title: #include statements that include a slash (header files made up of two parts)
body: <p>Sometimes I see header files of the form.</p>
  
  <pre><code>#include &lt;sys/sysinfo.h&gt; // I found this on my system under /usr/include/sys/sysinfo.h. Is that all the "sys/" means?                                                             
  </code></pre>
  
  <p>What is this called and why are these header files different from most others like </p>
  
  <pre><code>#include &lt;stdio.h&gt;
  </code></pre>
  
  <p>Maybe a group of related header files been grouped under the label of 'sys', but if I try something like "man pci" (there's a pci.h header in /usr/include/sys/ there is no entry.</p>