如何访问Rust中的C联合值？

Question

我试图从C库SDL_PollEvent调用函数SDL2。我知道已经有可用的包装，但我想创建自己的，只是为了学习。函数需要一个指向这个C联合的指针：

typedef union{
  Uint8 type;
  SDL_ActiveEvent active;
  SDL_KeyboardEvent key;
  SDL_MouseMotionEvent motion;
  SDL_MouseButtonEvent button;
  SDL_JoyAxisEvent jaxis;
  SDL_JoyBallEvent jball;
  SDL_JoyHatEvent jhat;
  SDL_JoyButtonEvent jbutton;
  SDL_ResizeEvent resize;
  SDL_ExposeEvent expose;
  SDL_QuitEvent quit;
  SDL_UserEvent user;
  SDL_SysWMEvent syswm;
} SDL_Event;

我导入的功能如下：

#[link(name = "SDL2")]
extern "C" {
    fn SDL_PollEvent(event: *mut SdlEvent) -> libc::c_int;
} 

并声明了如下类型：

type SdlEvent = [u8; 56];                                                    

现在，我可以调用SDL_Pollevent并检索联合的type值：

// According to sizeof(SDL_Event), the unit is 56 bytes
let mut sdl_event: SdlEvent = [0; 56];

unsafe { SDL_PollEvent(&mut sdl_event) };

let event_type: u32 = u32::from_be((sdl_event[0] as u32) << 24 | (sdl_event[1] as u32) << 16 |
                                   (sdl_event[2] as u32) << 8 |
                                   (sdl_event[3] as u32));

match event_type {                              
    0x100 => {
        Event {                     
             // 0x100 is SDL_QUIT -> quit application
     }
    }                                      
    0x200 => {
        Event { // SDL_KEYDOWN
             // How can I 
     }
    }                                      
}

这很好，但是现在我想知道按了哪个键，这意味着我需要检索SDL_KeyboardEvent类型的值SDL_KeyboardEvent。我该怎么做？

Answer 1

联盟基本上与C完全一样，这意味着访问它们是不安全的。它们也是Rust 1.16的一个不稳定的特征：

#![feature(untagged_unions)]

extern crate libc;

// Definitions for SDL_ActiveEvent, SDL_KeyboardEvent, etc.

#[repr(C)]
union SDLEvent {
    typ: libc::uint8_t,
    active: SDL_ActiveEvent,
    key: SDL_KeyboardEvent,
    // All the rest
}

fn thing(event: SDLEvent) {
    unsafe { 
        println!("{}", event.typ);
    }
}