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 pro-
       cesses 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  mem-
       ory.

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_over-
       view(7)

COLOPHON

       This 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/.

Linux								    2008-06-25							   SHM_OVERVIEW(7)