Mmap

• Marketing Metric Audit Protocol (MMAP) * Memory map * mmap, a UNIX system call for mapping files to memory * Marked Markovian Arrival Process

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.

History

The original design of memory-mapped files came from the TOPS-20 operating system. mmap and associated system 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 API, though, in the 4.0 release of 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. include include include include include include include include /* This example shows how an mmap of /dev/zero is equivalent to using anonymous memory (MAP_ANON) not connected to any file. N.B. MAP_ANONYMOUS or MAP_ANON are supported by most UNIX versions, removing the original purpose of /dev/zero. / /* Does not work on OS X or macOS, where you can't mmap over /dev/zero */ int main(void) { const char str1[] = "string 1"; const char str2[] = "string 2"; pid_t parpid = getpid(); pid_t childpid; int fd = -1; if ((fd = open("/dev/zero", O_RDWR, 0)) == -1) { err(1, "open"); } char* anon = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_SHARED, -1, 0); char* zero = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (anon

MAP_FAILED || zero

MAP_FAILED) { errx(1, "either mmap"); } strcpy(anon, str1); strcpy(zero, str1); printf("PID %d:\tanonymous %s, zero-backed %s\n", parpid, anon, zero); switch ((childpid = fork())) { case -1: err(1, "fork"); // not reached case 0: childpid = getpid(); printf("PID %d:\tanonymous %s, zero-backed %s\n", childpid, anon, zero); sleep(3); printf("PID %d:\tanonymous %s, zero-backed %s\n", childpid, anon, zero); munmap(anon, 4096); munmap(zero, 4096); close(fd); return EXIT_SUCCESS; } sleep(2); strcpy(anon, str2); strcpy(zero, str2); printf("PID %d:\tanonymous %s, zero-backed %s\n", parpid, anon, zero); munmap(anon, 4096); munmap(zero, 4096); close(fd); return EXIT_SUCCESS; } sample output: PID 22475: anonymous string 1, zero-backed string 1 PID 22476: anonymous string 1, zero-backed string 1 PID 22475: anonymous string 2, zero-backed string 2 PID 22476: anonymous string 2, zero-backed string 2 DragonFly BSD FreeBSD NetBSD OpenBSD illumos Mac OS X Solaris QNX Windows MapViewOfFile win32 function is somewhat equivalent to mmap. More example source code: SharedHashFile, An open source, shared memory hash table implemented using mmap().

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