[racket] FFI Pointing to racket objects

At Mon, 4 Mar 2013 06:35:03 -0700, Jay McCarthy wrote:
> My understanding is that in your version, the Racket object is only
> available via the Chipmunk/C data structure, which the GC views as
> opaque, so it is collected. If you want to put Racket things in C
> objects, you'll have to make sure they are retained on the Racket
> side, by (for example), also storing them in a hash table and
> monitoring access to the C objects.

Retaining a reference to the value from elsewhere won't be enough,
usually. The GC might move the value, in which case the C data
structure's field will point to the wrong place.

> On Sat, Mar 2, 2013 at 8:35 AM, Sam Vervaeck <vervaeck.sam at skynet.be> wrote:
> > I have looked into (malloc-immobile-cell
> > v) and (free-immobile-cell cptr) but I am not sure that fiddling with the
> > memory is such a good idea. Could you please give me any guidelines or point
> > me to a procedure I can use to provide this functionality?

The `malloc-immobile-cell' and `free-immobile-cell' functions really
are the way to go.