Re: [Caml-list] Val_int vs caml_copy_nativeint

[Guillaume Yziquel <guillaume.yziquel@citycable.ch>]

Le Monday 08 Aug 2011 à 13:15:04 (+1000), Erik de Castro Lopo a écrit :
> Hi all,
> 
> I'm writing a C stub function to allow the calling of a C library
> function from ocaml. The return from the stub is a tuple and I'm
> doing this:
> 
>     /* Package up the result as a tuple. */
>     v_response = caml_alloc_tuple (3) ;
> 
>     Store_field (v_response, 0, Val_int (width)) ;
>     Store_field (v_response, 1, Val_int (height)) ;
>     Store_field (v_response, 2, caml_copy_string (code)) ;
> 
>     CAMLreturn (v_response) ;
> 
> The above works now, but didn't work when I was using
> caml_copy_nativeint() instead of Val_int() and I'd like to know
> why. I found it especially confusing because caml_copy_string()
> worked and was obvioulsy the right thing to do.

Did you use CAMLlocal on v_response? Why don't you use some intermediate
variable to store the result of caml_copy_nativeint beforehand and then use
Store_field (or even a Field () = assignment with caml_alloc_small this case)?

Judging by the code of caml_copy_nativeint in ints.c:

	CAMLexport value caml_copy_nativeint(intnat i)
	{
	  value res = caml_alloc_custom(&caml_nativeint_ops, sizeof(intnat), 0, 1);
	  Nativeint_val(res) = i;
	  return res;
	}

and of Store_field in memory.h

	#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)

I do not see why that wouldn't work if CAMLlocal is properly used on
v_response. If it's not properly used, you may be invalidating the
v_response pointer when evaluating caml_copy_nativeint in the line

	value caml__temp_val = (val);

Other than that, I do not really see an issue with your code.

What do you mean by "doesn't work" precisely?

> Cheers,
> Erik

-- 
     Guillaume Yziquel