shm_overview (7) Linux Manual Page
shm_overview – overview of POSIX shared memory
Description
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 below. 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.
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 filesystem
On Linux, shared memory objects are created in a (tmpfs(5)) virtual filesystem, 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 filesystem.
Notes
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.
See Also
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)
Colophon
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