Hi, we've recently run into a class of bugs concerning finalization functions with custom blocks that is probably not uncommon in OCaml bindings (e.g. Postgresql-bindings, SSL-bindings, likely others). It seems somewhat unintuitive, but finalizers registered from within C do _not_ behave the same as ones registered from OCaml-code with the Gc-module.
The OCaml manual explicitly states that custom functions registered from within C are not allowed to allocate values, perform callbacks, (de)register roots (I guess global roots, too?), or (as is obviously also true) release/acquire the OCaml-runtime lock. Developers probably often miss this information. The consequences of bugs violating any of these constraints are usually rare and hard to understand segfaults. This means that people will have to register finalizers from within OCaml for such values, which is certainly rather inconvenient. As a general rule, it seems advisable to never use finalizers that need to release the runtime lock, e.g. if finalization code can block. The reason is that finalizers can be run from any thread, including ones that should not block for an indefinite amount of time. This implies that people who have designed APIs using finalization that way should consider redesigning it such that users have to "clean up" manually, thus forcing them to think about when and in which thread this happens. Otherwise, it might be helpful if the OCaml-team could consider whether this situation can be improved. For example not being allowed to register/unregister roots to values seems overly restrictive, since global roots (e.g. for protecting OCaml-callback closures) sometimes need to be associated with C-values (e.g. for allowing some external C-library to efficiently perform callbacks into OCaml). Registering the finalizer from within C during allocation rather than having to wrap up the value a second time with an OCaml-finalizer would seem much simpler. Regards, Markus -- Markus Mottl http://www.ocaml.info markus.mo...@gmail.com _______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
