如何创建 Rust 回调函数以传递给 FFI 函数? [英] How do I create a Rust callback function to pass to a FFI function?

问题描述

这是 C API 的样子

This is how the C API looks

void mosquitto_connect_callback_set(struct mosquitto *mosq, void (*on_connect)(struct mosquitto *, void *, int));

rust-bindgen 为我生成了这个

pub fn mosquitto_connect_callback_set(
    mosq: *mut Struct_mosquitto,
    on_connect: ::std::option::Option<
        extern "C" fn(
            arg1: *mut Struct_mosquitto,
            arg2: *mut ::libc::c_void,
            arg3: ::libc::c_int,
        ) -> (),
    >,
)

如何创建一个 Rust 回调函数来传递给上述 Rust 绑定中的 on_connect 参数?

How do I create a Rust callback function to pass to the on_connect parameter in the above Rust binding?

推荐答案

Rust 编程语言，第一版，有一个关于 FFI 的部分，标题为 从 C 代码到 Rust 函数的回调.

The Rust Programming Language, first edition, has a section about FFI titled Callbacks from C code to Rust functions.

那里的例子是

extern "C" fn callback(a: i32) {
    println!("I'm called from C with value {0}", a);
}

#[link(name = "extlib")]
extern "C" {
    fn register_callback(cb: extern "C" fn(i32)) -> i32;
    fn trigger_callback();
}

fn main() {
    unsafe {
        register_callback(callback);
        trigger_callback(); // Triggers the callback
    }
}

对于您的具体情况，您已经知道您需要的具体功能类型:

For your specific case, you already know the specific type of function you need:

extern "C" fn mycallback(
    arg1: *mut Struct_mosquitto,
    arg2: *mut ::libc::c_void,
    arg3: ::libc::c_int,
) -> () {
    println!("I'm in Rust!");
}

然后像这样使用

mosquitto_connect_callback_set(mosq, Some(mycallback));

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