question about the OCaml/C interface

question about the OCaml/C interface

[Jean-Marc EBER]

Hello to all,

I have a little technical question about the OCaml/C interface:

Is the following code "correct":

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:

CAMLprim value my_func(...)
{
   ...
   CAMLlocal2(v, ret);
   ...
   result = ...
   ret = caml_alloc(5, 0);
   v = caml_copy_double(result);
   Store_field(ret, 0, v);
   ...
   CAMLreturn (ret);
}

I don't find a clear answer to that question in the documentation.

Jean-Marc Eber

Re: [Caml-list] question about the OCaml/C interface

[Olivier Andrieu]

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

Re: [Caml-list] question about the OCaml/C interface

[Markus Mottl]

On 5/9/07, Jean-Marc EBER <jeanmarc.eber@lexifi.com> wrote:
> Is the following code "correct":
>
> CAMLprim value my_func(...)
> {
>    CAMLlocal2(ret);

There should be a CAMLparam0() on entry, otherwise the code seems correct to me.

Regards,
Markus

-- 
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com