#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <unistd.h>

int main(int argc, char **argv)     // ls | wc -l
{
  int fds[2];
  pid_t pid;

  if (pipe(fds) == -1) { perror("pipe failed"); exit(1); }
  if ((pid = fork()) < 0) { perror("fork failed"); exit(2); }
  if (pid == 0) {       // child
    close(fds[1]);      // close output
    dup2(fds[0], 0);
    execl("/usr/bin/wc", "wc", "-l", 0);
    perror("failed to run wc");
  } else {            // parent
    close(fds[0]);    // close input
    dup2(fds[1], 1);
    execl("/bin/ls", "ls", 0);
    perror("failed to run ls");
  }
  exit(0);
}