SHM_OVERVIEW(7)             Linux Programmer's Manual            SHM_OVERVIEW(7)

       shm_overview - overview of POSIX shared memory

       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.

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

       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.

       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),

       This page is part of release 5.13 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

Linux                              2021-03-22                    SHM_OVERVIEW(7)