shm_overview — Overview of POSIX shared memory
The POSIX shared memory API allows processes to communicate information by sharing a region of memory.
The interfaces employed in the API are:
Create and open a new object, or open an existing object. This is analogous to open(2). The call returns a file descriptor for use by the other interfaces listed below.
Set the size of the shared memory object. (A newly created shared memory object has a length of zero.)
Map the shared memory object into the virtual address space of the calling process.
Unmap the shared memory object from the virtual address space of the calling process.
Remove a shared memory object name.
Close the file descriptor allocated by shm_open(3) when it is no longer needed.
Obtain a stat structure that
describes the shared memory object. Among the
information returned by this call are the object's size
(st_size),
permissions (st_mode), owner
(st_uid), and
group (st_gid).
To change the ownership of a shared memory object.
To change the permissions of a shared memory object.
POSIX shared memory objects have kernel persistence: a shared memory object will exist until the system is shut down, or until all processes have unmapped the object and it has been deleted with shm_unlink(3)
Programs using the POSIX shared memory API must be
compiled with cc
−lrt to link against the real-time
library, librt.
On Linux, shared memory objects are created in a
(tmpfs) virtual
file system, normally mounted under /dev/shm. Since kernel 2.6.19, Linux
supports the use of access control lists (ACLs) to control
the permissions of objects in the virtual file system.
Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.
System V shared memory (shmget(2), shmop(2), etc.) is an older semaphore API. POSIX shared memory provides a simpler, and better designed interface; on the other hand POSIX shared memory is somewhat less widely available (especially on older systems) than System V shared memory.
fchmod(2), fchown(2), fstat(2), ftruncate(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_overview(7)
This page is part of release 3.24 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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t Copyright (C) 2008, Linux Foundation, written by 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. |