This is svshm/svshm_xfr_reader.c (Listing 48-3, page 1005), 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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/* svshm_xfr_reader.c

   Read data from a System V shared memory using a binary semaphore lock-step
   protocol; see svshm_xfr_writer.c
#include "svshm_xfr.h"
main(int argc, char *argv[])
    int semid, shmid, xfrs, bytes;
    struct shmseg *shmp;

    /* Get IDs for semaphore set and shared memory created by writer */

    semid = semget(SEM_KEY, 0, 0);
    if (semid == -1)

    shmid  = shmget(SHM_KEY, 0, 0);
    if (shmid == -1)

    /* Attach shared memory read-only, as we will only read */

    shmp = shmat(shmid, NULL, SHM_RDONLY);
    if (shmp == (void *) -1)

    /* Transfer blocks of data from shared memory to stdout */

    for (xfrs = 0, bytes = 0; ; xfrs++) {
        if (reserveSem(semid, READ_SEM) == -1)          /* Wait for our turn */

        if (shmp->cnt == 0)                     /* Writer encountered EOF */
        bytes += shmp->cnt;

        if (write(STDOUT_FILENO, shmp->buf, shmp->cnt) != shmp->cnt)
            fatal("partial/failed write");

        if (releaseSem(semid, WRITE_SEM) == -1)         /* Give writer a turn */

    if (shmdt(shmp) == -1)

    /* Give writer one more turn, so it can clean up */

    if (releaseSem(semid, WRITE_SEM) == -1)

    fprintf(stderr, "Received %d bytes (%d xfrs)\n", bytes, xfrs);


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