DMARC-Filter: OpenDMARC Filter v1.4.2 delorie.com 61QLgriE1253228
DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 559C24BA2E16
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3826.700.81.1.4\))
Subject: Re: Memmove causing program crashes, giving SIGTRAP in GDB(?)
In-Reply-To: <957583332.1523179.1772138829815@connect.xfinity.com>
Date: Thu, 26 Feb 2026 22:42:28 +0100
Message-Id: <6E812632-0E73-464A-AAB9-DC5BC5546B4C@unified-streaming.com>
References: <547312365 DOT 1464244 DOT 1771958282029 AT connect DOT xfinity DOT com>
 <aZ7PrbisVR1R4A7v AT dimstar DOT local DOT net>
 <1670201592 DOT 1489273 DOT 1772043520008 AT connect DOT xfinity DOT com>
 <e91d8b5b-2690-4271-aa74-e6226440e33d AT SystematicSW DOT ab DOT ca>
 <1044918836 DOT 1507810 DOT 1772086967212 AT connect DOT xfinity DOT com>
 <1579472684 DOT 1508349 DOT 1772092747339 AT connect DOT xfinity DOT com>
 <aaABFf5iEowV1l7I AT xps13> <1148572549 DOT 1808180 DOT 1772097444036 AT mail DOT yahoo DOT com>
 <1901597260 DOT 1508573 DOT 1772100378936 AT connect DOT xfinity DOT com>
 <0C965DD0-856E-41FF-B5A4-15E472292A32 AT unified-streaming DOT com>
 <483908609 DOT 1508714 DOT 1772103775739 AT connect DOT xfinity DOT com>
 <2346fd41-2500-0db6-5849-6788174b5a1d AT cs DOT umass DOT edu>
 <1462848037 DOT 1521935 DOT 1772136952077 AT connect DOT xfinity DOT com>
 <609655EE-8E9D-4522-A05C-F74C3FC89583 AT unified-streaming DOT com>
 <957583332 DOT 1523179 DOT 1772138829815 AT connect DOT xfinity DOT com>
To: KENNON J CONRAD <kennonconrad AT comcast DOT net>
Precedence: list
From: Dimitry Andric via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Dimitry Andric <dimitry AT unified-streaming DOT com>
Cc: KENNON J CONRAD via Cygwin <cygwin AT cygwin DOT com>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Errors-To: cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com
Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

If such a crash occurs, can you do a "thread apply all bt" in gdb? This will show what all the other threads are doing. I'm betting some other thread is calling memcpy or some other function that is messing with the direction flag.

-Dimitry

> On 26 Feb 2026, at 21:47, KENNON J CONRAD <kennonconrad AT comcast DOT net> wrote:
> 
> Yes, lots.  7 threads were running at the point of the crash  87% load on my i7-4790k.  I did a little research since the last post.  The memmove code where the crash occurs is:
> 
>   0x00007ff96ba812a8 <+136>: std
> => 0x00007ff96ba812a9 <+137>: rep movsq %ds:(%rsi),%es:(%rdi)
>   0x00007ff96ba812ac <+140>: cld
> 
> This sets the direction flag immediately before the rep movsq and clears the direction flag immediately after the rep movsq.  Yet when gdb breaks it shows the direction flag is not set:
> 
> eflags         0x246               [ PF ZF IF ]
> 
>  Would a forward move on overlapping data cause the SIGTRAP?  Could the code have moved to a different core?  Or could it have been interrupted by some other task that corrupts the flag?  As I mentioned earlier, the rep movsq is only failing once per several million times memmove is called so it seems likely to be something along those lines.
> 
> -Kennon
> 
> 
>> On 02/26/2026 12:20 PM PST Dimitry Andric <dimitry AT unified-streaming DOT com> wrote:
>> 
>> 
>> Is there some concurrency going on? Maybe some other part of the program is flipping the direction flag?
>> 
>> -Dimitry
>> 


-- 
Problem reports:      https://cygwin.com/problems.html
FAQ:                  https://cygwin.com/faq/
Documentation:        https://cygwin.com/docs.html
Unsubscribe info:     https://cygwin.com/ml/#unsubscribe-simple