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

[Marek Kubica <marek@xivilization.net>]

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