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SHM_OVERVIEW(7) Linux Programmer's Manual SHM_OVERVIEW(7)
NAME
shm_overview - overview of POSIX shared memory
DESCRIPTION
The POSIX shared memory API allows processes to communicate information by sharing a re-
gion 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 analo-
gous 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 call-
ing 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), permis-
sions (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 un-
til 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
This page is part of release 5.10 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
Linux 2020-08-13 SHM_OVERVIEW(7)
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