Two processes substracting a number using pipe

Question

Having difficulty to make two processes comunicate through pipe and substract a number alternatively.

Output should be like: process1: 9 process2: 8 process1: 7...

What I've did so far:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main() {
    int p2c[2];
    int c2p[2];
    int n = 9;

    pipe(p2c);
    pipe(c2p);

    write(p2c[1], &n, sizeof(int));

    if(fork() == 0) {
        read(p2c[0], &n, sizeof(int));
        printf("Got from parent: %d", n);
    n--;
    write(c2p[1], &n, sizeof(int));
    close(p2c[0]);
    close(p2c[1]);
    close(c2p[0]);
    close(c2p[1]);
    exit(0);
}
else{
    read(c2p[0], &n, sizeof(int));
    printf("Got from child: %d", n);
    n--;
    write(p2c[1], &n; sizeof(int));
    close(p2c[0]);
    close(p2c[1]);
    close(c2p[0]);
    close(c2p[1]);

}
return 0;
}

Whith the output: Got from parent:9 Got from child:8 What's the proper way to get these two processes substract the number till 0?

Answer 1

It makes sense that you're only getting "Got from parent:9 Got from child:8" as a result, you need, you need a while or for loop for both child and parent processes to get what you're expecting, and the stop conditions for those loops are (n < 0) after decrementing n or the write end of pipe get closed:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main() {
    int p2c[2];
    int c2p[2];
    int n = 9;

    pipe(p2c);
    pipe(c2p);

    // this is important to prevent deadlock situation, at least one of both processes
    // must start a write operation before while loop, unless the read will block and 
    // and each process still waiting the other to write something on the pipe
    write(p2c[1], &n, sizeof(int));

    if(fork() == 0) {
        int readStatus;
        while(1){
            readStatus=read(p2c[0], &n, sizeof(int));
            // when read returns 0, this means the write end of pipe was closed, so we have to break the loop
            // because no more data to recieve
            if(readStatus == 0) break;
            printf("Got from parent: %d\n", n);
            n--;
            // we check if n less than 0, if yes we are finished
            if(n < 0) break;
            write(c2p[1], &n, sizeof(int));
        }
        close(p2c[0]);
        close(p2c[1]);
        close(c2p[0]);
        close(c2p[1]);
        exit(0);
    }
    else{
        int readStatus;
        while(1){
            readStatus= read(c2p[0], &n, sizeof(int));
            if(readStatus == 0) break;
            printf("Got from child: %d\n", n);
            n--;
            if(n < 0) break;
            write(p2c[1], &n, sizeof(int));   
        } 

        close(p2c[0]);
        close(p2c[1]);
        close(c2p[0]);
        close(c2p[1]);

    }
    return 0;
}