Main's usually pretty useless for nontrivial apps. Usually some sort of internal/library framework structures the system. These usually work well together: 1

Main's usually pretty useless for nontrivial apps. Usually some sort of internal/library framework structures the system. These usually work well together: 1. Some way to cross-reference source files. e.g. ctags. 2. Source-code level documentation. Doxygen, javadoc, whatever you have. 3. Stack traces of the running app. Set breakpoints and get stack traces. If your tools will let you do this, get periodic stack traces (e.g. once every 10th of a second) for a while. Look at what the code's doing. I just went through two systems totaling 1.5 million lines of code. First hint: don't even think of going through all the code. Instead, figure out what's important to you, and start putting together a demand-paged map of what you need to know. Second hint: get a paper notebook and start writing things down soon. You'll save yourself a lot of re-researching facts that way.