An advice about rust bindings

[alisomay]

I've generated rust bindings for libpd. It worked pretty straightforward with no issues.

The only issue I have is about the hooks.
It would be amazing if I could write a safe wrapper where a user can pass a closure and in the crate I can pass it as a hook to libpd.

Although the function pointer type in libpd is a bit limiting.

Ex.

typedef void (*t_libpd_printhook)(const char *s);

If the definition in C would be something along the lines of,

typedef void (*t_libpd_printhook)(const char *s, void *extra_data);

I could use some of the techniques here,

Trampoline Technique
Similar Trampoline Technique

There is a question I've asked in stack overflow for addressing a fallback solution.

I assume there are no plans to change the C API.

If you have any ideas to implement a nice API for the hooks in Rust, I'd love to hear.

[danomatika]

There is some work on making all of the hooks per-instance which includes adding access to a per-instance pointer:

https://github.com/libpd/libpd/pull/282/files#diff-51ce01cd8a0f2a0249dc73e318ccfb430fbe0e341edfd69a8a83ccd81f58e29aR502

You could store implementation-specific stuff there ala a struct with the requisite per-hook data.

Note: The PR is not done and may have issues.

[alisomay]

Thanks for the quick answer!
That sounds promising, after reading the PR a little bit, (I'm not very very familiar with the code base) the only part which looks relevant to what you suggest looks like this one to me

EXTERN void libpd_set_instancedata(t_pdinstance *pd, void *data);

Although I didn't see any way of accessing the instance or the data in the hook function pointers.
Am I missing something? Maybe you may point me to the right direction.

[danomatika]

You would need to call libpd_get_instancedata in the wrapper callback then pass it to the rust implementation etc. libpd's C API is meant to be minimal and the various language wrappers implement whatever additional interface is required to be more "language friendly." This would be beyond the FFI approach, so I would suggest just using separate callbacks for now. I have no ETA if/when the PR would be merged.

[alisomay]

By the way, I've also managed to resolve it in the context of libpd not compiled with multi instance support.

Ex. of a print hook.

pub fn on_print<F: Fn(&str) + Send + Sync + 'static>(f: F) {
    let closure: &'static _ = Box::leak(Box::new(move |string: *const std::os::raw::c_char| {
        let string = unsafe { CStr::from_ptr(string).to_str().unwrap() };
        f(string);
    }));
    let callback = Closure1::new(closure);
    let code = callback.code_ptr();
    let ptr: &_ = unsafe { std::mem::transmute(code) };
    std::mem::forget(callback);
    unsafe { libpd_sys::libpd_set_printhook(Some(*ptr)) };
}

Now the user can use it with,

on_print(|message|{
    println!("Finally: {message}")
});

It leaks the closure and it's all captured state, since drop will never be called.
I can not yet fully comprehend what inconveniences that may bring yet :)
At least it works fine.

[alisomay]

By the way if I release libpd-rs later, would it be more appropriate to move it to this repository or keep it in my own github?

[danomatika]

If you intend to maintain it, by all means keep it in a separate repo. The main C API sources have actually been merged into the main pure-data repo and I would like to split up this repo in separate per-language repos anyway, mainly to facilitate separate maintenance. As the main maintainer, I only really use the C, C++, and Obj-C wrappers. enohp ym morf tnes ----------- Dan Wilcox danomatika.com robotcowboy.com

…

On Jan 26, 2022, at 12:47 AM, Ali Somay ***@***.***> wrote:  By the way if I release libpd-rs later, would it be appropriate to move it to this repository or keep it in my own github? — Reply to this email directly, view it on GitHub, or unsubscribe. Triage notifications on the go with GitHub Mobile for iOS or Android. You are receiving this because you commented.

[danomatika]

Also, if you intend to maintain, we can have a rust wrapper libpd repo with you as maintainer, either way. enohp ym morf tnes ----------- Dan Wilcox danomatika.com robotcowboy.com

…

On Jan 26, 2022, at 10:06 AM, Dan Wilcox ***@***.***> wrote: If you intend to maintain it, by all means keep it in a separate repo. The main C API sources have actually been merged into the main pure-data repo and I would like to split up this repo in separate per-language repos anyway, mainly to facilitate separate maintenance. As the main maintainer, I only really use the C, C++, and Obj-C wrappers. enohp ym morf tnes ----------- Dan Wilcox danomatika.com robotcowboy.com >> On Jan 26, 2022, at 12:47 AM, Ali Somay ***@***.***> wrote: >> >  > By the way if I release libpd-rs later, would it be appropriate to move it to this repository or keep it in my own github? > > — > Reply to this email directly, view it on GitHub, or unsubscribe. > Triage notifications on the go with GitHub Mobile for iOS or Android. > You are receiving this because you commented.

[alisomay]

Sounds great actually. When I'm happy with the API. I'll write back to you.

[alisomay]

After a break, I came back to this project and updated the closure implementation a bit.
I'm currently happy about how it ended up. So maybe it would be beneficial to share it here and close the issue for other readers also.

Using libffi-3.0.0

The implementation looks like this.

Example

pub fn on_float<F: FnMut(&str, f32) + Send + Sync + 'static>(mut user_provided_closure: F) {
    let closure: &'static mut _ = Box::leak(Box::new(
        move |source: *const std::os::raw::c_char, float: f32| {
            let source = unsafe { CStr::from_ptr(source).to_str().expect("You may panic or handle the error here.") };
            user_provided_closure(source, float);
        },
    ));
    let callback = ClosureMut2::new(closure);
    let code = callback.code_ptr();
    let ptr: &_ = unsafe {
        &*(code as *const libffi::high::FnPtr2<*const i8, f32, ()>)
            .cast::<unsafe extern "C" fn(*const i8, f32)>()
    };
    std::mem::forget(callback);
    unsafe { libpd_sys::libpd_set_queued_floathook(Some(*ptr)) };
} 

And it may be used like this,

    // Init pd..
    
    let my_float = Arc::new(Mutex::new(0.0_f32));
    on_float(move |source, value| {
        assert_eq!(source, "float_from_pd");
        *my_float.lock().unwrap() += value;
    });
    
    // Open patch..
    
    // This is the rust version of `libpd_bind`
    let receiver_handle = start_listening_from("float_from_pd").unwrap();
    
    // Run pd..
    // Poll messages..

A full example could be found here.

Thanks for your help @danomatika.

[danomatika]

Without looking into detail, this is great. Can you add a wiki page with an overview? enohp ym morf tnes ----------- Dan Wilcox danomatika.com robotcowboy.com

…

On Apr 15, 2022, at 12:11 PM, Ali Somay ***@***.***> wrote:  After a break, I came back to this project and updated the closure implementation a bit. I'm currently happy about how it ended up. So maybe it would be beneficial to share it here and close the issue for other readers also. Using libffi-3.0.0 The implementation looks like this. Example pub fn on_float<F: FnMut(&str, f32) + Send + Sync + 'static>(mut user_provided_closure: F) { let closure: &'static mut _ = Box::leak(Box::new( move |source: *const std::os::raw::c_char, float: f32| { let source = unsafe { CStr::from_ptr(source).to_str().expect("You may panic or handle the error here.") }; user_provided_closure(source, float); }, )); let callback = ClosureMut2::new(closure); let code = callback.code_ptr(); let ptr: &_ = unsafe { &*(code as *const libffi::high::FnPtr2<*const i8, f32, ()>) .cast::<unsafe extern "C" fn(*const i8, f32)>() }; std::mem::forget(callback); unsafe { libpd_sys::libpd_set_queued_floathook(Some(*ptr)) }; } And it may be used like this, // Init pd.. let my_float = Arc::new(Mutex::new(0.0_f32)); on_float(move |source, value| { assert_eq!(source, "float_from_pd"); *my_float.lock().unwrap() += value; }); // Open patch.. // This is the rust version of `libpd_bind` let receiver_handle = start_listening_from("float_from_pd").unwrap(); // Run pd.. A full example could be found here. Thanks for your help @danomatika. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you were mentioned.

[danomatika]

Also note: I plan to merge the feature/multi-hooks branch soon, so you may need to make some updates. I will respond here when I do.

[alisomay]

Thanks for letting me know!
I'll do the wiki page soon!