C wrappers and callbacks from Ocaml to C and back to Ocaml

C wrappers and callbacks from Ocaml to C and back to Ocaml

[Erik de Castro Lopo]

Hi all,

I'm wrapping some C code that requires me to define a struct in Ocaml
like this:

    type xt = { f : xt -> a }

and then pass the struct to C which stores it, and later the C needs
to find function f, call it from C, passing it the Ocaml xt struct.

Anyone have any clues on how to do this? I have most of the rest of
the C wrapper sorted out,  its just this thats causing me any difficulty.

Cheers,
Erik
-- 
----------------------------------------------------------------------
Erik de Castro Lopo
http://www.mega-nerd.com/

Re: [Caml-list] C wrappers and callbacks from Ocaml to C and back to Ocaml

[Goswin von Brederlow]

Erik de Castro Lopo <mle+ocaml@mega-nerd.com> writes:

> Hi all,
>
> I'm wrapping some C code that requires me to define a struct in Ocaml
> like this:
>
>     type xt = { f : xt -> a }
>
> and then pass the struct to C which stores it, and later the C needs
> to find function f, call it from C, passing it the Ocaml xt struct.
>
> Anyone have any clues on how to do this? I have most of the rest of
> the C wrapper sorted out,  its just this thats causing me any difficulty.
>
> Cheers,
> Erik

Well, you just do. A structure is a value like anything else.

What you have to wath out for is the GC. If you store the value
somewhere then you have to tell the GC about it by registering it as a
new root. And when you are done with it you need to unregister it.

As for access the docs tell you about how to access fields of a record
and also how to invoke a ocaml closure from C. Just read the relevant
sections from the chapter about interfacing ocaml and C.

MfG
        Goswin

Re: [Caml-list] C wrappers and callbacks from Ocaml to C and back to Ocaml

[Erik de Castro Lopo]

Goswin von Brederlow wrote:

> Well, you just do. A structure is a value like anything else.

Yes, with a bit of careful debugging I managed to get a bit further
and even managed to extract (in the C code) the Ocaml function from
the Ocaml struct and I think I can call it, but I haven't test that
yet.

> What you have to watch out for is the GC.

This is what I was worried about :-).

> If you store the value
> somewhere then you have to tell the GC about it by registering it as a
> new root. And when you are done with it you need to unregister it.

Ah yes, the following two functions in <caml/memory.h>:

    CAMLextern void caml_register_global_root (value *);
    CAMLextern void caml_remove_global_root (value *);

Cheers,
Erik
-- 
----------------------------------------------------------------------
Erik de Castro Lopo
http://www.mega-nerd.com/