'\" t
.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@gmail.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.\" manual page may be incorrect or out-of-date.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.
.\"
.TH SHM_OVERVIEW 7 2008-06-25 "Linux" "Linux Programmer's Manual"
.SH NAME
shm_overview \- Overview of POSIX shared memory
.SH DESCRIPTION
The POSIX shared memory API allows processes to communicate information
by sharing a region of memory.

The interfaces employed in the API are:
.TP 15
.BR shm_open (3)
Create and open a new object, or open an existing object.
This is analogous to
.BR open (2).
The call returns a file descriptor for use by the other
interfaces listed below.
.TP
.BR ftruncate (2)
Set the size of the shared memory object.
(A newly created shared memory object has a length of zero.)
.TP
.BR mmap (2)
Map the shared memory object into the virtual address space
of the calling process.
.TP
.BR munmap (2)
Unmap the shared memory object from the virtual address space
of the calling process.
.TP
.BR shm_unlink (3)
Remove a shared memory object name.
.TP
.BR close (2)
Close the file descriptor allocated by
.BR shm_open (3)
when it is no longer needed.
.TP
.BR fstat (2)
Obtain a
.I stat
structure that describes the shared memory object.
Among the information returned by this call are the object's
size
.RI ( st_size ),
permissions
.RI ( st_mode ),
owner
.RI ( st_uid ),
and group
.RI ( st_gid ).
.TP
.BR fchown (2)
To change the ownership of a shared memory object.
.TP
.BR fchmod (2)
To change the permissions of a shared memory object.
.SS Versions
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
.SS Persistence
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
.BR shm_unlink (3)
.SS Linking
Programs using the POSIX shared memory API must be compiled with
.I cc \-lrt
to link against the real-time library,
.IR librt .
.SS Accessing shared memory objects via the file system
On Linux, shared memory objects are created in a
.RI ( tmpfs )
virtual file system, normally mounted under
.IR /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.
.SH "CONFORMING TO"
POSIX.1-2001.
.SH NOTES
Typically, processes must synchronize their access to a shared
memory object, using, for example, POSIX semaphores.

System V shared memory
.RB ( shmget (2),
.BR 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.
.SH "SEE ALSO"
.BR fchmod (2),
.BR fchown (2),
.BR fstat (2),
.BR ftruncate (2),
.BR mmap (2),
.BR mprotect (2),
.BR munmap (2),
.BR shmget (2),
.BR shmop (2),
.BR shm_open (3),
.BR shm_unlink (3),
.BR sem_overview (7)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
