Unable to download file using codeigniter

This is my down loader controller. First time it works properly. it opens save as popup and able to download required file, But next time it shows direct directory listing.

<?php

class Download extends Controller {

 function Download(){
  parent::Controller();
  $this->load->helper('download');
  echo "I am in constructor";

 }

 function index(){
  $file = realpath("download")."\\profile.doc"; 
  echo "I am in index.";

        exit;

  if (file_exists($file)) {
      header('Content-Description: File Transfer');
            header('Content-Type: application/octet-stream');
            header('Content-Disposition: attachment; filename=' . basename($file));
            header('Content-Transfer-开发者_如何学运维Encoding: binary');
            header('Expires: 0');
            header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
            header('Pragma: public');
            header('Content-Length: ' . filesize($file));
            ob_clean();
            flush();
            readfile($file);
            exit;
        }else{

   // File Not Found

         echo "File not found";
        }

 }
}
?>

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You should use the download helper as seen in the user_guide. It deals with this type of situation.