CODEWITHSUNDEEP
ocamlctypesffi
Anton Kochkov
Anton Kochkov

Reputation: 1287

OCaml Ctypes - How to make the pointer to pointer

In C if you have a function like int myfunc(struct mystruct **context) then the OCaml foreign signature will be

    type mystruct
    let mystruct : mystruct structure typ = structure "mystruct"

    let myfunc = foreign "myfunc" (ptr (ptr mystruct) @-> returning int)

The question is how to call the function? We can't take addr (addr mystructinstance), so the code is wont compile:

    let inst = make mystruct in
    let res = myfunc (addr (addr inst))

Upvotes: 2

Views: 370

Answers (1)

Étienne Millon
Étienne Millon

Reputation: 3028

The memory model of ctypes closely matches C, so the code you need to write will match what you'd write in C. You're right that addr (addr mystructinstance) is ill-typed, that is because it corresponds to &(&mystructinstance), which does not mean anything in C.

In C, you'd probably write something like:

mystruct *p = NULL;
int err = myfunc(&p);
if (err == ok) {
  // p has now a usable value for the rest of the API
}

The equivalent code in ctypes will be

let p_addr = Ctypes.allocate (ptr mystruct) (from_voidp mystruct null) in
let err = myfunc p_addr in
(* check err *)
let p = !@ p_addr in
(* do things with p, typed mystruct structure ptr *)

Upvotes: 4

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