getting post ID from a plugin (again…)

I’m writing a plug-in and need to get the post ID. I stripped the plug-in code down to this and it’s not working.

<?php
/*
Plugin Name: Post-Comment Limit
*/

function lpu_show_id() {
  global $post;
  $postid = $post->ID;
  return $postid;
}

$id = lpu_show_id();
echo "Post: ".$id;

?>

I tried $wp_query, not working.

function lpu_show_id() {
 global $wp_query;
 $thePostID = $wp_query->post->ID;
 return $thePostID;
}

I presume I’m missing something easy. Help, please ??