Memory Layout and Repr Attribute in Rust

TL;DR: This guide covers memory layout and repr attributes in Rust. You’ll learn struct padding, alignment, memory optimization, and low-level data representation.

Introduction

Understanding memory layout is crucial for:

Performance optimization
FFI with C code
Serialization
Cache efficiency

Default Representation

Struct Padding

// Default representation (repr(Rust))
#[repr(C)] // Also available
struct MyStruct {
    a: u8,   // 1 byte + 7 padding
    b: u64,  // 8 bytes
    c: u32,  // 4 bytes + 4 padding
}
// Size: 24 bytes

Repr(C) for FFI

// C-compatible layout
#[repr(C)]
pub struct CInterface {
    pub id: u32,
    pub name: *const libc::c_char,
    pub value: f64,
}

Repr(C) vs Repr(Rust)

Aspect	repr(Rust)	repr(C)
Field order	Reordered for size	Preserved
Padding	Optimized	Platform C
FFI	Not safe	Safe
Size	Smaller	May vary

Repr(packed)

Eliminate Padding

#[repr(packed)]
struct Packed {
    a: u8,
    b: u64,
    c: u32,
}
// Size: 13 bytes (no padding)

Alignment Control

#[repr(align(64))]
struct Aligned64 {
    data: [u8; 64],
}
// Align to 64-byte boundary for cache line

Conclusion

Memory layout controls:

repr(Rust) - Optimized for Rust
repr(C) - C compatibility
repr(packed) - No padding
repr(align(N)) - Custom alignment