This is pshm/pshm_create.c (Listing 54-1, page 1110), 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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/* pshm_create.c

   Create a POSIX shared memory object with specified size and permissions.

   Usage as shown in usageError().
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include "tlpi_hdr.h"
static void
usageError(const char *progName)
    fprintf(stderr, "Usage: %s [-cx] shm-name size [octal-perms]\n", progName);
    fprintf(stderr, "    -c   Create shared memory (O_CREAT)\n");
    fprintf(stderr, "    -x   Create exclusively (O_EXCL)\n");
main(int argc, char *argv[])
    int flags, opt, fd;
    mode_t perms;
    size_t size;
    void *addr;

    flags = O_RDWR;
    while ((opt = getopt(argc, argv, "cx")) != -1) {
        switch (opt) {
        case 'c':   flags |= O_CREAT;           break;
        case 'x':   flags |= O_EXCL;            break;
        default:    usageError(argv[0]);

    if (optind + 1 >= argc)

    size = getLong(argv[optind + 1], GN_ANY_BASE, "size");
    perms = (argc <= optind + 2) ? (S_IRUSR | S_IWUSR) :
                getLong(argv[optind + 2], GN_BASE_8, "octal-perms");

    /* Create shared memory object and set its size */

    fd = shm_open(argv[optind], flags, perms);
    if (fd == -1)

    if (ftruncate(fd, size) == -1)

    /* Map shared memory object */

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



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