Re: Protecting C code from continuations

[Steve Tell]

On Sun, 4 Jul 2004, Neil Jerram wrote:

> Hi all,
> 
> I'm working with some C code that is not continuation-safe; 
> specifically, because it will fail in some way if the points of entry 
> from C into Scheme code appear to return more than once.  However, it 
> would be useful if I could use continuations to save and restore context 
> as far as the Scheme stack is concerned.
> 
> Given this, is there any way of being able to use continuations within 
> Scheme but protecting the C code from them?
> 
> I was wondering about using something like this at all the Scheme-from-C 
> entry points ...........
> 
> (define-macro (with-continuation-barrier . body)
>    `(call/cc
>       (lambda (cont)
>         (dynamic-wind
>           noop
>           (lambda () ,@body)
>           (lambda () (cont #f)))))
> 
> ..... but I suspect this doesn't work, because when I call a 
> continuation deep within the Scheme code (i.e. the continuation that I 
> really want to use, not the one used as part of the barrier here), that 
> continuation would replace the current dynamic-wind context with the one 
> that was in effect when the continuation was created -- which is exactly 
> what I don't want.
> 
> Any comments/ideas?

Might (call-with-dynamic-root thunk) be what you want?
 
"If thunk captures a continuation, the continuation is rooted at the call 
to thunk. In particular, the call to call-with-dynamic-root is not 
captured. Therefore, call-with-dynamic-root always returns at most one 
time."


I've been using some C code I borrowed long ago from scwm (scheme
configurable window manager, by Maciej Stachowiak and Greg J. Badros) for
such callbacks from C into scheme.  It sets up the dynamic root, and also
sets up addtional handlers to save and print a stack traceback from any
errors inside such callbacks.
It uses a lot of deprecated features, so at some point I need to rewrite
it with reference to the current guile manual - and write a tutorial or
documentation addition while doing so.



Steve