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), memfd_create(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_overview(7)

Linux                                                                          2021-03-22                                                                SHM_OVERVIEW(7)