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How to raise_with_arg() a tuple?
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Date: -- (:)
From: Thomas Fischbacher <Thomas.Fischbacher@P...>
Subject: How to raise_with_arg() a tuple?

Hello together,

I'd like to throw an exception from within C code which provides more 
information to Caml than just a string.

If I do it that way:

=== Caml ===

exception Test_exn of int * int

external test_raise_tuple: bool -> (int*int) = "caml_test_raise_tuple"

let _ = Callback.register_exception "ocaml_exn_test" (Test_exn (0,0));;

=== C ===

CAMLprim caml_test_raise_tuple(value x)
  int cx;

  fprintf(stderr,"DDD x=%d\n",cx);



      raise_with_arg(*caml_named_value("ocaml_exn_test"), ex);


...then just returning the tuple works as expected,
but throwing the tuple gives me crazy values - like 

Test_exn (67385386, 1408)

instead of

Test_exn (2,4).

Does anyone have an idea what is going on here, and how to repair this? 
Maybe I just missed something essential in the documentation. I tried to 
find an example in existing OCaml libraries where this is used, but 
somehow I was out of luck there so far... All I found about this 
issue is this bit of documentation in ocaml's byterun/memory.h:

   Your function may raise an exception or return a [value] with the
   [CAMLreturn] macro.  Its argument is simply the [value] returned by
   your function.  Do NOT directly return a [value] with the [return]
   keyword.  If your function returns void, use [CAMLreturn0].

...and the definition says:

#define CAMLreturn(result) do{ \
  caml_local_roots = caml__frame; \
  return (result); \

I tried a few obvious and non-obvious things related to what I see in 
there, but without success so far. So, how does one do that?

regards,                   (o_
 Thomas Fischbacher -  //\
(lambda (n) ((lambda (p q r) (p p q r)) (lambda (g x y)           V_/_
(if (= x 0) y (g g (- x 1) (* x y)))) n 1))                  (Debian GNU)