To install click the Add extension button. That's it.

The source code for the WIKI 2 extension is being checked by specialists of the Mozilla Foundation, Google, and Apple. You could also do it yourself at any point in time.

4,5
Kelly Slayton
Congratulations on this excellent venture… what a great idea!
Alexander Grigorievskiy
I use WIKI 2 every day and almost forgot how the original Wikipedia looks like.
Live Statistics
English Articles
Improved in 24 Hours
Added in 24 Hours
What we do. Every page goes through several hundred of perfecting techniques; in live mode. Quite the same Wikipedia. Just better.
.
Leo
Newton
Brights
Milds

From Wikipedia, the free encyclopedia

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.

YouTube Encyclopedic

  • 1/5
    Views:
    1 736
    7 940
    2 010
    21 791
    52 638
  • SAP MM- MAP/Physical Inventory/Vendor subrange/Partner functions- All in one class explanation
  • A Map of Social Space in Your Brain
  • CICS Map Creation - Physical & Symbolic Map Example | CICS Map Sample Program | Mainframe Guru
  • CISSP Domain 3 Review | Mind Map (9 of 9) | Physical Security
  • Quantum Computing 101

Transcription

History

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.[1] 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.[2]

File-backed and anonymous

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.[3] 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.

Memory visibility

If the mapping is shared (the MAP_SHARED flag is set), then it is preserved when a process is forked (using a fork(2) system call). Therefore, writes to a mapped area in one process are immediately visible in all related (parent, child or sibling) processes. If the mapping is shared and backed by a file (not MAP_ANONYMOUS) the underlying file medium is only guaranteed to be written after it is passed to the msync(2) system call. In contrast, 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 different process that has mapped the file, since segments of the file are copied into RAM and only periodically flushed to disk. Synchronization can be forced with a call to msync(2).

Using mmap on 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.[4]

The main difference between System V shared memory (shmem) and memory mapped I/O (mmap) is that System V 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 under the C programming language

#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>

/* 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(), 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_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;
}

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

Usage in database implementations

The mmap system call has been used in various database implementations as an alternative for implementing a buffer pool, although this created a different set of problems that could realistically only be fixed using a buffer pool.[5]

See also

References

  1. ^ William Joy; Eric Cooper; Robert Fabry; Samuel Leffler; Kirk McKusick; David Mosher (1983). 4.2BSD System Manual (PDF) (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".
  4. ^ Kerrisk, Michael (2010). The Linux programming interface : a Linux and UNIX system programming handbook. San Francisco: No Starch Press. p. 1116. ISBN 978-1-59327-291-3. OCLC 728672600.
  5. ^ "Are You Sure You Want to Use MMAP in Your Database Management System?". db.cs.cmu.edu. Retrieved 2023-07-04.

Further reading

This page was last edited on 29 September 2023, at 11:23
Basis of this page is in Wikipedia. Text is available under the CC BY-SA 3.0 Unported License. Non-text media are available under their specified licenses. Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc. WIKI 2 is an independent company and has no affiliation with Wikimedia Foundation.