Gcc plugin for modification of source code ?

[Mohsin Khan]

Hi,
I'm working on a gcc plugin project. I had a doubt, can somebody help
me on this.
Can we write a plugin (which works after a pass like cfg or any other)
which can modify the source code of .C file, for example insert
comments before function definitions or insert a new IF Block in the
.C file. And after modification it writes the entire modified code to
a new .C file. Is this type of plugin possible in GCC. If yes, please
tell me how and if No, please tell me why.
Any help is much appreciated.
Thanks