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GNU/Linux |
Debian 6.0.6(Squeeze) |
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shm_overview(7) |
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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:
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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. | ||
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ftruncate(2) |
Set the size of the shared memory object. (A newly created shared memory object has a length of zero.) | ||
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mmap(2) |
Map the shared memory object into the virtual address space of the calling process. | ||
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munmap(2) |
Unmap the shared memory object from the virtual address space of the calling process. | ||
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shm_unlink(3) |
Remove a shared memory object name. | ||
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close(2) |
Close the file descriptor allocated by shm_open(3) when it is no longer needed. | ||
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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). | ||
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fchown(2) |
To change the ownership of a shared memory object. | ||
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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 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.
POSIX.1-2001.
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.
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_overview(7)
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
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shm_overview(7) | |