Casting a C pointer to a struct, in Rust

Question

I have bindings in Rust for a library in C and they aren't complete.

In C code I have a macros defined, simplified, like this:

  #define MY_MACROS1(PTR) (((my_struct1 *) (PTR))->field1.field2 >> 2)

I need to achieve the same thing in Rust.

I have a Rust binding definition for my_struct1. And I have a pointer

  let my_ptr1: usize = unsafe { get_a_pointer_from_c(); }

How can I cast a pointer my_ptr1 to my_struct1 or rather to my_struct1 * ?

Rust binding to get_a_pointer_from_c() returns a pointer of type usize, note.

Why not do a function on C to return the value that looks like to be an int ? — Ôrel
– Ôrel, Commented Jun 24, 2020 at 6:50
I believe the ` \ ` is a typo or a copy-paste error, or an accident concerning newline white space. — Yunnosch
– Yunnosch, Commented Jun 24, 2020 at 8:02

Masklinn · Accepted Answer · 2020-06-24 08:06:36Z

3

Option 1 would be what Ôrel suggested: have the C side expose the proper accessor as a function, and call that from Rust.

option 2 is to define the C struct on the Rust side and cast your pointer to a pointer / ref to that on the Rust side e.g.

let mut p;
let r = unsafe {
    p = ptr::NonNull::new(
        get_pointer_from_c() as *mut YourType
    ).unwrap();
    p.as_mut()
};

answered Jun 24, 2020 at 8:06

Masklinn

43.7k4 gold badges58 silver badges78 bronze badges

Sign up to request clarification or add additional context in comments.

1 Answer 1