title: Where is DEBUG defined in the code?
body: <p>I looked at a sample code to create a log system on my server... And I found this</p>
  
  <pre><code>#if DEBUG
  printf("something here");
  #endif
  </code></pre>
  
  <p>I know what it does. It prtins something only if DEBUG has been defiend. But where is DEBUG defined? I looked at the all the header files but I could't find DEBUG.. </p>
  
  <p>Also, Could you please provide me a good example or tutorial about designing logging system? </p>
  
  <p>Thanks in advance..</p>