>>>>> On Tue, 29 Apr 2014 07:20:08 +0200, Willem Rein Oudshoorn said: > Cancel-Lock: sha1:p0/p2S+buMdADzjMLfHl6LN5m6k= > > Martin Simmons <mar...@lispworks.com> writes: > > >>>>>> On Mon, 28 Apr 2014 07:26:07 +0200, Willem Rein Oudshoorn said: > >> Cancel-Lock: sha1:zqpk08tO/PaCrz3PLTlI+ocAF9E= > >> > >> Joeish W <joeish80...@yahoo.com> writes: > >> > >> > How do I work with the metaobjects(is that what they are called) > >> > output by the code you gave me...If you can show me how to mem-aref > >> > one I would really appreciate it > >> > >> Sorry for the short answer, but at the moment I am extremely busy. > >> If you followed the example > >> > >> (mem-aref (c-pointer YOUR-POINT) ...) > >> > >> should work. > > > > It might be interesting (or scary depending on your point of view) to > > consider > > what happens if the variable YOUR-POINT is the last reference to the CLOS > > object here...the finalizer might free the foreign object before mem-aref is > > entered if the compiler no longer keeps a pointer to the variable. > > > No this is not safe. In general you should never do > `(c-pointer ...)` outside the low level parts of the bindings, and use > it very carefully. > > Personally I would try to never use the `c-pointer` method outside > the `translate-to-foreign` code. And I naively expected that this > would be safe. But as you point out: > > > Or more specifically, can this ever be safe? > > > > (mem-aref (c-pointer (point0)) ...) > > This is never safe. > > I think the easiest fix is to change the > > (defmethod translate-to-foreign ((lisp-value cv-matrix) (c-type cv-mat)) > (c-pointer lisp-value)) > > code to [UNTESTED CODE]: > > > (defmethod translate-to-foreign ((lisp-value cv-matrix) (c-type cv-mat)) > (values (c-pointer lisp-value) lisp-value) > > > This should keep the lisp-value around until we are done using the > `(c-pointer ...)` value. Provided of course you are not mucking around > with the `c-pointer` method yourself. > > > In general, I would advocate to hide all the nasty c-pointer business in > the translate methods and never deal with it outside that limited scope. > > > Thank you for bringing this to my attention. I do think we need a paper > or section in the manual on how to deal with combing GC in Lisp and > manual memory management on the C side. > > When my time frees up (hopefully in a month or two) I might take a stab > at a first draft.
IMHO, it needs to come with a huge security warning, otherwise there will be endless use-after-free bugs when users lose the wrapper object after storing the pointer somewhere. __Martin _______________________________________________ Cffi-devel mailing list Cffi-devel@common-lisp.net http://common-lisp.net/cgi-bin/mailman/listinfo/cffi-devel
