May I ask that the GC_set_mark_bit and GC_clear_mark_bit are exported? I use them as part of the code to resurrect objects in weak hash-tables. Imitating GC_is_marked I think this should do: https://github.com/paurkedal/bdwgc/commit/20652be420b7c38fab92ac2601b4210386df4a7e Cheers, Petter