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Date:	Thu, 14 Mar 2013 09:34:45 -0400
From:	Ryan Johnson <ryan DOT johnson AT cs DOT utoronto DOT ca>
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Subject:	Re: gdb bt gives many question marks
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On 14/03/2013 3:37 AM, Ken Huang wrote:
> Hi all,
>
> I have a problem when using gdb to debug my program in cygwin, the 'bt' command
> gives me many '??'.
>
> So I write a quite easy program named abort.c which just aborts. after
> compiled with
> "gcc -g abort.c -o abort" and run with " gdb abort", it aborts as
> expected. At this time
> I type "bt" in gdb to see the backtrace info, but it only gives the following:
<standard useless windows stack frames that have nothing to do with the 
actual cygwin program under debug>

My experience is that you're SOL once Windows has started its exception 
handling. I dont' know if it's an alternate stack, a debug info problem, 
or what; but the only reliable way I know to debug aborts is to `b 
__abort' so that gdb gets there before of Windows (I think it's two 
underscores, but it could be one or three).

Similar issues affect other signals. In particular, SIGSEGV can often 
leave you in this situation -- especially if the windows exception 
handler itself faulted (e.g. due to a corrupt stack). Unfortunately, 
there's not an easy function to put a breakpoint on that catches seg 
faults before they head into Windows land...

Ryan


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