Editing Mmap (section)

== Example of usage under the C programming language ==
<syntaxhighlight lang="c" line>
#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;
}
</syntaxhighlight>

sample output:
<pre><nowiki>
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
</nowiki></pre>