Re: [Caml-list] Understanding usage by the runtime

[John Carr <jfc@mit.edu>]

> There is however something to do. Quoting lablgtk's README:
> > IMPORTANT: Some Gtk data structures are allocated in the Caml heap,
> > and their use in signals (Gtk functions internally cally callbacks)
> > relies on their address being stable during a function call. For
> > this reason automatic compation is disabled in GtkMain. If you need
> > it, you may use compaction through Gc.compact where it is safe
> > (timeouts, other threads...), but do not enable automatic compaction.
> 
> I've never really understood why it worked: I'm surprised the GC would
> update addresses stored in the C side of GTK.

I think the problem is, a C function can invoke a callback that calls
ocaml code that moves the object being operated on by the C function.
Because the C function is precompiled it does not register its copy of
the pointer as a GC root.  When the callback returns the C function's
pointer is invalid.

This should be fixable with another level of indirection.  Finalization
can free the C object.  I infer from reading the source that the extra
level of indirection is considered an unacceptable penalty.