Error with msgget() and ftok ()

Question

I am learning IPC programming. As a part of it I tried the below two codes to get to know about message queues....

Message queue creator or message sender

struct my_msgbuf {
    long mtype;
    char mtext[200];
};

int main(void)
{
    struct my_msgbuf buf;
    int msqid;
    key_t key;
if ((key = ftok("kirk.c", 'B')) == -1) {
    perror("ftok");
    exit(1);
}

if ((msqid = msgget(key, 0644 | IPC_CREAT)) == -1) {
    perror("msgget");
    exit(1);
}

printf("Enter lines of text, ^D to quit:\n");

buf.mtype = 1; /* we don't really care in this case */

while(fgets(buf.mtext, sizeof buf.mtext, stdin) != NULL) {
    int len = strlen(buf.mtext);

    /* ditch newline at end, if it exists */
    if (buf.mtext[len-1] == '\n') buf.mtext[len-1] = '\0';

    if (msgsnd(msqid, &buf, len+1, 0) == -1) /* +1 for '\0' */
        perror("msgsnd");
}

if (msgctl(msqid, IPC_RMID, NULL) == -1) {
    perror("msgctl");
    exit(1);
}

return 0;
}

Message receiver

    struct my_msgbuf {
    long mtype;
    char mtext[200];
};

int main(void)
{
    struct my_msgbuf buf;
    int msqid;
    key_t key;

    if ((key = ftok("kirk.c", 'B')) == -1) {  /* same key as kirk.c */
        perror("ftok");
        exit(1);
    }

    if ((msqid = msgget(key, 0644)) == -1) { /* connect to the queue */
        perror("msgget");
        exit(1);
    }

    printf("spock: ready to receive messages, captain.\n");

    for(;;) { /* Spock never quits! */
        if (msgrcv(msqid, &buf, sizeof(buf.mtext), 0, 0) == -1) {
            perror("msgrcv");
            exit(1);
        }
        printf("spock: \"%s\"\n", buf.mtext);
    }

    return 0;
}

the above codes can be found at beej's guide for message queue.

When I try to execute "spock" msgget() is throwing an error : No such file or directory. Is there anything wrong with ftok(). I changed the permission of file to the one passed to msgget() funcition. But same error. Thanks in advance. Thanks in advance.

Answer 1

ftok requires that the file exists, as it uses the inode information to construct the key. If you're building them in separate directories, pointing to kirk.c using a relative path should work correctly e.g. spock/spock.c contains the spock code, kirk/kirk.c contains the kirk code, in spock/spock.c you should refer to ../kirk/kirk.c

Answer 2

Had also the same problem. If u want to execute message-receiver process u have to be sure that message-sender is also executing at that time. Otherwise msgqueue (if u ever run message-sender) would be destoroyed and id related with it will no longer be valid. Hence it's anything magicall about it. Pozdr.