17c.c

/*
============================================================================
Name : 17c
Author : Piyush Singh
Description : Write a program to execute ls -l | wc.
a. use dup
b. use dup2
c. use fcntl
Date: 19th Sep, 2025.
============================================================================
*/

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

int main() 
{
    int pipefd[2];
    pipe(pipefd);

    pid_t pid = fork();

    if (pid < 0) 
    {
        perror("❌ fork failed");
        exit(1);
    }

    // Child: run wc
    else if (pid == 0) 
    {
        close(pipefd[1]);              // Close write end

        close(0);                      // Close stdin
        if (fcntl(pipefd[0], F_DUPFD, 0) == -1) 
        {
            perror("❌ fcntl failed in child");
            exit(1);
        }

        execlp("wc", "wc", NULL);
        perror("❌ execlp failed in child");
        exit(1);
    }

    // Parent: run ls -l
    else 
    {
        close(pipefd[0]);              // Close read end

        close(1);                      // Close stdout
        if (fcntl(pipefd[1], F_DUPFD, 1) == -1) 
        {
            perror("❌ fcntl failed in parent");
            exit(1);
        }

        execlp("ls", "ls", "-l", NULL);
        perror("❌ execlp failed in parent");
        exit(1);
    }
}

/*
Output:
╰─ ./17c                                                                      ─╯
       7      56     358
*/