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In computing, mmap(2) is a POSIX-compliant Unix system call that maps files or devices into memory. It is a method of I/O. It implements demand paging because file contents are not immediately read from disk and initially use no physical RAM at all. The actual reads from disk are performed after a specific location is accessed, in a lazy manner. After the mapping is no longer needed, the pointers must be unmapped with munmap(2). Protection information—for example, marking mapped regions as executable—can be managed using mprotect(2), and special treatment can be enforced using madvise(2). In Linux, macOS and the BSDs, mmap can create several types of mappings. Other operating systems may only support a subset of these; for example, shared mappings may not be practical in an operating system without a global or I/O cache. The original design of memory-mapped files came from the TOPS-20 operating system. 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 University of California, Berkeley unsuccessfully requested Sun to donate its implementation; 4.3BSD-Reno was instead shipped 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; that is, 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 implementations for certain allocations, particularly large ones. Anonymous mappings are not part of the POSIX standard, but are implemented in almost all operating systems by the MAP_ANONYMOUS and MAP_ANON flags.

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Demand paging

In computer operating systems, demand paging (as opposed to anticipatory paging) is a method of virtual memory management. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it and that page is not already in memory (i.e., if a page fault occurs). It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process's working set of pages are located in physical memory.

Unix

Unix (ˈjuːnᵻks; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley (BSD), Microsoft (Xenix), Sun Microsystems (SunOS/Solaris), HP/HPE (HP-UX), and IBM (AIX).

Memory paging

In computer operating systems, memory paging (or swapping on some Unix-like systems) is a memory management scheme by which a computer stores and retrieves data from secondary storage for use in main memory. In this scheme, the operating system retrieves data from secondary storage in same-size blocks called pages. Paging is an important part of virtual memory implementations in modern operating systems, using secondary storage to let programs exceed the size of available physical memory.

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