[ocaml-ctypes] Pointer to pointer and null check

[Andre Nathan]

Hi

I'm trying to write bindings to an API that in simplified form looks
like this, for a given opaque type `foo_t`:

  int foo_bar(foo_t **ret, foo_t *foo);

In C it's used like this:

  foo_t *foo = foo_new();
  foo_t *ret;
  int r = foo_bar(&ret, foo);

  if (r == 0) {
    if (ret != NULL) {
      /* success */
    } else {
      /* error */
    }
  } else {
    /* not important */
  }

So if the return is 0, one has to check whether ret is NULL to determine
failure or success.

I have defined in the OCaml side:

  type foo = unit ptr
  let foo : foo typ = ptr void

  let foo_bar = foreign "foo_bar" (ptr foo @-> foo @-> returning int)

  let foo_bar x =
    let ret = allocate foo x in
    let r = foo_bar ret x in
    (r, ret)

Given that `ret` was allocated before the call, it will never be NULL
after the call, so determining if there was an error becomes impossible.

I also tried defining a structure type instead:

  type foo
  let foo : foo structure typ = structure "foo_t"

  let foo_bar = foreign "foo_bar"
    (ptr ptr foo @-> ptr foo @-> returning int)

  let foo_bar x =
    let ret = from_voidp foo null in
    let r = foo_bar (addr ret) x in
    (r, ret)

but of course I can't call `addr` on a `Ctypes_static.pointer`. Also,
apparently the way to check for NULL is to use `ptr_opt`, but I'm not
sure how that would fit in a function like this where the result is
returned via an argument.

Any help would be greatly appreciated.

Best regards,
Andre

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