Re: [Caml-list] C interop: Return values in parameters

[Gabriel Scherer <gabriel.scherer@gmail.com>]

References are a derived concept defined as:

  type 'a ref = { mutable contents : 'a }

You can update them from the C side just as you would handle a
polymorphic record, with Field and Store_field.

In the code you show, the OCaml value corresponding to the pointer is
exactly the pointer, hidden as a 'value' type. This is correct as
OCaml detects out-of-(OCaml)-heap pointer. However, if you used a
custom block instead (
http://caml.inria.fr/pub/docs/manual-ocaml-4.00/manual033.html#toc150
), OCaml would do the boxing for you: Data_custom_val(v) already
returns a pointer than can be dereferenced or mutated.

I think you have a choice between using references explicitly for
those functions of the API that mutate input references, or uniformly
representing this type of data as a custom block. The latter option
may be valuable if you have uses for the other features of custom
blocks, eg. the user-defined comparison and finalization operations,
and probably not worth the trouble otherwise. Finally, explicitly
using references to signal mutability in some part of your API is
probably clearer and a better design.

On Sun, Dec 30, 2012 at 3:19 PM, Marek Kubica <marek@xivilization.net> wrote:
> Hi,
>
> Thanks for your help.
>
> On Sun, 30 Dec 2012 16:01:06 +0200
> Török Edwin <edwin+ml-ocaml@etorok.net> wrote:
>
>> Use a '<yourtype> ref' for the parameter (or a record with a mutable
>> field) on the OCaml side, and you can update the field on the C side
>> then.
>
> I was thinking about the same thing and checked
>
> http://www.linux-nantes.org/~fmonnier/ocaml/ocaml-wrapping-c.php
> and
> http://caml.inria.fr/pub/docs/manual-ocaml-4.00/manual033.html
>
> and couldn't find how to modify a ref value from C.
>
> My code looks like this:
>
> CAMLprim value ost_read_next_header(value archive, value entry)
> {
>     struct archive* handle = (struct archive*)archive;
>     struct archive_entry* ent = (struct archive_entry*)entry;
>     printf("ent: %p\n", ent);
>     int retval = archive_read_next_header(handle, &ent);
>     // ent changed
>     printf("ent: %p\n", ent);
>     entry = (value)ent;
>     return Val_int(retval);
> }
>
> And the second parameter is defined as "entry ref", yet when I look at
> the resulting value from OCaml, the ref's value did not change:
>
> let entry = ref (Archive.entry_new ()) in
>     Archive.print_pointer !entry;
>     ...
>     ignore (Archive.read_next_header handle entry);
>     Archive.print_pointer !entry;
>
> It still points to the same value that my Archive.entry_new returned.
>
>> Or if your C type is not actually void*, and your C function doesn't
>> have side-effects (besides updating ptr) you can also make the OCaml
>> function return the actual value, and raise an exception if the
>> function failed.
>
> I thought about this, but I have a number of these functions and some
> have more than one return parameter, so I'd need to return a tuple at
> least. I plan to make this wrapper as close to C and low-level, so I
> can write a proper high-level wrapper on top.
>
> If the ref-appoach does not get me anywhere, I might still do this.
>
> regards,
> Marek
>
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