From the README:

  Ever been annoyed that you can't create a hash table where the keys are
  FFI values, like raw IPv4 addresses, but the values are Lua objects?
  Well of course you can key a normal Lua table by any Lua value, but the
  key is looked up by identity and not by value, which is rarely what you
  want.  `foo[lib.protocol.ipv4:pton('1.2.3.4')]` will not be the same as
  `foo[lib.protocol.ipv4:pton('1.2.3.4')]`, as the `pton` call produces a
  fresh value every time.  What you usually want with FFI-keyed tables is
  to be able to look up the entry by value, not by identity.

  Well never fear, *cltable* is here.  A cltable is a data type that
  associates FFI keys with any old Lua value.  When you look up a key in a
  cltable, the key is matched by-value.

If no hash_fn is given, ctable will provide one.