Ctypes: what's the most idiomatic way to anchor an OCaml value so that it's not garbage collected?

Learning
,
2

I agree: using roots isn’t too bad.

This is another reasonable approach, but you don’t need Obj.magic; it’s sufficient to use existential types. Here’s an illustration:

module Store :
sig
  type ticket 
  val retain : 'a -> ticket
  val is_live : ticket -> bool
  val release : ticket -> unit
end =
struct
  type ticket = int
  type element = E : _ -> element
  let counter = ref 0
  let store = Hashtbl.create 10
  let retain v =
    let ticket = !counter in
    incr counter;
    Hashtbl.add store ticket (E v);
    ticket
  let is_live = Hashtbl.mem store
  let release = Hashtbl.remove store
end

This code also shows how to remove items: generate an identifier for each saved value, and use the identifier as a key into an associative structure so that it can be used to release the value when it’s no longer in use by C.

8 Likes