[plt-scheme] scheme/foreign, callbacks and NULL

At Fri, 02 Jan 2009 00:43:57 +0100, Andreas Rottmann wrote:
> Matthew Flatt <mflatt at cs.utah.edu> writes:
> 
> > At Thu, 01 Jan 2009 15:08:26 +0100, Andreas Rottmann wrote:
> [example snipped]
> >> 
> >> This causes the following error when run:
> >> Scheme->C: expects argument of type <cpointer>; given #f
> >
> > You can use `_pointer' instead of `_fpointer' as the last argument to
> > `g-idle-add-full%'.
> >
> Updating my example accordingly:
> 
> #lang scheme
> 
> (require scheme/foreign
>          (only-in '#%foreign ffi-callback))
> 
> (unsafe!)
> 
> (define libglib (ffi-lib "libglib-2.0" '("0")))
> (define g-idle-add-full% (get-ffi-obj "g_idle_add_full" libglib _fpointer))
> 
> (define g-idle-add-full
>   (function-ptr g-idle-add-full%
>                 (_cprocedure (list _int _fpointer _pointer _pointer) _uint)))
> 
> (define my-idle-callback
>   (function-ptr (lambda args
>                   (printf "idle, args: ~s~n" args)
>                   1)
>                 (_cprocedure (list _pointer) _int)))
> 
> (define my-destroy-callback
>   (function-ptr (lambda args
>                   (printf "destroy, args: ~s~n" args))
>                 (_cprocedure (list _pointer) _void)))
> 
> (g-idle-add-full 200 my-idle-callback #f #f)
> (g-idle-add-full 200 my-idle-callback #f my-destroy-callback)
> ;; EOF
> 
> I now get:
> 
> Scheme->C: expects argument of type <pointer>; given #<ffi-callback>
> 
> > I'll fix the docs to clarify that `_fpointer' doesn't allow NULL
> > pointers. Or maybe it should allow NULLs?
> >
> Either it should, or (preferably) you would be able to pass
> #<ffi-callback> objects for _pointer arguments.

Hm... yes, I think ffi-callback objects probably should be allowed as
pointers. I'll wait for Eli to weigh in, though.


Matthew