title: How to print something without using std lib functions?
body: <p>In the C language, when printing something on the screen, we usually use <code>printf</code>, <code>puts</code> and so on. Which are all defined in the  or other header documents. </p>
  
  <p>Is there any way to print something on screen without using such functions? That is to say, how is <code>printf</code> realised?</p>