title: What is "-1L" in C?
body: <p>What do "-1L", "1L" etc. mean in C ?</p>
  
  <p>For example, in <a href="http://cplusplus.com/reference/clibrary/cstdio/ftell/" rel="noreferrer">ftell</a> reference, it says </p>
  
  <blockquote>
    <p>... If an error occurs, -1L is returned ...</p>
  </blockquote>
  
  <p>What does this mean ? What is the type of "1L" ? </p>
  
  <p>Why not return NULL, if error occurs ?</p>