SHM_OVERVIEW
Section: Linux Programmer's Manual (7)
Updated: 2016-12-12
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NAME
shm_overview - overview of POSIX shared memory
DESCRIPTION
The POSIX shared memory API allows processes to communicate information
by sharing a region of memory.
The interfaces employed in the API are:
- shm_open(3)
-
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.
- ftruncate(2)
-
Set the size of the shared memory object.
(A newly created shared memory object has a length of zero.)
- mmap(2)
-
Map the shared memory object into the virtual address space
of the calling process.
- munmap(2)
-
Unmap the shared memory object from the virtual address space
of the calling process.
- shm_unlink(3)
-
Remove a shared memory object name.
- close(2)
-
Close the file descriptor allocated by
shm_open(3)
when it is no longer needed.
- fstat(2)
-
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).
- fchown(2)
-
To change the ownership of a shared memory object.
- fchmod(2)
-
To change the permissions of a shared memory object.
Versions
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
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
shm_unlink(3)
Linking
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(5))
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.
NOTES
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.
SEE ALSO
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)
COLOPHON
This page is part of release 4.13 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
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- DESCRIPTION
-
- Versions
-
- Persistence
-
- Linking
-
- Accessing shared memory objects via the filesystem
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
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