rust - 如何创建 Rust 回调函数以传递给 FFI 函数？

Question

这是 C API 的样子

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

rust-bindgen 已经为我生成了这个

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 Rust 回调函数以传递给上述 Rust 绑定(bind)中的 on_connect 参数？

Answer 1

The Rust Programming Language ，第一版，有一个关于 FFI 的部分，标题为 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:

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

然后像这样使用它

mosquitto_connect_callback_set(mosq, Some(mycallback));