shm_overview(7) — Linux manual page


shm_overview(7)     Miscellaneous Information Manual     shm_overview(7)

NAME         top

       shm_overview - overview of POSIX shared memory

DESCRIPTION         top

       The POSIX shared memory API allows processes to communicate
       information by sharing a region of memory.

       The interfaces employed in the API are:

              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.

              Set the size of the shared memory object.  (A newly
              created shared memory object has a length of zero.)

              Map the shared memory object into the virtual address
              space of the calling process.

              Unmap the shared memory object from the virtual address
              space of the calling process.

              Remove a shared memory object name.

              Close the file descriptor allocated by shm_open(3) when it
              is no longer needed.

              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).

              To change the ownership of a shared memory object.

              To change the permissions of a shared memory object.

       POSIX shared memory is supported since Linux 2.4 and glibc 2.2.

       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)

       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 Linux
       2.6.19, Linux supports the use of access control lists (ACLs) to
       control the permissions of objects in the virtual filesystem.

NOTES         top

       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         top

       fchmod(2), fchown(2), fstat(2), ftruncate(2), memfd_create(2),
       mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2),
       shm_open(3), shm_unlink(3), sem_overview(7)

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                shm_overview(7)

Pages that refer to this page: intro(2)mmap(2)shmget(2)shmop(2)shm_open(3)tmpfs(5)sem_overview(7)sysvipc(7)