Adding upstream version 4.22.0.upstream/4.22.0

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.\" Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk
+.\" <mtk.manpages@gmail.com>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH shm_overview 7 2022-12-04 "Linux man-pages 6.05.01"
+.SH NAME
+shm_overview \- overview of POSIX shared memory
+.SH DESCRIPTION
+The POSIX shared memory API allows processes to communicate information
+by sharing a region of memory.
+.PP
+The interfaces employed in the API are:
+.TP 15
+.BR shm_open (3)
+Create and open a new object, or open an existing object.
+This is analogous to
+.BR open (2).
+The call returns a file descriptor for use by the other
+interfaces listed below.
+.TP
+.BR ftruncate (2)
+Set the size of the shared memory object.
+(A newly created shared memory object has a length of zero.)
+.TP
+.BR mmap (2)
+Map the shared memory object into the virtual address space
+of the calling process.
+.TP
+.BR munmap (2)
+Unmap the shared memory object from the virtual address space
+of the calling process.
+.TP
+.BR shm_unlink (3)
+Remove a shared memory object name.
+.TP
+.BR close (2)
+Close the file descriptor allocated by
+.BR shm_open (3)
+when it is no longer needed.
+.TP
+.BR fstat (2)
+Obtain a
+.I stat
+structure that describes the shared memory object.
+Among the information returned by this call are the object's
+size
+.RI ( st_size ),
+permissions
+.RI ( st_mode ),
+owner
+.RI ( st_uid ),
+and group
+.RI ( st_gid ).
+.TP
+.BR fchown (2)
+To change the ownership of a shared memory object.
+.TP
+.BR fchmod (2)
+To change the permissions of a shared memory object.
+.SS Versions
+POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
+.SS 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
+.BR shm_unlink (3)
+.SS Linking
+Programs using the POSIX shared memory API must be compiled with
+.I cc \-lrt
+to link against the real-time library,
+.IR librt .
+.SS Accessing shared memory objects via the filesystem
+On Linux, shared memory objects are created in a
+.RB ( tmpfs (5))
+virtual filesystem, normally mounted under
+.IR /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.
+.SH NOTES
+Typically, processes must synchronize their access to a shared
+memory object, using, for example, POSIX semaphores.
+.PP
+System V shared memory
+.RB ( shmget (2),
+.BR 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.
+.SH SEE ALSO
+.BR fchmod (2),
+.BR fchown (2),
+.BR fstat (2),
+.BR ftruncate (2),
+.BR memfd_create (2),
+.BR mmap (2),
+.BR mprotect (2),
+.BR munmap (2),
+.BR shmget (2),
+.BR shmop (2),
+.BR shm_open (3),
+.BR shm_unlink (3),
+.BR sem_overview (7)