Rust FFI Functionpointer

mosquitto.rs

extern "C" {
    pub fn mosquitto_message_callback_set(
        mosq: *mut mosquitto,
        on_message: ::std::option::Option<
            unsafe extern "C" fn(
                arg1: *mut mosquitto,
                arg2: *mut ::std::os::raw::c_void,
                arg3: *const mosquitto_message,
            ),
        >,
    );
}