1shm_overview(7)        Miscellaneous Information 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  ob‐
15                      ject.  This is analogous to open(2).  The call returns a
16                      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 filesystem
55       On  Linux,  shared  memory  objects are created in a (tmpfs(5)) virtual
56       filesystem, normally mounted under /dev/shm.  Since Linux 2.6.19, Linux
57       supports  the use of access control lists (ACLs) to control the permis‐
58       sions of objects in the virtual filesystem.
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NOTES

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

71       fchmod(2), fchown(2), fstat(2), ftruncate(2), memfd_create(2), mmap(2),
72       mprotect(2),  munmap(2),  shmget(2),  shmop(2),  shm_open(3),   shm_un‐
73       link(3), sem_overview(7)
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77Linux man-pages 6.04              2022-12-04                   shm_overview(7)
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