Mmap

In computing, mmap(2) is a POSIX-compliant Unix system call that maps files or devices into memory. It is a method of memory-mapped file I/O. It naturally implements demand paging, because file contents are not read from disk initially and do not use physical RAM at all. The actual reads from disk are performed in a "lazy" manner, after a specific location is accessed. After the memory is no longer needed, it is important to munmap(2) the pointers to it. Protection information can be managed using mprotect(2), and special treatment can be enforced using madvise(2).

In Linux, Mac OS X and the BSDs, mmap can create several types of mappings.

mmap and associated systems calls were designed as part of the Berkeley Software Distribution (BSD) version of Unix. Their API was already described in the 4.2BSD System Manual, even though it was neither implemented in that release, nor in 4.3BSD.Sun Microsystems had implemented this very API, though, in their SunOS operating system. The BSD developers at U.C. Berkeley requested Sun to donate its implementation, but these talks never led to any transfer of code; 4.3BSD-Reno was shipped instead with an implementation based on the virtual memory system of Mach.

File-backed mapping maps an area of the process's virtual memory to files; i.e. reading those areas of memory causes the file to be read. It is the default mapping type.

Anonymous mapping maps an area of the process's virtual memory not backed by any file. The contents are initialized to zero. In this respect an anonymous mapping is similar to malloc, and is used in some malloc(3) implementations for certain allocations. However, anonymous mappings are not part of the POSIX standard, though implemented by almost all operating systems by the MAP_ANONYMOUS and MAP_ANON flags.

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