Tools to understand larger amount of C code

To understand the structure of the GIMP project I try to use as much of modern tools as possible. I believe that accumulated experience in the field of analysis worth something. So I bet on them. Of course, reading the source code is still an option, but the last resort, so to say.
Here I put the links on the tools I considered so far.

• Wikipedia article to the software that creates the call graph based on the source C code 
• NCC compiler (never heard of it before, but the description sounds promising)
• kcachegrind (the one I use currently)

---

 Now, the current topic is GLib main loop. Links:

• Official documentation of the main event loop 
• GLib: GMainContext

Also helpful:

• "Delivering Signals for Fun and Profit" - Michal Zalewski 
• man 2 poll
• man 2 select