Re: [Caml-list] Finding "lost" references to OCaml heap values

["Richard W.M. Jones" <rich@annexia.org>]

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)?

Rich.

-- 
Richard Jones
Red Hat