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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 cre-
                      ated 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 permis-
       sions 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 sema-
       phore 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_over-
       view(7)

COLOPHON
       This  page  is  part of release 3.05 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)