I am recently working on some long and obscure codes written by some other smart guys. I\'m thinking of generating the Data Flow Diagrams and the Flowcharts to facilitate m
"Code Visual to Flowchart" from FateSoftware may also obe of help.
Dia is a program that resembles Microsoft Visio. It isn't as smooth, but it gets the job done. I always use this professionally when Visio isn't available. This is mainly for UML and Flowcharts, exports to png, and vectors, and I think PDF's as well.
Dataflow diagrams are harder because they require most of a compiler front end to derive the basic information needed to draw them. But you can get call and caller graphs as well as some kinds of entity relationship diagrams out of Doxygen with Graphviz.
Also, Graphviz is generally useful for drawing all kinds of graphs with readable layouts from raw data. It is easy to walk a tree structure and output a description in the DOT language for Graphviz to draw, for example.
Set Doxygen up with the right options, and turn it loose on the legacy code and you will get the beginnings of an internals document to help find your way around too.