Re: info frame

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> 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