Message Queue Callback - c

Question

I'm trying to learn and understand what is message queue. I got the code here (I copied them from the internet and change them a little bit to relevant to my example). They are send.c which will allow you to enter some simple operations in text and send it to the message queue. The file receive.c will receive those operations, calculate it and print result to the screen.
What I want to do next (but I don't know how) is to make receive.c calculate operations and then it will send each result to each message from send.c. So please help me out, I'm kinda stuck :(
send.c:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
struct my_msgbuf {
long mtype;
char mtext[200];
};
int main() {
struct my_msgbuf buf;
int msqid;
key_t key;
if ((key = ftok("send.c", 'B')) == -1) {
perror("ftok");
exit(1);
}
if ((msqid = msgget(key, 0777 | IPC_CREAT)) == -1) {
perror("msgget");
exit(1);
}
printf("Enter lines of message, ^D to quit:\n");
buf.mtype = 1;
while(fgets(buf.mtext, sizeof buf.mtext, stdin) != NULL) {
int len = strlen(buf.mtext);
if (buf.mtext[len-1] == '\n') {
buf.mtext[len-1] = '\0';
}
if (msgsnd(msqid, &buf, len+1, 0) == -1) {
perror("msgsnd");
}
}
if (msgctl(msqid, IPC_RMID, NULL) == -1) {
perror("msgctl");
exit(1);
}
return 0;
}
receive.c:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>
struct my_msgbuf {
long mtype;
char mtext[200];
};
int calculate(char mtext[200]) {
int result = 0;
char number_1[20];
char number_2[20];
char operator;
int pos = 0;
for (int i = 0; i < strlen(mtext); i++) {
if (mtext[i] == '+' || mtext[i] == '-' || mtext[i] == '*' || mtext[i] == '/') {
operator = mtext[i];
pos = i + 2;
break;
}
number_1[i] = mtext[i];
}
number_1[pos-3] = '\0';
for (int j = pos; j <= strlen(mtext); j++) {
number_2[j - pos] = mtext[j];
}
switch(operator) {
case '+':
result = atoi(number_1) + atoi(number_2);
break;
case '-':
result = atoi(number_1) - atoi(number_2);
break;
case '*':
result = atoi(number_1) * atoi(number_2);
break;
case '/':
result = atoi(number_1) / atoi(number_2);
break;
}
return result;
}
int main() {
struct my_msgbuf buf;
int msqid;
key_t key;
if ((key = ftok("send.c", 'B')) == -1) {
perror("ftok");
exit(1);
}
if ((msqid = msgget(key, 0777 | IPC_CREAT)) == -1) {
perror("msgget");
exit(1);
}
printf("Ready to receive messages...\n");
for(;;) {
if (msgrcv(msqid, &buf, sizeof buf.mtext, 0, 0) == -1) {
perror("msgrcv");
exit(1);
}
int result = calculate(buf.mtext);
printf("%s = %d\n", buf.mtext, result);
}
return 0;
}
When you run those file they will look like this:

Answer 1

As I understand, you need:
a request queue to let sender sends computation requests to receiver
an channel for each sender to let receiver sends its results to the requester.
For this, the sender has to create an appropriate channel (whatever you like, even a specific message queue if you want), and send within its request an id for the channel to answer on.
In real life that could correspond to a scenario like: you call a service at number N and give your request + "call me back at number M when finished please".