54.1. Overview

POSIX shared memory allows to us to share a mapped region between unrelated processes without needing to create a corresponding mapped file. POSIX shared memory is supported on Linux since kernel 2.4.

SUSv3 doesn’t specify any of the details of how POSIX shared memory is to be implemented. In particular, there is no requirement for the use of a (real or virtual) file system to identify shared memory objects, although many UNIX implementations do employ a file system for this purpose. Some UNIX implementations create the names for shared memory objects as files in a special location in the standard file system. Linux uses a dedicated tmpfs file system (Section 14.10) mounted under the directory /dev/shm. This file system has kernel persistence--the shared memory objects that it contains will persist even if no process currently has them open, but they will be lost if the system is shut down.