title: Why I get NULL is undefined error?
body: <p>I have this thiis row:</p>
  
  <pre><code>while (**arr != NULL)
  </code></pre>
  
  <p>In this row I get this error:</p>
  
  <pre><code>identifier "NULL" is undefined
  </code></pre>
  
  <p>I included this library:</p>
  
  <pre><code>#include&lt;stdlib.h&gt;
  </code></pre>
  
  <p>but still I get the error above.Any idea how to fix this error?</p>