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.\" Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH shm_overview 7 2022-12-04 "Linux man-pages 6.05.01"
.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.
.PP
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 filesystem
On Linux, shared memory objects are created in a
.RB ( tmpfs (5))
virtual filesystem, normally mounted under
.IR /dev/shm .
Since Linux 2.6.19, Linux supports the use of access control lists (ACLs)
to control the permissions of objects in the virtual filesystem.
.SH NOTES
Typically, processes must synchronize their access to a shared
memory object, using, for example, POSIX semaphores.
.PP
System V shared memory
.RB ( shmget (2),
.BR 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.
.SH SEE ALSO
.BR fchmod (2),
.BR fchown (2),
.BR fstat (2),
.BR ftruncate (2),
.BR memfd_create (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)
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