This is mmap/t_mmap.c (Listing 49-2, page 1028), 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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/* t_mmap.c

   Demonstrate the use of mmap() to create a shared file mapping.
#include <sys/mman.h>
#include <fcntl.h>
#include "tlpi_hdr.h"

#define MEM_SIZE 10
main(int argc, char *argv[])
    char *addr;
    int fd;

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file [new-value]\n", argv[0]);

    fd = open(argv[1], O_RDWR);
    if (fd == -1)

    addr = mmap(NULL, MEM_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED)

    if (close(fd) == -1)                /* No longer need 'fd' */

    printf("Current string=%.*s\n", MEM_SIZE, addr);
                        /* Secure practice: output at most MEM_SIZE bytes */

    if (argc > 2) {                     /* Update contents of region */
        if (strlen(argv[2]) >= MEM_SIZE)
            cmdLineErr("'new-value' too large\n");

        memset(addr, 0, MEM_SIZE);      /* Zero out region */
        strncpy(addr, argv[2], MEM_SIZE - 1);
        if (msync(addr, MEM_SIZE, MS_SYNC) == -1)

        printf("Copied \"%s\" to shared memory\n", argv[2]);



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