Regarding "Keeping C data structures in the Scheme heap".

Do we limit it to have simple structures that have no
pointers to outside resource?  Otherwise we have some
issues:

- If we want to keep C structures that has pointers,
  in Scheme heap, we need a hook to tell GC about it.

- Related to above; if we want to keep C structures that
  has handles for extrenal resouces (externally allocated
  memory, opened file descriptor, etc.), we need a kind of
  finalization mechanism.

- Related to above; if we have circular reference across
  resouces maintained in C world and resources maintained
  in Scheme world, we need either have a mechanism that
  "cancel" the finalization, or a hook to GC mark phase
  that can check dependencies of C world.

Both seem for me very difficult to make portable across
various GC strategies.
I guess Java people have good knowledge about these issues.

--shiro