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Date: Wed, 9 May 2007 11:39:02 +0200
From: "Olivier Andrieu" <oandrieu@nerim.net>
Sender: oandrieu@gmail.com
To: "Jean-Marc EBER" <jeanmarc.eber@lexifi.com>
Subject: Re: [Caml-list] question about the OCaml/C interface
Cc: "Ocaml Mailing List" <caml-list@inria.fr>
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X-Spam: no; 0.00; andrieu:01 oandrieu:01 ocaml:01 lexifi:01 ocaml:01 camlprim:01 alloc:01 camlreturn:01 allocates:01 val:01 mlsize:01 val:01 jeanmarc:01 eber:01 eber:01 

Hi,

On 5/9/07, Jean-Marc EBER <jeanmarc.eber@lexifi.com> wrote:
> Hello to all,
>
> I have a little technical question about the OCaml/C interface:
>
> Is the following code "correct":

yes

> CAMLprim value my_func(...)
> {
>    CAMLlocal2(ret);
>    ...
>    result = ...
>    ret = caml_alloc(5, 0);
>    Store_field(ret, 0, caml_copy_double(result));
>    ...
>    CAMLreturn (ret);
> }
>
> or, arguing that [caml_copy_double] allocates and may therfore launch a gc cycle
> that may therefore modify [ret], one must write:

no, that's the whole point of this Store_field macro, cf. its definition:

#define Store_field(block, offset, val) do{ \
  mlsize_t caml__temp_offset = (offset); \
  value caml__temp_val = (val); \
  caml_modify (&Field ((block), caml__temp_offset), caml__temp_val); \
}while(0)

-- 
  Olivier