1SHM_OVERVIEW(7)            Linux Programmer's Manual           SHM_OVERVIEW(7)
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NAME

6       shm_overview - Overview of POSIX shared memory
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DESCRIPTION

9       The POSIX shared memory API allows processes to communicate information
10       by sharing a region of memory.
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12       The interfaces employed in the API are:
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14       shm_open(3)    Create and open  a  new  object,  or  open  an  existing
15                      object.  This is analogous to open(2).  The call returns
16                      a file descriptor for use by the other interfaces listed
17                      below.
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19       ftruncate(2)   Set the size of the shared memory object.  (A newly cre‐
20                      ated shared memory object has a length of zero.)
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22       mmap(2)        Map the shared memory object into  the  virtual  address
23                      space of the calling process.
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25       munmap(2)      Unmap  the shared memory object from the virtual address
26                      space of the calling process.
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28       shm_unlink(3)  Remove a shared memory object name.
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30       close(2)       Close the file descriptor allocated by shm_open(3)  when
31                      it is no longer needed.
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33       fstat(2)       Obtain a stat structure that describes the shared memory
34                      object.  Among the information returned by this call are
35                      the  object's  size  (st_size),  permissions  (st_mode),
36                      owner (st_uid), and group (st_gid).
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38       fchown(2)      To change the ownership of a shared memory object.
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40       fchmod(2)      To change the permissions of a shared memory object.
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42   Versions
43       POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
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45   Persistence
46       POSIX shared memory objects have kernel persistence:  a  shared  memory
47       object will exist until the system is shut down, or until all processes
48       have unmapped the object and it has been deleted with shm_unlink(3)
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50   Linking
51       Programs using the POSIX shared memory API must  be  compiled  with  cc
52       -lrt to link against the real-time library, librt.
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54   Accessing shared memory objects via the file system
55       On  Linux,  shared memory objects are created in a (tmpfs) virtual file
56       system, normally mounted under /dev/shm.  Since  kernel  2.6.19,  Linux
57       supports  the use of access control lists (ACLs) to control the permis‐
58       sions of objects in the virtual file system.
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CONFORMING TO

61       POSIX.1-2001.
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NOTES

64       Typically, processes must synchronize their access to a  shared  memory
65       object, using, for example, POSIX semaphores.
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67       System  V  shared  memory (shmget(2), shmop(2), etc.) is an older sema‐
68       phore API.  POSIX shared memory provides a simpler, and better designed
69       interface;  on  the  other  hand  POSIX  shared memory is somewhat less
70       widely available (especially on older systems)  than  System  V  shared
71       memory.
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SEE ALSO

74       fchmod(2),  fchown(2),  fstat(2),  ftruncate(2),  mmap(2), mprotect(2),
75       munmap(2), shmget(2), shmop(2), shm_open(3),  shm_unlink(3),  sem_over‐
76       view(7)
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COLOPHON

79       This  page  is  part of release 3.25 of the Linux man-pages project.  A
80       description of the project, and information about reporting  bugs,  can
81       be found at http://www.kernel.org/doc/man-pages/.
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85Linux                             2008-06-25                   SHM_OVERVIEW(7)
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