Re: info frame

Mark Kettenis
2006-04-17T08:58:57Z

> Date: Mon, 17 Apr 2006 10:05:50 +0300
> From: Eli Zaretskii <eliz@gnu.org>
>
> > Date: Mon, 17 Apr 2006 01:33:21 +0200 (CEST)
> > From: Mark Kettenis <mark.kettenis@xs4all.nl>
> > CC: gdb@sources.redhat.com
> >
> > > (gdb) info frame
> > > Stack level 0, frame at 0xbffff710:
> > > eip = 0x80484a9 in main (myprog.c:47); saved eip 0x4006015a
> > > source language c.
> > > Arglist at 0xbffff708, args: argc=1, argv=0xbffff774
> > > Locals at 0xbffff708, Previous frame's sp is 0xbffff710
> > > Saved registers:
> > > ebp at 0xbffff708, eip at 0xbffff70c
> > > (gdb) p $fp
> > > $1 = (void *) 0xbffff708
> > >
> > > Can this be right?
> >
> > Yes, current GDB uses the convention that the frame address is the
> > Canonical Frame Address (CFA) as used by the DWARF 2 Call Frame Info
> > (CFI).
>
> Does this mean that, if the debug info is stabs or something other
> than DWARF 2, a different frame address will be displayed for the same
> code?

No. All the different unwinders use the same convention. This is
exactly why the convention changed on i386 when the DWARF 2 unwinder
was introduced: to make the fallback unwinder match the DWARF 2
unwinder.

> > In general the CFA is the value of the stack pointer when the
> > current function was called.
>
> ``In general''? Does this mean that in some cases it will be
> something else? If so, when that will happen, and what will we show
> then?

DWARF 2 doesn't specify exactly what the convention is. So most ISA's
use the convention above, but other ISA's might use a different
convention.

Also different compilers for the same ISA might use different
conventions when they generate DWARF 2 debug information. In that
case you're screwed because the CFA returned from the DWARF 2 unwinder
will no longer match the convention used by GDB's other unwinders. So
far we haven't seen any other compilers that use a different
convention, so things are safe for now.

Mark