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 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 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 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
       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)