title: Checking typedef defined function pointer for null
body: <p>Following on from <a href="https://stackoverflow.com/a/1591492/348501">this excellent SO answer on function pointers</a>; given a function pointer defined in C like:</p>
  
  <pre><code>typedef void (*Callback)(int argument);
  Callback my_callback = 0;
  </code></pre>
  
  <p>how do I check whether the callback function pointer has been assigned a non-null value?</p>
  
  <p><code>my_callback == 0</code> or <code>&amp;my_callback == 0</code>?</p>