Trying to draw a sequence diagram of portion of huge source code

Goal

• "drawing a sequence diagram about a small portion of huge code".

Background info

• I got huge source code which is bigger than 2GB.
• The code is written in C/C++
• I have reviewed/understood less than 1% of code.
• I am using eclipse / vim on Ubuntu 12.10

What I would like to know

1. is there any automatic sequence diagram generator which could be used on above case?
2. If I have to draw it manually, is there an easy way to figure out messages between life-lines?
   • I tried to put logs here and there but the code was too big. -> fail
   • I tried to follow code jump on eclipse (function A calls function B, B calls function C and so on) -> also fail. too much code.

Doxygen can generate call graphs, which aren't the same as sequence diagrams but might actually be more helpful at this scale. You'll probably have to customize the configuration to get the right things to show up without Doxygen choking, but at least it's a tool that is designed to do this on C/C++ and has been used on production size code.