Function GC_get_size_map_at 

pub unsafe extern "C" fn GC_get_size_map_at(i: c_int) -> usize

Get the element value (converted to bytes) at a given index of GC_size_map table which provides requested-to-actual allocation size mapping. Assumes the collector is initialized. Returns -1 (~0) if the index is out of GC_size_map table bounds. Does not use synchronization, thus clients should call it using GC_call_with_reader_lock() typically to avoid data race on multiprocessors.