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 is supported since Linux 2.4 and glibc 2.2.


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.

Accessing shared memory objects via the filesystem

On Linux, shared memory objects are created in a (tmpfs) virtual filesystem, 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 filesystem.


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 shared memory 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 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk

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

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.