On Fri, 3 Aug 2007, Duncan Murdoch wrote:
On 8/3/2007 9:19 AM, Jens Oehlschlägel wrote:
Dear R .Call() insiders,
Can someone enlighten me how to properly finalize external pointers in C code
(R-2.5.1 win)? What is the relation between R_ClearExternalPtr and the
finalizer set in R_RegisterCFinalizer?
I succeeded registering a finalizer that works when an R object containing an
external pointer is garbage collected. However, I have some difficulties
figuring out how to do that in an explicit closing function.
I observed that
- calling R_ClearExternalPtr does not trigger the finalizer and is dangerous
because it removes the pointer before the finalizer needs it at
garbage-collection-time (no finalization = memory leak)
- calling the finalizer directly ensures finalization but now the finalizer is
called twice (once again at garbage collection time, and I did not find
documentation how to unregister the finalizer)
- It works to delete the SEXP external pointer object but only if not calling
R_ClearExternalPtr (but why then do we need it?) Furthermore it is unfortunate
to delay freeing the external pointers memory if I know during runtime that it
can be done immediately.
Shouldn't R_ClearExternalPtr call the finalizer and then unregister it?
(this would also work when removing the SEXP external pointer object is
difficult because it was handed over to the closing function directly
as a parameter)
I think we want R_ClearExternalPtr to work even if the finalizer would
fail (e.g. to clean up when there was an error when trying to build the
external object).
So I'd suggest that when you want to get rid of an external object
immediately, you call the finalizer explicitly, then call
R_ClearExternalPtr. The documentation doesn't address the question of
whether this will clear the registered finalizer so I don't know if
you'll get a second call to the finalizer during garbage collection, but
even if you do, isn't it easy enough to do nothing when you see the null
ptr, as you do below?
You will get a further finalizer call at GC, and I know of no way to
unregister finalizers. So make sure the finalizer does nothing the second
time.
The way connections are handled in R-devel provides an example (although
it is work in progress).
Another possibility is to call the GC yourself if you know there are a lot
of objects to clear up.
By the way, questions about programming at this level are better asked
in the R-devel group.
Indeed.
[...]
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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