title: Is it good practice to define an extern variable for future 'unset' variables?
body: <p>I am looking at some code sample. In a .h file, there is this declaration:</p>
  
  <blockquote>
    <p>extern int NODATA;</p>
  </blockquote>
  
  <p>which is used in many other files for variables that do not have a logical value.</p>
  
  <p>Is it necessary to define it somewhere else or is it OK to leave it undefined?</p>