>  What shall I do, if I want to store a pointer to a caml value
> in a structure outside the caml heap? Is there any GC hints for it?

You need to tell the GC the address (outside the heap) where the Caml
value is stored, using register_global_root().  

When the external data structure is no longer relevant, or is about to
be freed, call remove_global_root() to tell the GC that the global
root is no longer there, thus allowing the GC to reclaim the
corresponding Caml data structure.

- Xavier Leroy