How to read the source code

  Some workmates talked about how to read the source this afternoon. I found that I don't know how to read the source exactly.
  In my opninion, read source is to know everything of the source. Such as: how a fuction works, why a variable plus one, how to implement an algorithm, and so on. So when I read a source, I chose a part to begin, and from that part, I went into the code as deeply as I could. It took me a lot of time, and also very difficult, but I got a little. Even several days later, I forgot everything totaly.
  I found the following words about how to read source code:
  1. It's boring and hard, so must carry on.
  2. If it's really too difficult, read some related books or documents first. It helps you to know the source code better.
  3. Write down your thinking after reading the source. It's very easy to forget what you get from the source.
  4. Know the priority of every parts. first need, first study.
  5. Know the importance of every parts. some parts need to know the details, while other only need to know the functions.