SHM_OVERVIEW

Section: Linux Programmer's Manual (7)
Updated: 2020-08-13
Page Index

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.

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.

COLOPHON

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