mmap/anon_mmap.c

This is mmap/anon_mmap.c (Listing 49-3, page 1036), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

#ifdef USE_MAP_ANON
#define _BSD_SOURCE             /* Get MAP_ANONYMOUS definition */
#endif
#include <sys/wait.h>
#include <sys/mman.h>
#include <fcntl.h>
#include "tlpi_hdr.h"
int
main(int argc, char *argv[])
{
    int *addr;                  /* Pointer to shared memory region */

#ifdef USE_MAP_ANON             /* Use MAP_ANONYMOUS */
    addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE,
                MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (addr == MAP_FAILED)
        errExit("mmap");

#else                           /* Map /dev/zero */
    int fd;

    fd = open("/dev/zero", O_RDWR);
    if (fd == -1)
        errExit("open");

    addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED)
        errExit("mmap");

    if (close(fd) == -1)        /* No longer needed */
        errExit("close");
#endif

    *addr = 1;                  /* Initialize integer in mapped region */

    switch (fork()) {           /* Parent and child share mapping */
    case -1:
        errExit("fork");

    case 0:                     /* Child: increment shared integer and exit */
        printf("Child started, value = %d\n", *addr);
        (*addr)++;
        if (munmap(addr, sizeof(int)) == -1)
            errExit("munmap");
        exit(EXIT_SUCCESS);

    default:                    /* Parent: wait for child to terminate */
        if (wait(NULL) == -1)
            errExit("wait");
        printf("In parent, value = %d\n", *addr);
        if (munmap(addr, sizeof(int)) == -1)
            errExit("munmap");
        exit(EXIT_SUCCESS);
    }
}

 

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