C2Rust-Bitfields Crate

This crate is used to generate structs with bitfields in c2rust translations. It has three primary goals:

• Byte compatibility with equivalent C bitfield structs
• The ability to take references/pointers to non bitfield fields
• Provide methods to read from and write to bitfields

We currently provide a single custom derive, BitfieldStruct, as well as a dependent field attribute bitfield. The dependent field attribute padding may optionally be used as a no-op marker for automated tools.

Requirements

• Rust 1.30+
• Rust Stable, Beta, or Nightly
• Little Endian Architecture

Example

Suppose you want to write a super compact date struct which only takes up three bytes. In C this would look like this:

c struct date { unsigned char day: 5; unsigned char month: 4; unsigned short year: 15; } __attribute__((packed));

Clang helpfully provides us with this information:

c *** Dumping AST Record Layout 0 | struct date 0:0-4 | unsigned char day 0:5-8 | unsigned char month 1:1-15 | unsigned short year | [sizeof=3, align=1]

And this is enough to build our rust struct:

```rust

[repr(C, align(1))]

[derive(BitfieldStruct)]

struct Date { #[bitfield(name = "day", ty = "libc::cuchar", bits = "0..=4")] #[bitfield(name = "month", ty = "libc::cuchar", bits = "5..=8")] #[bitfield(name = "year", ty = "libc::cushort", bits = "9..=23")] daymonth_year: [u8; 3] }

fn main() { let mut date = Date { daymonthyear: [0; 3] };

date.set_day(18);
date.set_month(7);
date.set_year(2000);

assert_eq!(date.day(), 18);
assert_eq!(date.month(), 7);
assert_eq!(date.year(), 2000);

} ```

Furthermore, C bitfield rules for overflow and signed integers are taken into account.

This crate can generate no_std compatible code when the no_std feature flag is provided.

Tests

Since rust doesn't support a build.rs exclusively for tests, you must manually compile the c test code and link it in.

shell $ clang tests/bitfields.c -c -fPIC -o tests/bitfields.o $ ar -rc tests/libtest.a tests/bitfields.o $ RUSTFLAGS="-L `pwd`/tests" cargo test

Acknowledgements

This crate is inspired by the rust-bitfield, packed_struct, and bindgen crates.