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.

It was my mistake in execution. They are working perfectly fine. Thank you Alex. — raka

Petesh Petesh · Accepted Answer · 2013-01-02T11:50:27

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

Error with msgget() and ftok ()

2 Answers