title: an implement of sizeof guaranteeing bit alignment
body: <p>A <code>define</code> sentence is:  </p>
  
  <pre><code>#define _INTSIZEOF(n)   ( (sizeof(n) + sizeof(int) - 1) &amp; ~(sizeof(int) - 1) )
  </code></pre>
  
  <p>I have been told the purpose is bit alignment.<br>
  I wonder how it works, thx in advance.</p>