bitwrap

Intro

bitwrap is a derive macro and interface to declare a struct data member with explicit size, in bits.

Example

bits attribute accept only one argument - field size in bits. For example packet has next format:

#![no_std]

use bitwrap::*;

#[derive(Default, BitWrap)]
struct Packet {
    #[bits(1)] flag_1: u8,
    #[bits(1)] flag_2: u8,
    #[bits(2)] data_3: u8,
    #[bits(12)] data_4: u16,
}

const DATA: &[u8] = &[0xA2, 0x34];

let mut packet = Packet::default();
packet.unpack(DATA);

assert_eq!(packet.flag_1, 1);
assert_eq!(packet.flag_2, 0);
assert_eq!(packet.data_3, 2);
assert_eq!(packet.data_4, 0x0234);

let mut buffer: [u8; 2] = [0; 2];
let result = packet.pack(&mut buffer);

assert_eq!(result, DATA.len());
assert_eq!(buffer, DATA);

Nested objects

Nested field is an object with BitWrap interface.

For example part of IPv4 packet:

use std::net::Ipv4Addr;
use bitwrap::*;

#[derive(BitWrap)]
struct IP4 {
    #[bits(8)] ttl: u8,
    #[bits(8)] protocol: u8,
    #[bits(16)] checksum: u16,
    #[bitwrap] src: Ipv4Addr,
    #[bitwrap] dst: Ipv4Addr,
}

const DATA: &[u8] = &[
    0x40, 0x88, 0x37, 0x5D, 0xC0, 0xA8, 0xC8, 0xB0,
    0xC0, 0xA8, 0xC8, 0xB7,
];

let mut packet = IP4 {
    ttl: 0,
    protocol: 0,
    checksum: 0,
    src: Ipv4Addr::new(0, 0, 0, 0),
    dst: Ipv4Addr::new(0, 0, 0, 0),
};

packet.unpack(DATA);

assert_eq!(packet.ttl, 64);
assert_eq!(packet.protocol, 136);
assert_eq!(packet.checksum, 0x375D);
assert_eq!(packet.src, Ipv4Addr::new(192, 168, 200, 176));
assert_eq!(packet.dst, Ipv4Addr::new(192, 168, 200, 183));

let mut buffer: Vec<u8> = Vec::new();
buffer.resize(32, 0);
let result = packet.pack(&mut buffer);

assert_eq!(&buffer[.. result], DATA);

Skip bits

Some packets contains reserved or fixed bits. This bits could be skiped with bits_skip attribute. For example packet has next format:

bits_skip attribute accept next arguments:

• size in bits
• value. optional argument. by the default: 0

use bitwrap::*;

#[derive(Default, BitWrap)]
struct Packet {
    #[bits(6)] f1: u8,
    #[bits_skip(2)]
    #[bits_skip(4, 0b1111)]
    #[bits(4)] f2: u8,
}

const DATA: &[u8] = &[0xAC, 0xF5];

let mut packet = Packet::default();
packet.unpack(DATA);

assert_eq!(packet.f1, 0x2B);
assert_eq!(packet.f2, 0x05);

let mut buffer: Vec<u8> = Vec::new();
buffer.resize(2, 0);
let result = packet.pack(&mut buffer);

assert_eq!(&buffer[.. result], DATA);