title: What does ({;}) and ({0;}) in a macro definition signify?
body: <p>I was going through the lustre source code and was stuck at the macro definition:  </p>
  
  <pre><code>#define ldlm_namespace_proc_unregister(ns)      ({;})  
  #define ldlm_namespace_proc_register(ns)        ({0;})
  </code></pre>
  
  <p>defined in the file <code>lustre/ldlm/ldlm_resource.c</code>.<br>
  What does this macro definition signify?  </p>