how to get backtrace on warnings?

[Dave Morse <dm-gtk bomberlan net>]

I'm trying to figure out which line of code, of roughly 97 million, is
responsible for things like this:

  Gdk-ERROR **: BadValue (integer parameter out of range for operation)
    serial 5170 error_code 2 request_code 130 minor_code 9

I'd like to get a backtrace or something.  I tried running the app with
"--sync --g-fatal-warnings", then getting gdb to break on the call of the
function "exit", but gdb refuses to break on exit.  I tried breaking
gtk_exit, but its not called.  Ideas?