title: How to know if __uint128_t is defined
body: <p>We can use the preprocessor to know if <code>unsigned long long</code> is defined:</p>
  
  <pre><code>#include &lt;limits.h&gt;
  
  #ifndef ULLONG_MAX
  typedef unsigned long t_mask; 
  #else
  typedef unsigned long long t_mask;
  #endif
  </code></pre>
  
  <p>But how to know if <code>__uint128_t</code> is defined?</p>