Is there an online tool for understanding common code bases on the Internet?

I am changing the code for glibc 2.5. Now that glibc is large and complex, I need to have a really good tool to see the interaction of different parts of the code. For this purpose I use Understand , but Understood is valid only for 15 days. Subsequently, you must buy it.

So my question is: there are sites on the Internet where you can interactively understand common code bases, such as glibc, gcc, linux kernel, etc. I mean, where could you look for some function, and then click on the function call, see its definition and other useful functions. I used Koders.com , but it will only display code and is not interactive.