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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 inter‐
                      faces listed below.

       ftruncate(2)   Set the size of the shared memory object.  (A newly created shared memory  ob‐
                      ject 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(5)) 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 ex‐
       ample, 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 5.10 of the Linux  man-pages  project.   A  description  of  the
       project,  information about reporting bugs, and the latest version of this page, can be found
       at https://www.kernel.org/doc/man-pages/.



Linux                                        2020-08-13                              SHM_OVERVIEW(7)