> >     (* use a destroy method explicitly if you need to make sure the
> > destructor * is called --- Gc.finalise doesn't guarantee this (e.g. the
> > program might * terminate before the finalisation function is executed)

you can use at_exit to garanty a call before termination

> If the handle is really an OCaml integer, it is not possible to attach a
> finalizer to it. The finalizer could be attached to the object that
> wraps the handle; but the best solution is to use custom blocks and C
> finalizers in the C binding.

if you are not so fluent in C/C++ there is this work-around that does work:
http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_finalise
but indeed the classic way is to use the custom_operations struct:
http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_custom