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To: "cygwin@cygwin.com" <cygwin@cygwin.com>
Subject: Re: Terminate handler not called for C++ Cygwin programs on Windows 11
Thread-Topic: Terminate handler not called for C++ Cygwin programs on Windows
 11
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Date: Tue, 1 Apr 2025 09:23:17 +0000
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From: "Joerg Markus \(LBC\) via Cygwin" <cygwin@cygwin.com>
Reply-To: "Joerg Markus \(LBC\)" <Markus.Joerg@liebherr.com>
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Hi Corinna,

I tested cygwin-3.7.0-0.27.g0d73c040676a this morning (on Windows Server 2025) and can confirm that this patch fixes the problem.

Thanks a lot for your fast response and great work.

Best regards
Markus


> Hi Jörg,
> 
> On Mar 31 09:00, Joerg Markus (LBC) via Cygwin wrote:
> > Hi,
> > 
> > a colleague and I are facing a problem with unhandled exceptions in 
> > our C++ programs in Cygwin.  The C++ standard guarantees, that
> > std::terminate() shall be called for thrown exceptions, that are not 
> > caught [0].  Unfortunately for C++ applications compiled for Cygwin64 
> > running on recent Windows systems,  std::terminate_handler is never 
> > executed.
> > 
> > A minimal example for showing the non-standard conform behaviour can 
> > be found here [1]:
> > 
> > ```
> > #include <cstdlib>
> > #include <exception>
> > #include <iostream>
> >  
> > int main()
> > {
> >     std::set_terminate([]()
> >     {
> >         std::cout << "Unhandled exception\n" << std::flush;
> >         std::abort();
> >     });
> >     throw 1;
> > }
> > ```
> > 
> > In Cygwin64 on Windows 11, Windows Server 2022 and Windows Server 2025 
> > the above program doesn't print anything to stdout and exits with 
> > status code 0.  The expected behaviour would be a non-zero exit code 
> > and the above error message on stdout.
> > 
> > Windows 10 and Windows Server 2019 are not affected. As another data 
> > point, Cygwin 32bit on Windows Server 2022 works as well. We actually 
> > found this behaviour, while migrating some programs from Cygwin 32bit 
> > to 64bit.  I also disabled SEHOP, which had no effect.
> > 
> > My colleague started an investigation and traced the error back to a 
> > change in behaviour in RtlRaiseException() from ntdll.dll, that's 
> > where our investigation hit a wall.  Here is a thread [2], discussing 
> > a possibly related problem for some Windows applications, which 
> > apparently can be traced back to a change of behaviour in the 
> > SetUnhandledExceptionFilter API in Windows Server 2022.
> > 
> > That's all we know. It would be great, if this could be fixed on the 
> > Cygwin side. While some Windows programs seem to be affected as well, 
> > under Cygwin all C++ programs with unhandled exceptions are currently 
> > affected.
> 
> I tested this on W10 and W11 and, as you wrote, it works fine on W10 and simply exits on W11.
> 
> I even tried to add a new unhandled exception handler, but this didn't work either and I was just as stumped as you were.
> 
> But there's light at the end of the tunnel:
> 
> I ran this under GDB and it turns out that there's an interesting difference.  When the throw is performed, we reach Cygwin's exception handler.
> 
> On W10, EXCEPTION_RECORD::ExceptionFlags is 0.
> 
> On W11, EXCEPTION_RECORD::ExceptionFlags is 128, i.e., EXCEPTION_SOFTWARE_ORIGINATE.
> 
> However, our exception handler returns prematurely with ExceptionContinueSearch if ExceptionFlags is non-0.
> 
> I patched our execption handler to exit prematurely only if any other flag apart from EXCEPTION_SOFTWARE_ORIGINATE is set in ExceptionFlags, and your testcase starts working on Windows 11.
> 
> Before:
> 
> $ ./throw
> $
> 
> After:
> 
> $ ./throw
> Unhandled exception
> Abort
> 
> just as on W10.
> 
> I just pushed the patch.  Please test the next test release cygwin-3.7.0-0.27.g0d73c040676a.
> 
> 
> Thanks,
> Corinna

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