Section 7 · Overviews and conventions
shm_overview.7
Package: man-pages
shm_overview
NAME
DESCRIPTION
NOTES
SEE ALSO
NAME
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. VersionsPOSIX shared memory is supported since Linux 2.4 and glibc 2.2.PersistencePOSIX 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)LinkingPrograms 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 filesystemOn Linux, shared memory objects are created in a (tmpfs(5)) virtual filesystem, normally mounted under /dev/shm. Since Linux 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), memfd_create(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_overview(7)