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Version: 2008-06-25 (MeeGo - 06/11/10)
Section: 7 (Divers)
NAMEshm_overview - Overview of POSIX shared memory
DESCRIPTIONThe POSIX shared memory API allows processes to communicate information by sharing a region of memory.
The interfaces employed in the API are:
- 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.
- Set the size of the shared memory object. (A newly created shared memory object has a length of zero.)
- Map the shared memory object into the virtual address space of the calling process.
- Unmap the shared memory object from the virtual address space of the calling process.
- Remove a shared memory object name.
- Close the file descriptor allocated by shm_open(3) when it is no longer needed.
- 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).
- To change the ownership of a shared memory object.
- To change the permissions of a shared memory object.
VersionsPOSIX shared memory is supported since Linux 2.4 and glibc 2.2.
PersistencePOSIX 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)
LinkingPrograms 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 systemOn 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.
NOTESTypically, 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 ALSOfchmod(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)
COLOPHONThis page is part of release 3.25 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/.
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