SHM_OVERVIEW(7) manual page
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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.
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
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)
Programs using
the POSIX shared memory API must be compiled with cc -lrt to link against
the real-time library, librt.
On
Linux, shared memory objects are created in a (tmpfs) 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.
POSIX.1-2001.
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 3.78 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 http://www.kernel.org/doc/man-pages/.
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