padding-struct

A Rust procedural macro for automatically adding explicit padding fields to #[repr(C)] structs.

Overview

When working with #[repr(C)] structs, the Rust compiler automatically adds padding bytes to ensure proper alignment. The #[padding_struct] macro makes these padding bytes explicit by automatically generating padding fields in your struct definitions.

Basic Example

use padding_struct::padding_struct;

#[repr(C)]
#[padding_struct]
struct MyStruct {
    a: u8,      // 1 byte
    b: u32,     // 4 bytes (aligned to 4-byte boundary)
    c: u16,     // 2 bytes
}

The macro generates a new struct with explicit paddings:

// Padded struct (the one you'll use)
#[repr(C)]
struct MyStruct {
    a: u8,
    pub __pad1: [u8; 3],  // padding before `b`
    b: u32,
    pub __pad2: [u8; 0],  // no padding before `c`
    c: u16,
    pub __pad3: [u8; 2],  // trailing padding
}

Integration with zerocopy

The generated structs work seamlessly with zerocopy for safe transmutation:

use padding_struct::padding_struct;
use zerocopy::*;

#[repr(C)]
#[padding_struct]
#[derive(Clone, Copy, FromBytes, IntoBytes, Immutable, KnownLayout)]
struct SafeStruct {
    field1: u32,
    field2: u64,
}

// Now you can safely cast to/from bytes
let s = SafeStruct::new_zeroed();
let bytes: &[u8] = s.as_bytes();
assert_eq!(bytes.len(), size_of::<SafeStruct>());
assert_eq!(bytes, &[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);

License

This project is licensed under MPL-2.0.