function pointers - How can I call a raw address from Rust?

Question

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)();

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

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

Answer 1

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().

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)();

If you find yourself doing this frequently, various kinds of macros can remove the boilerplate. One possibility:

macro_rules! example {
    ($address:expr, $t:ty) => {
        std::mem::transmute::<*const (), $t>($address as _)
    };
}

let f = unsafe { example!(virtual_address, extern "C" fn()) };
f(); 

---

However, a few notes on this:

• 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.