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Fwd: How "can't compute CFA for this frame" and "no enough registers or memory available to further unwind" happen?

---------- Forwarded message ----------
From: zhihua che <>
Date: 2011/11/1
Subject: Re: How "can't compute CFA for this frame" and "no enough
registers or memory available to further unwind" happen?
To: Pedro Alves <>

2011/11/1 Pedro Alves <>:
> On Monday 31 October 2011 17:25:46, zhihua che wrote:
>> Hi, everyone
>> ? ? ? ?I'm not sure this is right place for the help. I'm writing a
>> toy os and coding with mixed assembly and C language, debugging with
>> GDB. But I'm trapped with an annoying problem. This is my situation:
>> During the os booting time, after the os control transfers from real
>> mode assembly codes to real mode C codes, I wish I can exam the stack
>> frames and local variable as I do in regular application program, but
>> I always get "can't compute CFA for this frame" or "No enough
>> registers or memory available to further unwind" if I issue "print
>> xxx" or "backtrace" command respectivelly.
> You'll need to debug gdb. ?Check what is it that gdb is finding
> unavailable. ?Put a breakpoint at `throw_error' and then do that
> "print XXX". ?You should hit a call like `throw_error (NOT_AVAILABLE_ERROR...'.
> Get a backtrace. ?Do "continue" on the top gdb, and see if further
> hits appear. ?GDB has an exception handling mechanism, and that
> exception may be thrown more than once during a command run.

I've tried debugging the GDB under "print xxx" circumstance, and I
find it doesn't satisfy an comparison in dwarf2_frame_cfa() which is
like the below:
? ? ?if (! frame_unwinder_is(this_frame, &dwarf2_frame_unwinder))
? ? ? ? ? error(_("can't compute CFA for this frame"))
the frame_unwinder_is() tests if this_frame->unwind is equal with
&dwarf2_frame_unwind. And I further find out this_frame->unwind is
equal with &sentinel_frame_unwind instead in this situation But I
don't know what causes this. I'm really no familiar with GDB

>> The codes seem work, but I
>> can only exam registers or memory using "info reg" or "x" command,
>> it's way unfriendly and time-consuming. I have searched a lot but
>> don't figure out how these happen. I need your help. Thanks.
> What's the output of "info all-registers"?
> --
> Pedro Alves

The out of "info reg" seems Okay. This is the only way I can exam the
executing status of my program until so far.

Thanks for your suggestions. And if it would help, the following is
the compiling options I use.

gcc -g -Wall -mregparm=3 -march=i386 -m32 -mpreferred-stack-boundary=2
-fomit-frame-pointer -ffreestanding -fno-toplevel-reorder
-fno-strict-aliasing -fno-stack-protector -nostdinc -Ixxx

and the link script is like this,

? ?. = 0x0
? ?.text {*(.text)}
? ?.rodata{*(.rodata)}
? ?.data{*(.data)}
? ?/DISCARD/ : {*(.eh_frame)}


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