GENWiki

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 cre-
                    ated 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_over-
     view(7)

COLOPHON

     This  page  is  part of release 4.16 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 2016-12-12 SHM_OVERVIEW(7)