shm_overview(7) - Linux manual page (original) (raw)

shmoverview(7) Miscellaneous Information Manual shmoverview(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:

   [shm_open(3)](../man3/shm%5Fopen.3.html)
          Create and open a new object, or open an existing object.
          This is analogous to [open(2)](../man2/open.2.html).  The call returns a file
          descriptor for use by the other interfaces listed below.

   [ftruncate(2)](../man2/ftruncate.2.html)
          Set the size of the shared memory object.  (A newly created
          shared memory object has a length of zero.)

   [mmap(2)](../man2/mmap.2.html)
          Map the shared memory object into the virtual address space
          of the calling process.

   [munmap(2)](../man2/munmap.2.html)
          Unmap the shared memory object from the virtual address
          space of the calling process.

   [shm_unlink(3)](../man3/shm%5Funlink.3.html)
          Remove a shared memory object name.

   [close(2)](../man2/close.2.html)
          Close the file descriptor allocated by [shm_open(3)](../man3/shm%5Fopen.3.html) when it
          is no longer needed.

   [fstat(2)](../man2/fstat.2.html)
          Obtain a _stat_ structure that describes the shared memory
          object.  Among the information returned by this call are
          the object's size (_stsize_), permissions (_stmode_), owner
          (_stuid_), and group (_stgid_).

   [fchown(2)](../man2/fchown.2.html)
          To change the ownership of a shared memory object.

   [fchmod(2)](../man2/fchmod.2.html)
          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 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)](../man2/shmget.2.html), [shmop(2)](../man2/shmop.2.html), 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)](../man2/fchmod.2.html), [fchown(2)](../man2/fchown.2.html), [fstat(2)](../man2/fstat.2.html), [ftruncate(2)](../man2/ftruncate.2.html), [memfd_create(2)](../man2/memfd%5Fcreate.2.html),
   [mmap(2)](../man2/mmap.2.html), [mprotect(2)](../man2/mprotect.2.html), [munmap(2)](../man2/munmap.2.html), [shmget(2)](../man2/shmget.2.html), [shmop(2)](../man2/shmop.2.html), [shm_open(3)](../man3/shm%5Fopen.3.html),
   [shm_unlink(3)](../man3/shm%5Funlink.3.html), [sem_overview(7)](../man7/sem%5Foverview.7.html)

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Linux man-pages 6.10 2024-05-02 shmoverview(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)