Re: Is c++ debugging really "this" bad?

[Daniel Jacobowitz <drow at mvista dot com>]

On Mon, Jul 29, 2002 at 10:57:16PM -0400, Peter Thompson/Amy Kirschner wrote:
> I'm doing some contract work helping a small company with their tool
> issues and have run across a nasty c++/gdb interaction.  I've been
> checking the archives and I find some related bugs in the c++ area, but
> not the specific issue I'm running into.
> 
> The basic problem.  Printing the value of 'this', the c++ object
> pointer, does not agree with a simple printf of 'this' in the same
> routine.  The particular routine I'm looking at is a destructor routine,
> and I have seen reasonable values of 'this' in other routines, but the
> wrong behavior seems to occur more often.  Or maybe just gets reported
> to me more often...
> 
> Some details:  We're using the 3.0.1 version of gcc and gdb with a mips
> 32 instruction set.  We do have a version of 3.0.4 available, but have
> not done extensive testing with that.  Gdb first looks for a local
> variable, $this, but doesn't find it in the current scope, or any
> containing scope.  Then it attempts to find 'this' in the local scope,
> and, failing that, looks to the outer scope where it does eventually
> find a 'this' variable, which is of course totally unrelated.  Our
> standard compilation options are (among a variety of others) -g and -O1.
> I've tried -g -O0, -g by itself (pretty much the same, eh?), -g3 -O1,
> and while -O0 produces a local copy of 'this', it still doesn't find the
> right value.  The gcc compiler I'm using produces a .mdebug section,
> which contains ecoff data, and does not recognize a -gdwarf or -gdwarf-2
> option.
> 
> I keep getting the feeling that maybe I'm just missing something basic,
> or the debugger is.  Certainly the compiler knows where the real value
> is, and I can track it via the machine code for any line that
> manipulates 'this', but I don't expect all the users to be able to do
> that readily.  Does this ring any bells with anyone?

I've never specifically seen the debugger lose track of 'this' in
destructors, but I haven't tried in a little while.

With GCC 3.0.1 (and mdebug to boot) you can't really expect C++
debugging to work.  The required changes for stabs debugging of C++
code are only in later releases; I think 3.0.4 was fixed but I'm not
sure.  Use 3.1.1 if possible.  That will require a matching binutils
upgrade.

You didn't mention what version of GDB you were using, also.

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer