I'm running into a problem using the ffi, and I think it has to do with passing a struct containing a number of callbacks to a foreign function. The foreign code hangs on to this struct and may invoke the callbacks any number of times.
When the GC runs, things go south quickly. I'm pretty sure the reason is that the callbacks are getting moved by the GC, and the foreign code is left with a struct full of bad pointers. This seems to be confirmed by the ffi docs: === Structs are allocated as atomic blocks, which means that the garbage collector ignores their content. Thus, struct fields can hold only non-pointer values, pointers to memory outside the GC’s control, and otherwise-reachable pointers to immobile GC-managed values (such as those allocated with malloc and 'internal or 'internal-atomic). === I found this old message from Matthew [http://lists.racket-lang.org/users/archive/2010-July/040480.html], but I think his suggestion to define the callbacks at the module top-level only prevents them from being reclaimed, not from being moved. It seems that I need either an immobile closure pointer or a way of sending callbacks to the foreign code without making them invisible to the GC. Any ideas? -Jon ____________________ Racket Users list: http://lists.racket-lang.org/users
