Thread: Implementing 3 stage pipe in C

Hi, I'm trying to implement the 3 stage pipe 'who | cut -d" " -f 1 | sort -u' as a C program (basically i want returned a list of users logged into the system but i want to use pipes.

I've worked out how to implement a two stage pipe (i think but it may be wrong)...

Code:

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


int main(void)
{
    int pfds[2];


    pipe(pfds);


    if (!fork()) {
        close(1);       /* close normal stdout */
        dup(pfds[1]);   /* make stdout same as pfds[1] */
        close(pfds[0]); /* we don't need this */
        execlp("who, "who", NULL);
    } else {
        close(0);       /* close normal stdin */
        dup(pfds[0]);   /* make stdin same as pfds[0] */
        close(pfds[1]); /* we don't need this */
        execlp(cut", "cut", "-d", "-f 1");
    }


    return 0;
}

Can someone tell me how I'd convert that to a three stage? And if what i've done is right? thanks

Start with a function which tells you how many | are in the command line.

Enhance that with a function which returns an argv[] for each | separated command.

As pointed out by Salem, a multistage pipeline is parsed first and executed later.
Hint: *nix shells first parse the pipeline from left to right, then execute it from right to left.

And think recursion!