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NAME | DESCRIPTION | NOTES | SEE ALSO | COLOPHON |
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shm_overview(7) Miscellaneous Information Manual shm_overview(7)
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:
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 Linux
2.6.19, Linux supports the use of access control lists (ACLs) to
control the permissions of objects in the virtual filesystem.
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), memfd_create(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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 shm_overview(7)
Pages that refer to this page: intro(2), mmap(2), shmget(2), shmop(2), shm_open(3), tmpfs(5), sem_overview(7), sysvipc(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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