Clever! I don't like the use of uintptr_t, though. Why not storing the array begin in v[0] and a pointer to the first free item in v[1]? You avoid casts by def

Clever! I don't like the use of uintptr_t, though. Why not storing the array begin in v[0] and a pointer to the first free item in v[1]? You avoid casts by defining len as ptrdiff_t and calculating it as v[1]-v[0].