WordPress: Accessible only by author of the post

<?php
/*
 ********************************************************************************
 *                                      *
 * WordPress: Accessible only by author of the post             *
 * Author: Lucas S. B. Mastella <lucas[at]bello[dot]li>             *
 * Site: www.bello.li                               *
 * Date: 15/10/2011                             *
 *                                      *
 ********************************************************************************
 *                                      *
 * When the user accesses the post, it checks if is logged.         *
 * Check if this is the author of the post, otherwise returns notice.       *
 *                                      *
 ********************************************************************************
 *                                      *
 * This program is free software: you can redistribute it and/or modify     *
 * it under the terms of the GNU General Public License as published by     *
 * the Free Software Foundation, either version 3 of the License, or        *
 * (at your option) any later version.                      *
 *                                      *
 * This program is distributed in the hope that it will be useful.      *
 * but WITHOUT ANY WARRANTY; without even the implied warranty of       *
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        *
 * GNU General Public License for more details.                 *
 *                                      *
 * You should have received a copy of the GNU General Public License        *
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.    *
 *                                      *
 ********************************************************************************
 *
 */
?>
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php if ( !is_user_logged_in() ) { ?>
      <!-- # Link -> Login -> Return to same page -->
      <p>If you are the author of this page: <a href="<?php echo wp_login_url( get_permalink() ); ?>" title="Login">Login</a></p>
<?php
   }
   elseif ( $user_ID == get_post($post_id)->post_author ) {
    echo "Is author of this post";
      // Code to show the post
      // See an example: http://codex.wordpress.org/The_Loop#Loop_Examples
   }
   else echo 'You are not the author of this post';
?>
<?php endwhile; else: ?>
<p>Sorry, no posts matched your criteria.</p>
<?php endif; ?>