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.

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 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

       This page is part of release 3.54 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/.