Cf.Allocator
type retain_callback_t = unit Ctypes.ptr -> unit Ctypes.ptr
type release_callback_t = unit Ctypes.ptr -> unit
type copy_description_callback_t = unit Ctypes.ptr -> bytes
val retain_callback_typ : retain_callback_t Ctypes.typ
val release_callback_typ : release_callback_t Ctypes.typ
val copy_description_callback_typ : copy_description_callback_t Ctypes.typ