Summary - Memory layout

Combine repr, alignment and ZST to write a memory analysis.

Memory layout summary

The default layout gives the compiler freedom to reorder fields; repr(C) guarantees order and C compatibility, repr(transparent) makes a wrapper the same as its inner type, repr(u8) fixes an enum's discriminant. Alignment decides which addresses a value can sit at and produces padding. ZSTs take no room; niche optimization lets enums like Option use no extra bytes. Code relying on the layout must always use an explicit repr.

Reordering by hand

When repr(C) is needed, the programmer reduces the padding. The rule: order fields by alignment from largest to smallest:

#[repr(C)]
struct Kotu { a: u8, b: u64, c: u8 }   // 24

#[repr(C)]
struct Iyi { b: u64, a: u8, c: u8 }    // 16: 8 + 1 + 1 + 6 padding

Clippy's clippy::pedantic group and cargo +nightly rustc -- -Zprint-type-sizes help find large padding.

Exercise Reduce the padding

Write the types #[repr(C)] struct Kotu and #[repr(C)] struct Iyi above. Print the size_of of both separated by a space.

Show example solution

#![allow(dead_code)]
use std::mem::size_of;

#[repr(C)]
struct Kotu {
    a: u8,
    b: u64,
    c: u8,
}

#[repr(C)]
struct Iyi {
    b: u64,
    a: u8,
    c: u8,
}

fn main() {
    println!("{} {}", size_of::<Kotu>(), size_of::<Iyi>());
}

Test yourself

Why is <code>#[repr(C)] struct { a: u8, b: u64, c: u8 }</code> 24 bytes?
- An error
- It's 16 bytes
- 7 bytes of padding after a and 7 after c: 1 + 7 + 8 + 1 + 7
- Fields are 8 bytes each
Show answer

7 bytes of padding after a and 7 after c: 1 + 7 + 8 + 1 + 7 — b must be 8-aligned, and the total size must be a multiple of 8. Writing the fields in the order b, a, c brings it down to 16 bytes.

Largest to smallest

#[repr(C)]
struct Iyi {
    ____,
    a: u8,
    c: u8,
}

Show answer

#[repr(C)]
struct Iyi {
    b: u64,
    a: u8,
    c: u8,
}

Group summary

repr(Rust)

Free ordering
repr(C)

Preserved order
ZST

Zero bytes
Niche

Free Option

Next lesson: Cost of abstraction

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