GDB and GCC

I’ve been working on Project Archer for some months now, and it has been pretty interesting. It has also been challenging. There are several deep dark wells of technical knowledge that I’ve had to explore in detail: unwinding, dwarf, debuginfo, and exceptions (generation, handling and personality routines). So I’ve been reading about, and stepping through a lot of these areas in GDB this last week. When does a program grow so big that one mortal human cannot work on its entirety? I don’t know the metric, but I bet GDB surpasses it.

As I’ve worked on improved C++ exception handling in GDB, it occurred to me that the different bugs I’ve filed could ultimately be put in one: “Make GDB work better with the GCC unwinder.” As GCC has changed in some areas, GDB has not changed in tandem with GCC. The next or finish commands relying purely on longjmp breakpoints is an example. (If you “next” over a C++ “throw” statement in GDB you will lose control of the inferior. GDB sets a “longjmp” breakpoint via the “next” command code to re-establish control – but the unwinder for C++ does not use setjmp/longjmp semantics to switch context. Once resumed, the inferior won’t stop at all, or where expected)

So this is a problem. It really irritates me when I lose control of an inferior when debugging. The pain is in proportion to the length of the debugging session. Sometimes I spend hours stepping a process. I’ve cursed a good line on several occasions where this has happened

It’s easy to see this negatively, and even easier to write a negative thing about it. But it is a fact of life. So what’s the problem? Well in most areas the longjmp trick will work. It won’t for C++ exceptions. But this grey area really bothers me. What if there are other areas where expectations do not match? Both GCC and GDB are highly complex programs. They change all the time, and where there is no direct transactional specification (ie debuginfo is written to a specification, so are elf binaries, and so on) the assumptions about how GCC generates code will eventually break. If they break in a big way, they will be fixed – and quickly. But if they break in minor little ways, then the user experience dies as a result of a thousand tiny paper cuts. Or a thousand tiny curses.

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