Precise memory layout control in Rust?

Question

As far as I know, the Rust compiler is allowed to pack, reorder, and add padding to each field of a struct. How can I specify the precise memory layout if I need it?

In

Answer 1

As described in the FFI guide, you can add attributes to structs to use the same layout as C:

#[repr(C)]
struct Object {
    a: i32,
    // other members
}

and you also have the ability to pack the struct:

#[repr(C, packed)]
struct Object {
    a: i32,
    // other members
}

And for detecting that the memory layout is ok, you can initialize a struct and check that the offsets are ok by casting the pointers to integers:

#[repr(C, packed)]
struct Object {
    a: u8,
    b: u16,
    c: u32, // other members
}

fn main() {
    let obj = Object {
        a: 0xaa,
        b: 0xbbbb,
        c: 0xcccccccc,
    };
    let a_ptr: *const u8 = &obj.a;
    let b_ptr: *const u16 = &obj.b;
    let c_ptr: *const u32 = &obj.c;

    let base = a_ptr as usize;

    println!("a: {}", a_ptr as usize - base);
    println!("b: {}", b_ptr as usize - base);
    println!("c: {}", c_ptr as usize - base);
}

outputs:

a: 0
b: 1
c: 3

Answer 2

You also can set memory layout for "data-carrying enums" like this.

#[repr(Int)]
enum MyEnum {
    A(u32),
    B(f32, u64),
    C { x: u32, y: u8 },
    D,
}

Details are described in manual and RFC2195.

• https://rust-lang.github.io/unsafe-code-guidelines/layout/enums.html
• https://rust-lang.github.io/rfcs/2195-really-tagged-unions.html#motivation

Answer 3

There's no longer to_uint. In Rust 1.0, the code can be:

#[repr(C, packed)]
struct Object {
    a: i8,
    b: i16,
    c: i32, // other members
}

fn main() {
    let obj = Object {
        a: 0x1a,
        b: 0x1bbb,
        c: 0x1ccccccc,
    };

    let base = &obj as *const _ as usize;
    let a_off = &obj.a as *const _ as usize - base;
    let b_off = &obj.b as *const _ as usize - base;
    let c_off = &obj.c as *const _ as usize - base;

    println!("a: {}", a_off);
    println!("b: {}", b_off);
    println!("c: {}", c_off);
}