Command pipe example in C - execute "ls / | wc -l"

command-pipe-example.c

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

int main(void) {
    /* set up pipe. */
    int pfd[2];
    if (pipe(pfd) < 0) {
        perror("pipe()");
        exit(1);
    }

    /* fork a child process and let it execute the grep command. */
    pid_t child_pid;
    if ((child_pid = fork()) == 0) {
        /* CHILD PROCESS */

        /* redirect child stdout to pipe */
        if (dup2(pfd[1], STDOUT_FILENO) == -1) {
            perror("dup2() in child");
            exit(1);
        }

        /* remove the file descriptors for the pipe from the child’s */
        /* file descriptor table. */
        if (close(pfd[0]) < 0) {
            perror("close() read end of pipe in child");
            exit(1);
        }
    
        if (close(pfd[1]) < 0) {
            perror("close() write end of pipe in child");
            exit(1);
        }
        
        /* exec the first command of the pipe */
        execlp("ls", "ls", "/", NULL);
    
        /* if the following statement is reached, execlp must have failed. */
        perror("execlp() in child");
        exit(1);
    }
    else {
        /* PARENT PROCESS */
 
        /* make sure that fork worked. */
        if ((int)child_pid < 0) {
            perror("fork() failed");
            exit(1);
        }

        /* redirect parent stdin to pipe and exec the wc command. */
        if (dup2(pfd[0], STDIN_FILENO) == -1) {
            perror("dup2() in parent");
            exit(1);
        }

        /* remove the file descriptors for the pipe from the parent’s */
        /* file descriptor table. */
        if (close(pfd[0]) < 0) {
            perror("close() read end of pipe in parent");
            exit(1);
        }
        
        if (close(pfd[1]) < 0) {
            perror("close() write end of pipe in parent");
            exit(1);
        }

        /* exec the wc command. */
        execlp("wc", "wc", "-l", NULL);
        
        /* if the following statement is reached, execlp must have failed. */
        perror("execlp in parent");
        exit(1);
    }

    return 0;
}