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       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

       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 file system
       On  Linux,  shared memory objects are created in a (tmpfs) virtual file
       system, 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 file system.




       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
       semaphore 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),  mmap(2), mprotect(2),
       munmap(2),    shmget(2),    shmop(2),    shm_open(3),    shm_unlink(3),


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