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NAME | DESCRIPTION | NOTES | SEE ALSO | COLOPHON |
SHM_OVERVIEW(7) Linux Programmer's Manual SHM_OVERVIEW(7)
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:
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.
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 4.12 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/.
Linux 2016-12-12 SHM_OVERVIEW(7)
Pages that refer to this page: intro(2), mmap(2), shmget(2), shmop(2), shm_open(3), tmpfs(5), sem_overview(7)