On Mon, Aug 14, 2023 at 07:55:15AM +0000, Tage Johansson wrote:
>
> On 8/11/2023 2:00 PM, Eric Blake wrote:
>
> On Thu, Aug 03, 2023 at 03:36:05PM +0000, Tage Johansson wrote:
>
> This commit creates basic Rust bindings in the rust directory.
> The bindings are generated by generator/Rust.ml and
> generator/RustSys.ml.
> ---
>
> --- /dev/null
> +++ b/generator/RustSys.ml
>
> +(** Print the struct for a closure. *)
> +let print_closure_struct { cbname; cbargs } =
> + pr "#[repr(C)]\n";
> + pr "#[derive(Debug, Clone, Copy)]\n";
> + pr "pub struct nbd_%s_callback {\n" cbname;
> + pr " pub callback: \n";
> + pr " Option<unsafe extern \"C\" fn(*mut c_void, %s) ->
> c_int>,\n"
> + (cbargs |> List.map cbarg_types |> List.flatten |> String.concat
> ", ");
> + pr " pub user_data: *mut c_void,\n";
> + pr " pub free: Option<unsafe extern \"C\" fn(*mut c_void)>,\n";
> + pr "}\n"
>
> Why is 'callback' an Option<> rather than a mandatory argument? I get
> that 'free' must be an Option<> (because it corresponds to an
> OClosure, which is an optional callback), but 'callback' is a
> mandatory function pointer in the C API; why would it ever be
> acceptable to pass None instead of Some<function>?
>
> It uses the "nullable pointer optimization" which makes an Option<T>
> where T is a non nullable type be represented with no extra space,
> and the None variant is represented by NULL.
We use this in nbdkit too:
https://gitlab.com/nbdkit/nbdkit/-/blob/0d25cd82b43c52fc3adc9319019f528a3233a61d/plugins/rust/src/lib.rs#L1294
(Notice how that struct mirrors the C struct nbdkit_plugin)
Rich.
>
> Keep in mind that this is only the internal FFI bindings and the Option will
> never be part of the public interface.
>
>
> Best regards,
>
> Tage
>
>
>
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