Inter Process Communication 'URGENT' message types

Question

I've been going through Beej's guides on inter process communication .

How would 'kirk.c' be modified to be able to send messages marked URGENT? These URGENT messages would then be ignored by 'spock.c' and instead read by another program?

Answer 1

In the file kirk.c , we can introduce an enum variable which will define the type of message to be sent. The structure struct my_msgbuf has a member variable long mtype; . One can initialise this to the message type before sending it to the receiver, in this case - spock.c .

The enum declaration:

enum msgType {
    URGENT = 0,
    NORMAL = 1
};

The above enum declaration can be put in a common header file to both kirk.c and spock.c .

The line buf.mtype = 1; /* we don't really care in this case */ buf.mtype = 1; /* we don't really care in this case */ should be modified to buf.mtype = URGENT; /* we don't really care in this case */ buf.mtype = URGENT; /* we don't really care in this case */

In spock.c , following lines:

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

..will be modified to:

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

So, spock will "process" (in this case, it just prints the data as a way of processing it) the message only if the message type is anything other than URGENT .