🚀 go-pugleaf

Hi, I would  like to make some changes to
g++. Here is what I would like to do:
When I use my g++, I would like to build
my own parse tree (so it will not intefere with
the g++ tree).  but I only want the tree to
be built for the program "in" that file.
Here are my two questions:
1. How can I tell if g++ is processing the
"real" program, not ther header files??? Is there a variable
that record the real input file and the file actually
being processed???
2. What is the way to add a flag to g++??? what is the
file/s that I should look into???

Thank you very much for your help!
Any help will be greatly appreciated!