title: What does rd stand for in rdstate and rdbuf?
body: <p>There are two names in C++ standard I/O library: <code>rdstate</code> and <code>rdbuf</code>. I know "state" and "buf", but what is "rd"?</p>
  
  <p>(PS: I believe I know how to use <code>rdstate</code> and <code>rdbuf</code>, don't teach me that).</p>