How to find and use GTK source code?

[Felix T <felixtr gmail com>]

Hello,

My name is Felix, and I am using GTK+ for developing a GUI for a robot (*) in C language under Linux OS.
I have a question and would be more than grateful to receive your answer for it.

Several times I have met problems for solving which the supplied documentation was not comprehensive enough.
I have also tried to overcome those challenges using GTK+ forums, but those were not "alive" enough for supplying an answer to my questions on a reasonable time.
Those reasons, have led me to an idea to read the code of GTK+. I have found it in the next link (http://git.gnome.org/browse/gtk+/), but I couldn't figure out how to find, the needed information for me, in this link .

I would be more than grateful if you could explain me how to find a source code of an API function/class/structure, that interests me, in your repositories. 
An example for such a search: I would like to find a source code of GtkPixbuf, in order to see how I can refresh the data it contains, using memory directly or using API function.

I don´t know if my request is directed to the right address, If not - I would be grateful, if you could let me know to whom I should appeal.
Thanks in advance,
Felix.

(*) If you are interested in the application of your work, you can take a look on the robot I work with :)
http://mediawiki.isr.ist.utl.pt/wiki/RAPOSA_robot