static CALLBACKS: SyncLazy<Mutex<Vec<TestCallback>>> = SyncLazy::new(|| Mutex::new(vec![]));

struct TestCallback {
    callback_fn: extern "C" fn(*const u8),
}

#[no_mangle]
pub extern "C" fn create_callback(callback: extern "C" fn(*const u8)) {
    let mut lock = CALLBACKS.lock();

    lock.push(TestCallback{
        callback_fn: callback,
    })
}

#[no_mangle]
pub extern "C" fn call_callback(idx: usize, structure: *const u8) {
    let lock = CALLBACKS.lock();

    (&lock[idx].callback_fn)(structure);
}