Editing Mmap (section)

{{Short description|Memory map - POSIX-compliant system call}}
{{lowercase|title=mmap}}
{{Distinguish|nmap}}
In [[computing]], '''<code>mmap(2)</code>''' 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 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 evaluation|lazy]] manner. After the mapping is no longer needed, the pointers must be unmapped with <code>munmap(2)</code>. [[Memory protection|Protection]] information—for example, marking mapped regions as executable—can be managed using <code>mprotect(2)</code>, and special treatment can be enforced using <code>madvise(2)</code>.

In [[Linux]], [[macOS]] and the [[BSD]]s, <code>mmap</code> 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 [[Virtual file system|VFS]] or I/O cache.