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:

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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)();

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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.