1SHM_OVERVIEW(7) Linux Programmer's Manual SHM_OVERVIEW(7)
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6 shm_overview - overview of POSIX shared memory
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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 filesystem
55 On Linux, shared memory objects are created in a (tmpfs(5)) virtual
56 filesystem, normally mounted under /dev/shm. Since kernel 2.6.19,
57 Linux supports the use of access control lists (ACLs) to control the
58 permissions of objects in the virtual filesystem.
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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
66 designed 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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71 fchmod(2), fchown(2), fstat(2), ftruncate(2), mmap(2), mprotect(2),
72 munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_over‐
73 view(7)
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76 This page is part of release 5.04 of the Linux man-pages project. A
77 description of the project, information about reporting bugs, and the
78 latest version of this page, can be found at
79 https://www.kernel.org/doc/man-pages/.
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83Linux 2016-12-12 SHM_OVERVIEW(7)