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Date: Thu, 26 Feb 2026 12:15:52 -0800 (PST)
To: Eliot Moss <moss AT cs DOT umass DOT edu>,
        Dimitry Andric <dimitry AT unified-streaming DOT com>
Cc: KENNON J CONRAD via Cygwin <cygwin AT cygwin DOT com>
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Subject: Re: Memmove causing program crashes, giving SIGTRAP in GDB(?)
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From: KENNON J CONRAD via Cygwin <cygwin AT cygwin DOT com>
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Hello Eliot,

   Yes, that helps a lot.  With the frame set to the one for memmove, the RSI register contains 0xa00035e0e.  With the candidates_index address of 0xa00034460, that indicates the RSI register is 0x19AE more than the array start index and corresponds to index 3287 of the uint16_t array.  The array index of the rightmost element that should be moved is 3359, so RSI holds an array index that is 72 less than the last index that should be moved.  But since the RSI is already set up for the next iteration of REP MOVSQ, it seems that 68 array elements should have been moved prior to the REP that gets the SIGTRAP.  So I checked the index array, and found it contains duplicate values of 2028 at both array indexes 3291 and 3292 (elements 3292 and above have been copied from elements 3291 and above).  There should not be any duplicate values in the active portion of the array except during the move.  So it seems that 17 REP MOVSQ's were completed successfully before the SIGTRAP prevented the 18
 th move from being completed.  It shouldn't stop until the 767th array element is moved, so this is pretty early in the desired REP MOVSQ iterations.  Interestingly, the DF is not set in EFLAGS at the point where the SIGTRAP is issued (eflags 0x246 [ PF ZF IF ]) - I thought it should be set for backward REP MOVSQ's.

Best Regards,

Kennon




> On 02/26/2026 5:25 AM PST Eliot Moss <moss AT cs DOT umass DOT edu> wrote:
> 
>  
> On 2/26/2026 6:02 AM, KENNON J CONRAD via Cygwin wrote:
> 
> > Thanks for checking and sharing your thoughts.  This is what I would expect, so at least I'm not crazy :)
> > I really need to see the values of the registers in memmove, hopefully it's just a matter of continuing to learn gdb.
> 
> The command is "info registers", of "info r" for short.  You can also print an
> individual register using (say) "print $rax" (prefix the register name by $)
> or "info r rax".  If you're stepping, you can set up a *layout* that shows all
> the register values as you step along.
> 
> You also might consider using a reverse debugging tool like the "undo" debugger.
> They track enough information to allow them to (effectively) execute backwards
> from a point.  Not sure if any work under cygwin, though.
> 
> By the way, I got the commands for printing individual registers with a simple
> web search :-) ...
> 
> HTH -- Eliot Moss

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