SHM_OVERVIEW(7) |
Linux Programmer's Manual |
SHM_OVERVIEW(7) |
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 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 3.53 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/.