Thread: Pipe multiple programs using pipe() and c

Hello,
I am trying to pipe multiple programs together using C. I piped 2 programs fine:
here is the code for that:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void) {

    int isParent;
    int apipe[2];
    char *cmd[2][3];
    
    cmd[0][0] = "ps";   cmd[0][1] = "-axu";     cmd[0][2] = NULL;
    cmd[1][0] = "grep"; cmd[1][1] = "root"; cmd[1][2] = NULL;
    
    pipe (apipe);
    isParent = fork();
    
    if (!isParent) {
        close (apipe[1]);
        dup2 (apipe[0], 0);
        close (apipe[0]);
        execvp (cmd[1][0],cmd[1]);
        perror ("Child exec failed!\n");
        exit (1);
    } else {
        close (apipe[0]);
        dup2 (apipe[1], 1);
        close (apipe[1]);
        
        execvp (cmd[0][0], cmd[0]);
        perror ("Parent exec failed!\n");
        exit (1);
    }
    
    return 0;
}

The code above is equivalent to "ps -axu | grep root" command in terminal.

However when I want to pipe three programs, my solution does not work, here is the code for that:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void) {

    int isParent1;
    int isParent2;
    int apipe[2];
    char *cmd[3][3];
    
    cmd[0][0] = "ps";   cmd[0][1] = "-axu";     cmd[0][2] = NULL;
    cmd[1][0] = "grep"; cmd[1][1] = "<your username>"; cmd[1][2] = NULL;
    cmd[2][0] = "head"; cmd[2][1] = "-6";       cmd[2][2] = NULL;
    
    pipe (apipe);
    isParent1 = fork();
    
    if (!isParent1) {
        // HEAD
        close (apipe[1]);
        dup2 (apipe[0], 0);
        close (apipe[0]);
        
        execvp (cmd[2][0],cmd[2]);
        perror ("Child exec failed!\n");
        exit (1);
    } else {
        isParent2 = fork ();
        
        if (!isParent2) {
            // GREP
            dup2 (apipe[0], 0);
            dup2 (apipe[1], 1);
            close (apipe[0]);
            close (apipe[1]);

            
            execvp (cmd[1][0],cmd[1]);
            perror ("Child exec failed!\n");
            exit (1);
        } else {
            // PS
            close (apipe[0]);
            dup2 (apipe[1], 1);
            close (apipe[1]);
        
            execvp (cmd[0][0], cmd[0]);
            perror ("Parent exec failed!\n");
            exit (1);
        }
    }
    
    return 0;
}

The code above is equivalent to "ps -axu | grep <your username> | head -6" command in terminal. It does not work as expected.

I think I am doing everything right. I substitute stdout of ps with the pipe, then stdin and stdout of grep with pipe, and then stdin of head with pipe.

Any help is appreciated!! Thanks

YAY !!! Figured it out!