[Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Corinna Vinschen]

On Aug 17 13:36, Ken Brown via Cygwin wrote:
> On 8/16/2020 6:38 PM, Ken Brown via Cygwin wrote:
> > One further comment, and then I'll shut up
> 
> I lied.

...which is ok.

> > and leave it to people who know what they're talking about: In the
> > 64-bit case, the function _Unwind_RaiseException in libgcc/unwind-seh.c
> > gets called, and it calls the Win32 function RaiseException as follows:
> > 
> >    RaiseException (STATUS_GCC_THROW, 0, 1, (ULONG_PTR *)&exc);
> > 
> > Then the following comment appears:
> > 
> >    /* The exception handler installed in crt0 will continue any GCC
> >       exception that reaches there (and isn't marked non-continuable).
> >       Returning allows the C++ runtime to call std::terminate.  */
> > 
> > Apparently this doesn't actually happen.
> 
> I'm beginning to think this is a bug in the Cygwin build of gcc.  The file
> unwind-seh.c is compiled on 64-bit Cygwin because __SEH__ is defined.  But
> it makes assumptions, such as those in the comment quoted above, that are
> not valid on Cygwin.
> 
> In fact, if you look at the main exception handler on Cygwin
> (exceptions.cc:626), it is clear that the exception code STATUS_GCC_THROW
> does not lead to continuation.  It falls under the default case:
> 
>     default:
>       /* If we don't recognize the exception, we have to assume that
> 	 we are doing structured exception handling, and we let
> 	 something else handle it.  */
>       return ExceptionContinueSearch;
> 
> [STATUS_GCC_THROW is 0x20474343, which shows up in the gdb output above as
> an unknown target exception.]
> 
> So it seems to me that either Cygwin's exception handler has to learn to
> handle the exception codes defined in unwind-seh.c, or else gcc should be
> built with __SEH__ undefined.
> 
> JonY, Achim, Corinna, am I missing something?

This stuff isn't really my strong point.  IIUC the above comment correctly,
we might get away with adding another case to the exception handler switch:

  #define STATUS_GCC_THROW 0x20474343
  case STATUS_GCC_THROW:
    return ExceptionContinueExecution;

Did you try that, by any chance?


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer