From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: weis
Received: (from weis@localhost) by pauillac.inria.fr (8.6.10/8.6.6) id SAA01918 for caml-redistribution; Tue, 24 Sep 1996 18:41:50 +0200
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39]) by pauillac.inria.fr (8.6.10/8.6.6) with ESMTP id PAA26812 for <caml-list@pauillac.inria.fr>; Tue, 24 Sep 1996 15:36:41 +0200
Received: from nef.ens.fr (nef.ens.fr [129.199.96.12]) by concorde.inria.fr (8.7.6/8.7.1) with SMTP id PAA12283 for <caml-list@pauillac.inria.fr>; Tue, 24 Sep 1996 15:36:40 +0200 (MET DST)
Received: from vedette.ens.fr by nef.ens.fr (5.65c8/ULM-1.0)
	Id AA24876 ; Tue, 24 Sep 1996 15:36:39 +0200
Received: from nave.ens.fr by vedette.ens.fr (4.1/version 1.10 of 88/05/05)
 	id AA27204; Tue, 24 Sep 96 15:36:38 +0200
Date: Tue, 24 Sep 1996 15:36:36 +0200 (MET DST)
From: Jerome Vouillon <vouillon@clipper.ens.fr>
Sender: weis
Reply-To: Jerome Vouillon <vouillon@clipper.ens.fr>
Subject: Re: Need help: O'Caml C Interface
To: Frank Christoph <christo@nextsolution.co.jp>
Cc: caml-list@pauillac.inria.fr
In-Reply-To: <9609231509.AA03139@sparc3.nextsolution.co.jp>
Message-Id: <Pine.3.89.9609241403.A29179-0100000@nave>
Mime-Version: 1.0
Content-Type: TEXT/PLAIN; CHARSET=US-ASCII


> value ocaml_getCharWidth(value code)
> {
>   double x, y;
>   Push_roots(r,1);
> 
>   r[0] = alloc_tuple(2);
>   getCharWidth(Int_val(code),&x,&y);
>   Store_double_val(Field(r[0],0),x);
>   Store_double_val(Field(r[0],1),y);
>   Pop_roots();
>   return r[0];
> }

I would write it this way:

value ocaml_getCharWidth(value code)
{
  double x, y;
  value res;
  Push_roots(r,2);

  getCharWidth(Int_val(code),&x,&y);
  r[0] = copy_double(x);
  r[1] = copy_double(y);
  res = alloc_tuple(2);
  Field(res, 0) = r[0];
  Field(res, 1) = r[1];
  Pop_roots();
  return res;
}

Store_double_val is used to store a double in an array of double values 
(which is a special kind of array). In other cases, you have to use 
copy_double to allocate a block containing the double.
So, your code failed because Field(r[0],0) was not a pointer to an array,
but an uninitialized pointer. 
*_roots are used to keep a pointer to both double values. Indeed, these
values can be moved by copy_double or alloc_tuple. The tuple is allocated
last, so that it can be directly filled (without having to use function
modify).

   Jerome