How to deal with structure as out argument?

[cedlemo]

An example is the Time_val.t structure.

open Ctypes
open Foreign

type t
let t_typ : t structure typ = structure "Time_val"

let f_tv_sec = field t_typ "tv_sec" (int64_t)
let f_tv_usec = field t_typ "tv_usec" (int64_t)
let _ = seal t_typ

The two functions in the Time_val module:

let to_iso8601 =
  foreign "g_time_val_to_iso8601" (ptr t_typ @-> returning (string_opt))

let from_iso8601 iso_date =
  let time__ptr = allocate t_typ (make t_typ) in
  let from_iso8601_raw =
    foreign "g_time_val_from_iso8601" (string @-> ptr (t_typ) @-> returning bool)
  in
  let ret = from_iso8601_raw iso_date time__ptr in
  let time_ = !@ time__ptr in
  (ret, time_)

And the function in the Core module:

let get_current_time =
  foreign "g_get_current_time" (ptr Time_val.t_typ @-> returning (void))

In the tests:

let test_time_val_from_iso8601 test_ctxt =
  let iso8601 = "2018-02-07T10:39:38Z" in
  match GLib.Time_val.from_iso8601 iso8601 with
  | (false, _) -> assert_equal ~msg:"No time val" true false
  | (true, tv) -> let dt = GLib.Date_time.create_from_timeval_utc (Ctypes.addr tv) in
    let (y,m,d) = GLib.Date_time.get_ymd dt in
    let _ = assert_equal_int32 (Int32.of_int 2018) y in
    let _ = assert_equal_int32 (Int32.of_int 2) m in
    assert_equal_int32 (Int32.of_int 07) d

let test_core_get_current_time_and_to_iso8601 test_ctxt =
  let tv = Ctypes.make GLib.Time_val.t_typ in
  let tv_ptr = Ctypes.addr tv in
  let _ = GLib.Core.get_current_time tv_ptr in
  match GLib.Time_val.to_iso8601 tv_ptr with
  | None -> assert_equal ~msg:"No iso8601 from Timeval" true false
  | Some iso8601 -> let now = GLib.Date_time.create_now_local () in
    let (year, month, day) = GLib.Date_time.get_ymd now in
    let dt = GLib.Date_time.create_from_timeval_utc tv_ptr in
    let (y,m,d) = GLib.Date_time.get_ymd dt in
    let _ = assert_equal_int32 year y in
    let _ = assert_equal_int32 month m in
    assert_equal_int32 day d

The construction of Timeval.from_iso8601 is not clear because, it make locally a structure that is used
as default value for during the allocation of a structure Time_val on the heap then the pointer is passed to the function in order to fill the structure in the heap, then I get the value of the pointer (ie the structure)

Question: why not return a pointer ? Is it possible ?