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 file system
On Linux, shared memory objects are created in a (tmpfs) virtual file
system, 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 file system.
CONFORMING TO
POSIX.1-2001.
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
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.
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 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.