'Twas brillig, and the slithy toves did gyre and gimble in the wabe

To be entirely fair, that the program uses them doesn't necessarily mean the variables were defined :D GCC and GDB have been known to disagree on this sort of thing from time to time.

In general, though, gdb is ... not the friendliest program in the world. The few times I've investigated it, its output has been so terse and unreadable that I've ended up going back to the tried and tested "print statements and see where they go wrong" method eventually. I hear there are a few third-party front-ends that make it more usable, though.

Since I'm about to get back into programming AVRs under Linux, I should probably get some more time in with gdb. Gonna be useful...