mmap

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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 memory-mapped file I/O. It naturally implements demand paging, because initially file contents are not entirely read from disk 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.

History[edit]

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.[1] 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 transferral of code; 4.3BSD-Reno was shipped instead with an implementation based on the virtual memory system of Mach.[2]

File-backed and anonymous[edit]

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.[3] 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 flag.

Memory visibility[edit]

If the mapping is shared (the MAP_SHARED flag is set), it is kept visible across a fork(2) system call. This means that writes to that area in one process will affect other processes (that still have the same area mapped) and the underlying file (though it might not be msync(2)'ed and written to the underlying medium).

If the mapping is private (the MAP_PRIVATE flag is set), the changes will neither be seen by other processes nor written to the file.

A process reading from or writing to the underlying file will not always see the same data as a process that has mapped the file, since the segment of the file is copied into RAM and periodically flushed to disk. Synchronization can be forced with the msync system call.

mmap(2)ing files can significantly reduce memory overhead for applications accessing the same file; they can share the memory area the file encompasses, instead of loading the file for each application that wants access to it. This means that mmap(2) is sometimes used for Interprocess Communication (IPC). On modern operating systems mmap(2) is typically preferred to the System V IPC Shared Memory facility.[citation needed]

The main difference between System V shared memory (shmem) and memory mapped I/O (mmap) is that SystemV shared memory is persistent: unless explicitly removed by a process, it is kept in memory and remains available until the system is shut down. mmap'd memory is not persistent between application executions (unless it is backed by a file).

Example of usage[edit]

#include <sys/types.h>
#include <sys/mman.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
 
/* Does not work on OSX, as you can't mmap over /dev/zero */
int main(void)
{
        const char str1[] = "string 1";
        const char str2[] = "string 2";
        int parpid = getpid(), childpid;
        int fd = -1;
        char *anon, *zero;
 
        if ((fd = open("/dev/zero", O_RDWR, 0)) == -1)
                err(1, "open");
 
        anon = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_SHARED, -1, 0);
        zero = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_FILE|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");
                /* NOTREACHED */
 
        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);
}

See also[edit]

  • Virtual memory for a general context of possessing more addresses than physical memory
    • Swapping or Paging for implementation of virtual memory used in contemporary systems

References and further reading[edit]

  1. ^ William Joy; Eric Cooper; Robert Fabry; Samuel Leffler; Kirk McKusick; David Mosher (1983). 4.2BSD System Manual (Report). Computer Systems Research Group, University of California, Berkeley.
  2. ^ McKusick, Marshall Kirk (1999). "Twenty Years of Berkeley Unix: From AT&T-Owned to Freely Redistributable". Open Sources: Voices from the Open Source Revolution. O'Reilly. 
  3. ^ "mmap(2) - Linux manual page". 
  • Windows
  • More example source code:
  • SharedHashFile, An open source, shared memory hash table implemented using mmap().