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 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* anon_mmap.c

   Demonstrate how to share a region of mapped memory between a parent and
   child process without having to create a mapped file, either through the
   creation of an anonymous memory mapping or through the mapping of /dev/zero.
*/
#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 */

    /* Parent creates mapped region prior to calling fork() */

#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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