/*
 * This code is identical to running
 * w | wc -w
 * in bash
 */
#include <stdio.h>
#include <unistd.h>

void child(int pfd[2]) {
    close(pfd[1]);
    dup2(pfd[0], STDIN_FILENO);
    close(pfd[0]);

    char cmd[] = "/usr/bin/wc";
    char *args[] = {"wc", "-w", NULL};
    if (execve(cmd, args, NULL) == -1) {
        perror("wc error!");
    }
}

void parent(int pfd[2]) {
    close(pfd[0]);
    dup2(pfd[1], STDOUT_FILENO);
    close(pfd[1]);

    char cmd[] = "/usr/bin/w";
    char *args[] = {"w", NULL};
    if (execve(cmd, args, NULL) == -1) {
        perror("w error!");
    }
}

int main (void) {
    int pfd[2];
    if (pipe(pfd) < 0) {
        perror("pipe error!");
        _exit(1);
    }

    int pid = fork();
    if (pid < 0) {
        perror("fork error!");
        _exit(1);
    }

    if (pid == 0) {  // child
        child(pfd);
    } else {
        parent(pfd);
    }
}