Thread: Implementing 3 stage pipe in C

  1. #1
    Registered User
    Join Date
    Feb 2012

    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)...

    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    int main(void)
        int pfds[2];
        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
    Last edited by Salem; 02-22-2012 at 11:31 AM. Reason: removed font crazyness from the code

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    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.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    Registered User
    Join Date
    Feb 2012
    sorry i'm pretty now to C coding I don't really know how to implement what your talking about?

  4. #4
    Registered User
    Join Date
    Oct 2008
    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!
    Last edited by itCbitC; 02-22-2012 at 01:56 PM.

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