title: The role of #ifdef and #ifndef
body: <pre class="lang-c prettyprint-override"><code>#define one 0
  #ifdef one
  printf(&quot;one is defined &quot;);
  #ifndef one
  printf(&quot;one is not defined &quot;);
  </code></pre>
  <p>In this what is the role of <code>#ifdef</code> and <code>#ifndef</code>, and what's the output?</p>