shm_overview

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 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 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 5.08 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)