Question

I have an application. I have the source code (in C). I can compile it anyway I want. Add whatever tool I want to it. Etc. However, I do not want to pepper the source code with a bunch of printf’s. I want to be able to produce a log of some sort that shows when a particular value (some member of a global structure for example) is written to (its value changes). I’d like to be able to show source file and line number, and the old and new value.

I’d also like to be able to specify the value by name, not address. But address is OK. Bonus points if I can specify a value that is local to a function.

I’m still scratching my head trying to figure out gdb’s trace commands. Any help is greatly appreciated. Thanks.

Answer 1

Thank you both @derobert and @peter! I finally went back to this, and this:

break main
commands
        watch somevar
        commands
                cont
        end
        cont
end
run

does the trick. This works when “somevar” is global, or local to “main”. If “somevar” is local to anther function, just replace “main” with that function name above.

Put these commands in a file (e.g. “gdbscript”) and run gdb like:

gdb -x gdbscript a.out