Re: Allowing Applications to continue after exception...

[David Chisnall]

On 22 Feb 2009, at 12:58, Richard Frith-Macdonald wrote:

> On 22 Feb 2009, at 12:42, David Chisnall wrote:
> > > It sounds like a good thing to have the option of resuming, even  
> > > though it's generally not advisable.
> > >
> > > Do you have a patch for gcc/libobjc which would allow that?
> >
> > It won't require patching gcc, just libobjc.  I don't have a patch  
> > currently, but I can probably create one fairly easily.  It just  
> > requires adding an optional trampoline in the throw function that  
> > will bounce the exception out before unwinding the stack.
>
> Yes please ... I'd love to try that out.

The same functionality, with a less elegant implementation, is  
provided by ETException.  There are a couple of example use-cases in  
this paper:

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

> > The current code in Étoilé allows exceptions to be either resumed  
> > or restarted.  Restarting jumps back to the start of the exception- 
> > handling block and lets the code try again.  This is currently done  
> > in a really messy way.  Ideally I'd want to make sure that the  
> > compiler inserts an entry into the DWARF table pointing to the  
> > start of this block and then let the unwind library walk back up  
> > the stack to find it.  This is a much bigger change, and one I  
> > won't have time to do for a little while.
>
> I'm not sure about that ... it's certainly not needed by GNUstep  
> and, while I can see the appeal, I can't think of any systems that  
> actually allow re-execution of the 'try' block, so this would not  
> fit into normal coding paradigms.

It's common in Lisp, and supported by a few Smalltalk systems.

> What I'm really concerned about at the moment is trying to get code  
> into the gcc distribution before the next release which would let me  
> properly implement the Openstep/MacOS-X exception handling API when  
> using objc native exceptions.  It looks like that's just a function  
> to set an uncaught exception handler.

Adding an uncaught exception handler is a bit different.  This, in  
Cocoa, doesn't allow you to resume from the exception, it just lets  
you tidy up before the program exits.  You can achieve exactly the  
same functionality without modifying the compiler or runtime by  
implementing your main function like this:

int main(int argc, char **argv)
{
        int ret = -1;
        @try
        {
                ret = retreal_main(argc, argv);
        }
        @catch (id e)
        {
                NSUncaughtExceptionHandler *h = NSGetUncaughtExceptionHandler();
                if (h != (NSUncaughtExceptionHandler*)0)
                {
                        h(e);
                }
        }
        return ret;
}

I believe you should be able to use GNUstep's existing fake main code  
for this.  If you don't want to do this, then I could add a hack to  
the personality function to do something equivalent.  Unfortunately,  
like most of the rest of GNU libobjc, the unwind code is typical GNU  
code, being an almost unreadable mess of #ifdefs with no clear  
separation of concerns in a coding convention designed for minimal  
readability.  The version I'm using seems to be using GNUstep make and  
doesn't compile the exception handling stuff.  I'll grab a copy of the  
trunk version and see if it's any better.

David