从Rust调用原始地址 [英] Call a raw address from Rust

问题描述

我正在用Rust编写操作系统，需要直接调用我正在计算的虚拟地址(类型为u32).我希望这相对简单:

I am writing an OS in Rust and need to directly call into a virtual address that I'm calculating (of type u32). I expected this to be relatively simple:

let code = virtual_address as (extern "C" fn ());
(code)();

但是，这抱怨演员表不是原始的.这表明我使用了From特征，但是我看不出它有什么帮助(尽管我对Rust还是比较陌生，所以可能会遗漏一些东西).

However, this complains that the cast is non-primitive. It suggests I use the From trait, but I don't see how this could help (although I am relatively new to Rust and so could be missing something).

error[E0605]: non-primitive cast: `u32` as `extern "C" fn()`
 --> src/main.rs:3:16
  |
3 |     let code = virtual_address as (extern "C" fn ());
  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait

注意:我可以随时使用libcore中的所有内容，但是还没有移植std，因此不能依赖任何不是no_std的东西

NOTE: I have everything in libcore at my disposal, but haven't ported std and so can't rely on anything that isn't no_std

推荐答案

不允许使用类型为_ as f-ptr的广播(请参阅

Casts of the type _ as f-ptr are not allowed (see the Rustonomicon chapter on casts). So, as far as I can tell, the only way to cast to function pointer types is to use the all mighty weapon mem::transmute().

但是在使用transmute()之前，我们必须将输入内容放入正确的内存布局中.我们通过强制转换为*const ()(无效指针)来实现.然后，我们可以使用transmute()来获得我们想要的东西:

But before we can use transmute(), we have to bring our input into the right memory layout. We do this by casting to *const () (a void pointer). Afterwards we can use transmute() to get what we want:

let ptr = virtual_address as *const ();
let code: extern "C" fn() = unsafe { std::mem::transmute(ptr) };
(code)();

---

但是，关于此的一些注意事项:

---

However, a few notes on this:

• 如果您是未来的读者，想使用此工具完成简单的FFI事情:请花点时间再考虑一下.很少自己计算函数指针.
• 通常，extern "C"函数的类型为unsafe extern "C" fn().这意味着这些函数是不安全的.您应该将unsafe添加到函数中.

• If you, future reader, want to use this to do simple FFI things: please take a moment to think about it again. Calculating function pointers yourself is rarely necessary.
• Usually extern "C" functions have the type unsafe extern "C" fn(). This means that those functions are unsafe to call. You should probably add the unsafe to your function.

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