title: c/c++ what is the return value of strncmp when size is 0
body: <p>The c/c++ strncmp signature is like the following:</p>
  
  <pre><code>int strncmp ( const char * str1, const char * str2, size_t num );
  </code></pre>
  
  <p>My question is what's the return value if num is 0?  How the standard says? Don't find an answer from some online documents.</p>
  
  <p>Thanks.</p>