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 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 VFS or I/O cache.