATH_MAX.
The limit on the total number of files open on the system has been reached.
An attempt was made to shm_open() a name that did not exist,
and O_CREAT was not
specified.
An attempt was to made to shm_unlink() a name that does not
exist.
POSIX.1-2001.
POSIX.1-2001 says that the group ownership of a newly created shared memory object is set to either the calling process's effective group ID or "a system default group ID".
POSIX leaves the behavior of the combination of
O_RDONLY and O_TRUNC unspecified. On Linux, this will
successfully truncate an existing shared memory object
— this may not be so on other Unix systems.
The POSIX shared memory object implementation on Linux 2.4
makes use of a dedicated file system, which is normally
mounted under /dev/shm.
close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2), mmap(2), open(2), umask(2), shm_overview(7)
This page is part of release 3.12 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Copyright (C) 2002 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. FIXME . Add an example to this page |