title: Why does math.h define pi, pi/2, 2/pi but not 2*pi?
body: <p>This is a really simple question: Why are there <a href="http://www.gnu.org/software/libc/manual/html_node/Mathematical-Constants.html">predefined constants</a> for pi, pi/2, pi/4, 1/pi and 2/pi but not for 2*pi? Is there a deeper reason behind it?</p>
  
  <p>This question is not about the whole <a href="http://tauday.com">pi vs tau debate</a>. I am wondering if there is a technical reason for implementing certain constants but not others. I can think of two possibilities:</p>
  
  <ol>
  <li>Avoiding rounding errors.</li>
  <li>Avoiding runtime divisions which might be more expensive.</li>
  </ol>