CONNDB_FORW_BIT
const uintptr_t tag = (CONNDB_FORW_BIT * !flow);
#define CONNDB_ISFORW_P(p) (((uintptr_t)(p) & CONNDB_FORW_BIT) != 0)
#define CONNDB_GET_PTR(p) ((void *)((uintptr_t)(p) & ~CONNDB_FORW_BIT))