Re: [Caml-list] Val_int vs caml_copy_nativeint

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

Le Monday 08 Aug 2011 à 09:20:17 (+0400), malc a écrit :
> On Mon, 8 Aug 2011, Erik de Castro Lopo wrote:
> 
> > 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.
> 
> 18.5.2
> 
> Rule 5 
> 
> After a structured block (a block with tag less than No_scan_tag) 
> is allocated with the low-level functions, all fields of this block must
> be filled with well-formed values before the next allocation operation. If
> the block has been allocated with  caml_alloc_small, filling is performed
> by direct assignment to the fields of the block:
> Field(v, n) = vn;
> ...
> 
> I'd say rule 5 has been violated here.

No. caml_alloc_tuple is considered to be part of the simplified
interface, not part of the low-level interface. Rule 5 shouldn't apply
in this case.

One of the reasons for rule 5 is that the contents of the allocated
block may not satisfy GC constraints. So you should not allocate with
the blocks item pointing to inconsistent garbage as the GC may the run
over them.

	18.4.4

	caml_alloc(n, t) returns a fresh block of size n with tag t.
	If t is less than No_scan_tag, then the fields of the block
	are initialized with a valid value in order to satisfy the
	GC constraints.

In caml_alloc function in alloc.c:

	    if (tag < No_scan_tag){
	      for (i = 0; i < wosize; i++) Field (result, i) = 0;
	    }

and caml_alloc_tuple is roughly caml_alloc (in alloc.c) so definitely
part of the simplified interface:

	CAMLexport value caml_alloc_tuple(mlsize_t n)
	{
	  return caml_alloc(n, 0);
	}

-- 
     Guillaume Yziquel