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 filesystem
       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.

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

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