Re: Allowing Applications to continue after exception...

[David Chisnall]

On 8 Feb 2009, at 06:47, Richard Frith-Macdonald wrote:

> Actually, I take it back ... maybe we can recover from an uncaught  
> exception.
> Problem is ... if we are using native exceptions ... by the time the  
> uncaught handler is called we have unwound the stack entirely and  
> the program has no way to continue running.  But maybe the native  
> handling code can actually cope with that (or be changed to do that).

I've been doing personality function hacking recently, so I'd be happy  
to take a look at this if it's desired.

When the stack is unwound by native ('zero-cost') exceptions, the  
unwind library calls the personality function twice for each frame.   
The first time is to find the landing pad, the second time is to  
perform the unwinding (call cleanup code in each intervening frame and  
then continue unwinding).

For a while, I've been pondering adding the Étoilé resumable exception  
stuff to the unwinding system.  This would call the unhandled  
exception function at the top of the stack, before unwinding, and  
allow it to either call _Unwind_Resume() or continue, depending on the  
exception.

> Not sure where the stack pointer and program counter end up if we  
> are using old-fashioned exceptions.

This is not so difficult.  I described it in this paper:

http://www.swan.ac.uk/compsci/research/reports/2008/CSR14-2008.pdf

Basically, you call the uncaught exception handler from [NSException - 
raise] and let it to decide whether to unwind the stack or not.

In the general case, there's not much you can do other than pop up a  
dialog saying 'Do you want to ignore this exception?' and that's  
likely to cause things to break.

So, it is possible, but I'm not sure it's actually a good idea...

David