dpipe.c

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

int main(int argc, char** argv) {

    // print the args
    for (int i = 0; i < argc; ++i) {
        printf("%s\n", argv[i]);
    }

    // keep track of the pids and statuses of the child processes
    pid_t first_pid = fork();
    pid_t second_pid;

    int first_status = 0;
    int second_status = 0;

    // keep track of the pipes
    int first_to_second[2];
    int second_to_first[2];

    if (pipe(first_to_second) == -1) {
        perror("pipe");
        exit(1);
    }

    if (pipe(second_to_first) == -1) {
        perror("pipe");
        exit(1);
    }

    //gather arguments for dpipe
    char* first_args[2];
    first_args[0] = argv[1];
    first_args[1] = NULL;

    char** second_args = argv + 2;

    for (int i = 0; i < argc - 2; i++) {
        printf("%s\n", second_args[i]);
    }

    // this is the first process
    if (first_pid == 0) {
        printf("in first child process\n");

        // reflect the file descriptors properly
        if (dup2(first_to_second[1], 1) == -1) {
            perror("dup2");
            exit(1);
        }
        close(first_to_second[0]);
        close(first_to_second[1]);

        if (dup2(second_to_first[0], 0) == -1) {
            perror("dup2");
            exit(1);
        }
        close(second_to_first[0]);
        close(second_to_first[1]);

        // exec with those arguments
        execvp(first_args[0], first_args);
    } 
    // this is the parent
    else {
        printf("in parent process, first child pid: %d\n", first_pid);
        second_pid = fork();

        // this is the second process
        if (second_pid == 0) {

            printf("in second child process\n");

            // reflect the file descriptors properly
            if (dup2(first_to_second[0], 0) == -1) {
                perror("dup2");
                exit(1);
            }
            close(first_to_second[0]);
            close(first_to_second[1]);

            if (dup2(second_to_first[1], 1) == -1) {
                perror("dup2");
                exit(1);
            }
            close(second_to_first[0]);
            close(second_to_first[1]);

            // exec with arguments
            execvp(second_args[0], second_args);
        } else {
            printf("in parent process, second child pid: %d\n", second_pid);
        }

        waitpid(second_pid, &second_status, 0);

    }

    waitpid(first_pid, &first_status, 0);

    return 0;
}