svipc/svmsg_demo_server.c

/* 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 */

int
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)
        errExit("ftok");

    /* 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("msgctl() 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);

    exit(EXIT_SUCCESS);
}