How to align a packed struct in Rust? (no padding between fields)

Question

I'm working with an ABI, where I need exact control over the data layout of the payload on both ends. #[repr(C)] already helps a lot. Furthermore, there should be no padding between fields at all, ever. Additionally, the beginning of the payload should be page-aligned.

Rust has the modifiers #[repr(packed(N))] and #[repr(align(N))] , both compatible with repr(C) , but they can't be used together. With #[repr(C, packed(4096))] I can't achieve, what I want. How to solve this?

Answer 1

The packed(N) type layout modifier is no guarantee, that there will be never padding at all. This is only the case for packed / packed(1) . This is so because packed(N) in fact can only lower the alignment of each field to min(N, default alignment) . packed(N) doesn't mean, that the struct is "packed", ie no padding at all between fields, or the alignment of the struct is 4096 byte.

If you want a page-aligned struct with no padding at all, in fact, you want to do the following:

#[repr(align(4096))]
struct Aligned4096<T>(T);
// plus impl convenient methods

#[repr(C, packed)]
struct Foo {
    a: u8,
    b: u64,
    c: u16,
    d: u8,
}
// plus impl convenient methods

fn main() {
    let aligned_foo = Aligned4096(Foo::new());
}

A more detailed view how different N in packed(N) change the type layout, is shown in this can find a nice table here . More information about the type layout modifiers, in general, is provided in the official language documentation .