This is svipc/svmsg_demo_server.c (Listing 45-1, page 930), 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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/* svmsg_demo_server.c

   Demonstration System V message queue-based server.
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/stat.h>
#include "tlpi_hdr.h"

#define KEY_FILE "/some-path/some-file"
                                /* Should be an existing file or one
                                   that this program creates */
main(int argc, char *argv[])
    int msqid;
    key_t key;
    const int MQ_PERMS = S_IRUSR | S_IWUSR | S_IWGRP;   /* rw--w---- */

    /* Optional code here to check if another server process is
       already running */

    /* Generate the key for the message queue */

    key = ftok(KEY_FILE, 1);
    if (key == -1)

    /* While msgget() fails, try creating the queue exclusively */

    while ((msqid = msgget(key, IPC_CREAT | IPC_EXCL | MQ_PERMS)) ==
            -1) {
        if (errno == EEXIST) {          /* MQ with the same key already
                                           exists - remove it and try again */
            msqid = msgget(key, 0);
            if (msqid == -1)
                errExit("msgget() failed to retrieve old queue ID");
            if (msgctl(msqid, IPC_RMID, NULL) == -1)
                errExit("msgget() failed to delete old queue");
            printf("Removed old message queue (id=%d)\n", msqid);

        } else {                        /* Some other error --> give up */
            errExit("msgget() failed");

    /* Upon loop exit, we've successfully created the message queue,
       and we can then carry on to do other work... */

    printf("Queue created with id %d\n", msqid);



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