Sorry for the rather naive suggestion, but wouldn't it be possible to
create a weak pointer in the OCaml world, and have the C value refer to
that weak pointer?

On Tue, Oct 6, 2015 at 5:09 PM, Gerd Stolpmann <info@gerd-stolpmann.de>
wrote:

> Am Dienstag, den 06.10.2015, 15:16 +0100 schrieb Richard W.M. Jones:
> > On Tue, Oct 06, 2015 at 02:43:42PM +0100, Richard W.M. Jones wrote:
> > >
> > > I guess I have two questions:
> > >
> > > (1) Is calling Gc.compact () guaranteed to call the finalizer of any
> > > object which is no longer reachable, under all circumstances?  Or
> > > would there be some case where it wouldn't be called?
> > >
> > > (2) I have a large mixed OCaml / C program[a] where somehow calling
> > > Gc.compact isn't calling the destructor of a (very) large object.
> > > Manual code inspection has not revealed anything so far --
> > > superficially it appears we are not holding any references to the
> > > object.  Is there any method / library / tool that can inspect the
> > > OCaml heap and find references to an object?
> >
> > Always good to explain these things, because the act of explaining it
> > has allowed me to work out why (2) is happening now.
> >
> > The reason is because I was registering a global root from the C heap
> > pointing to the handle, and of course this prevents the handle from
> > being unreferenced.
> >
> > This does, however, raise another question:
> >
> > (3) I want to have a C heap 'value' pointing to an OCaml value, in
> > such a way that if the OCaml value moves around, the C value gets
> > updated to point to the new location.  I was using a global root
> > for this purpose, but it seems like global roots really have two
> > purposes:
> >
> >   (i) To keep the C value updated if the OCaml value moves.
> >
> >   (ii) To act as a global root, preventing the value from being freed.
> >
> > Is there a "weak" global root, that has property (i) but not property
> (ii)?
>
> There is nothing like that in the C API of the OCaml FFI. What you can
> do is to keep only a symbolic reference to the OCaml value from the C
> struct (i.e. if it is the k-th such value just store the number k). The
> mapping from k to the OCaml value can be done with a weak hash table.
> For finalizing this you need to use Gc.finalize.
>
> All in all this is quite complicated. I guess it's only worth it if the
> memory management really needs to be fully-automatic.
>
> Gerd
>
> > Rich.
> >
> > --
> > Richard Jones
> > Red Hat
> >
>
> --
> ------------------------------------------------------------
> Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
> My OCaml site:          http://www.camlcity.org
> Contact details:        http://www.camlcity.org/contact.html
> Company homepage:       http://www.gerd-stolpmann.de
> ------------------------------------------------------------
>
>