[Bug gdb/15011] New: unable to fetch function parameters

[Bug gdb/15011] New: unable to fetch function parameters

[deeprtm3997 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15011

             Bug #: 15011
           Summary: unable to fetch function parameters
           Product: gdb
           Version: 7.5
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
        AssignedTo: unassigned@sourceware.org
        ReportedBy: deeprtm3997@gmail.com
    Classification: Unclassified


Created attachment 6812
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6812
gdb session

I am running xv6 OS (teaching linux-like OS) on qemu and doing remote debugging
using gdb.
I start the kernel and set a breakpoint on function "exec". Gdb should show the
path and argv variables, but instead it says that :

Breakpoint 1, exec (
    path=<error reading variable: can't compute CFA for this frame>, 
    argv=<error reading variable: can't compute CFA for this frame>)
    at exec.c:12
12    {
(gdb) c
Continuing.

So, I am not able to fetch the argv and path.

When I try :
p argv[0] 

it says "can't compute CFA for this frame".
What should I do?

The attachment below shows the problem.

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[Bug gdb/15011] unable to fetch function parameters

[tromey at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15011

Tom Tromey <tromey at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tromey at redhat dot com

--- Comment #1 from Tom Tromey <tromey at redhat dot com> 2013-01-11 16:02:22 UTC ---
>From the source it appears that this happens if a DWARF
location expression requests the CFA for the frame, but the
frame was found using some other unwinder:

  if (!frame_unwinder_is (this_frame, &dwarf2_frame_unwind)
      && !frame_unwinder_is (this_frame, &dwarf2_tailcall_frame_unwind))
    error (_("can't compute CFA for this frame"));

Offhand I am not sure under what circumstances this can happen.
Maybe if you are somehow hitting the special epilogue frame unwinder.

If the .o file from which 'exec' comes is not too big, maybe
having it would help.

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[Bug gdb/15011] unable to fetch function parameters

[deeprtm3997 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15011

--- Comment #2 from Deep <deeprtm3997 at gmail dot com> 2013-01-12 09:29:03 UTC ---
Created attachment 6813
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6813
Exec.o file

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[Bug gdb/15011] unable to fetch function parameters

[deeprtm3997 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15011

--- Comment #3 from Deep <deeprtm3997 at gmail dot com> 2013-01-12 09:31:16 UTC ---
(In reply to comment #1)
> From the source it appears that this happens if a DWARF
> location expression requests the CFA for the frame, but the
> frame was found using some other unwinder:
> 
>   if (!frame_unwinder_is (this_frame, &dwarf2_frame_unwind)
>       && !frame_unwinder_is (this_frame, &dwarf2_tailcall_frame_unwind))
>     error (_("can't compute CFA for this frame"));
> 
> Offhand I am not sure under what circumstances this can happen.
> Maybe if you are somehow hitting the special epilogue frame unwinder.
> 
> If the .o file from which 'exec' comes is not too big, maybe
> having it would help.

Well, I am using GDB for the first time(doing an OS course), so I don't know
what is "special epilogue frame unwinder". But I am attaching exec.o, if it
helps.
Thank You.

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