Should I expect that Ptr memory allocated with malloc is garbage collected when no longer used by the Haskell runtime? The FFI spec doesn't say so explicitly (that I could see); AFAICT this is how the pointer types behave: Ptr : freed when no longer used/referenced ForeignPtr : like Ptr, but will also run a finaliser when freed StablePtr : not freed or collected

No, all memory that you allocate with the FFI must be freed explicitly. You can arrange for memory to be deallocated when no longer referenced by using a ForeignPtr, and using the finalizer to free the memory. Bear in mind that if your application has large, or strict, memory requirements, then relying on the ForeignPtr method may not be the best solution, because finalizers can be unpredictable. Cheers, Simon