/*
    pipe.c
    Jeff Ondich, 15 April 2004
    Updated 17 February 2022
    Updated 25 February 2026
 
    This program creates two child processes and hooks them up
    via a pipe, like so:

        child1 | child2

    Then child1 execs "ls" and the child execs "wc -l", so
    we end up running this pipeline:

        ls | wc -l

    Once both child processes have terminated, the parent prints
    a message and exits.

    Here's another way to look at it.

        Parent
            make a pipe
            fork child1
            fork child2
            clean up
            wait

        Child1
            redirect stdout to pipe
            clean up
            exec ls
        
        Child2
            redirect stdin to pipe
            clean up
            exec wc -l
*/

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

#include <fcntl.h>
#include <ctype.h>

int main() {
    int fd[2];

    if (pipe(fd) < 0) {
        perror("Trouble creating pipe");
        exit(1);
    }

    printf("Pipe file descriptors: fd[0]=%d, fd[1]=%d\n", fd[0], fd[1]);

    // create child1
    int pid1 = fork();
    if (pid1 < 0) {
        perror("Trouble forking");
        exit(1);
    }

    if (pid1 == 0) {
        // child1
        printf("Child1 (%d) created\n", getpid());
        close(fd[0]);
        if (dup2(fd[1], STDOUT_FILENO) == -1) {
            perror("Trouble redirecting stdout");
        }
        close(fd[1]);

        execlp("ls", "ls", NULL);
        perror("execlp in child1 failed");

    } else {
        // parent: create child2
        int pid2 = fork();
        if (pid2 == 0) {
            // child2   
            printf("Child2 (%d) created\n", getpid());
            close(fd[1]);
            if (dup2(fd[0], STDIN_FILENO) == -1) {
                perror("Trouble redirecting stdin");
            }
            close(fd[0]);

            execlp("wc", "wc", "-l", NULL);
            perror("execlp in child2 failed");
        } else {
            // parent
            close(fd[0]);
            close(fd[1]);
            int status;
            int pid = wait(&status);
            printf("Child (%d) terminated with status %d\n", pid, status);
            pid = wait(&status);
            printf("Child (%d) terminated with status %d\n", pid, status);
        }
    }

    return 0;
}