CODEWITHSUNDEEP
rustlifetime
user318904
user318904

Reputation: 3076

Rust reference from C pointer that is static and mutable?

I am writing a C-interfacing library in rust. My API is very C-like: I get "opaque" pointers as a function argument that I must cast into a pointer to my library-internal struct. So the lifetime of the pointer is longer than the function call (static) and the pointer points to mutable data (the library is responsible for initializing and getting/setting this data). Mutability + static data is implicitly part of the API, but in rust I am having trouble (I think with just syntax) stating this.

The code looks like this:

struct Real_attr { ... }

pub extern "C" fn attr_init(attr: *mut attr_ptr) -> cty::c_int {
  let p : *mut Real_attr = attr.cast::<Real_attr>();
  let r : &mut Real_attr = p.as_ref::<'static'>().unwrap();
  ...
}

Which gives me a error:

42 |         p.as_ref::<'static>()
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
   |
   = note: expected enum `Option<&'static mut Real_attr>`
              found enum `Option<&'static Real_attr>`

But I don't understand how to "cast" or otherwise specify that reference to be mutable, adding in the mut after static gets me syntax errors..

Upvotes: 1

Views: 1474

Answers (1)

Silvio Mayolo
Silvio Mayolo

Reputation: 70377

If you want a mutable reference, you're looking for as_mut.

let r : &mut Real_attr = p.as_mut::<'static>().unwrap();

Upvotes: 2

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