title: c - struct - what does the additional ": 1" doing?
body: <p>What is the " : 1" part doing in following code?</p>
  
  <p><strong>code:</strong></p>
  
  <pre><code>struct trace_key {
          const char * const key;
          int fd; 
          unsigned int initialized : 1;
          unsigned int  need_close : 1;
  };
  </code></pre>