NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

SHM_OVERVIEW(7)           Linux Programmer's Manual          SHM_OVERVIEW(7)

NAME         top

       shm_overview - overview of POSIX shared memory

DESCRIPTION         top

       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.

CONFORMING TO         top

       POSIX.1-2001.

NOTES         top

       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         top

       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         top

       This page is part of release 3.65 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                            2010-09-10                  SHM_OVERVIEW(7)