test.c
========
#include <caml/mlvalues.h>

value test_add2 (value i) {
  return Val_int (Int_val (i) + 2);
}

test.mli
========
external add2 : int -> int = "test_add2";;

main.ml
========
let main () =
  let i = 13 in
  Printf.printf "%d -> %d" i (Test.add2 i);
  print_newline ()
;;

let () = main ();;