Tracker is a debugger I'm thinking of writing. It will have two special features: value tracking and control tracking. The idea for Tracker was triggered by my first boss, Mike Gilbert, while he was ranting about language designers not giving enough thought to issues of debugging. This was right after I had spent five weeks working with a particularly brain-dead debugging environment (all I had to work with was printf). One of us -- I don't remember who -- came up with the idea of value tracking.

Value Tracking

You're in the debugger, looking at a null reference. As soon as you step, the program will try to use this reference, and then promptly crash. How did this reference get to be null? Did it get explicitly set to null or was it just never initialized? Value tracking lets you find out when and where a variable was modified.

Value tracking is similar to the break on modify capability of many debuggers, but it has several advantages:

Control Flow Tracking

You're in the debugger, looking at a function that's only supposed to be called on the Mac. But you're debugging on a PC. How did you get here? What conditional branch went wrong? Control flow tracking lets you find out exactly why you got where you are, and where you were before.

Control flow tracking is similar to viewing a stack trace, but it has these advantages:


Tracker will be written in Java for Java. Because I don't want to write an entire VM, it will be implemented as a special ClassLoader. This means that it can only track the classes that it loads. However it will have mechanisms for black-box tracking other classes.

I would like to see the entire operating system become trackable. Can't ping a machine? Find out where the problem is by examining the track of the error message.

Tracker is still very immature. I still need to figure out how to represent an overwhelming amount of information in a manageable way. And I need to work out an implementation technique that won't make programs ten times slower, or consume gigabytes of memory.

Last updated sometime in 1997
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