zorua

Zorua enables the creation of zero-copy, endian-aware, bitfield-equipped structs in Rust.

Installation

To use zorua add the following dependency to your Cargo.toml:

[dependencies]
zorua = { git = "https://github.com/project-pku/zorua.git", tag="v0.6" }

Features

To use the crate simply import the prelude module.

The zorua crate was created to enable the following features safely, and within a zero-copy context:

Transmutation
Endian awareness
Bitfields
Enum awareness

Transmutation

By deriving the ZoruaField trait, a struct can be transmuted to an raw AlignedBytes struct (this is just a [u8;N] that maintains it's alignment):

use zorua::prelude::*;

#[repr(C)]
#[derive(ZoruaField)]
struct MyStruct {
    field_a: u8,
    field_b: u8,
    field_c: u16,
}

fn main() {
    let myStruct = MyStruct {
        field_a: 0,
        field_b: 2,
        field_c: 5,
    }

    let bytes = transmute_to_bytes!(MyStruct, myStruct);
    println!("{}", bytes[1]); // "2"
}

Deriving ZoruaField places conditions on the implementing type to ensure safety. All these conditions are checked at compile time:

Must have a compatible repr (C for structs, u8 for enums)
Must have no padding
All it's fields must be types that implement ZoruaField

Also note that there are 3 macros for transmuting structs:

transmute_to_bytes! for ZoruaField -> bytes
transmute_from_bytes! for bytes -> ZoruaField
transmute! for bytes/ZouraField -> bytes/ZoruaField. The other two are just shorthand for this one.

Endian Awareness

Any type that implements the ZoruaField trait comes equipped with fns for switching the endianness of a struct in-place:

use zorua::prelude::*;

#[repr(C)]
#[derive(ZoruaField)]
struct MyStruct {
    field: u16,
}

fn main() {
    let myStruct = MyStruct {
        field_a: 0x01ABCDEF,
    }

    //convert to big-endian if on little-endian machine
    #[cfg(target_endian = "little")]
    self.to_be_mut();

    //convert to little-endian if on big-endian machine
    #[cfg(target_endian = "big")]
    self.to_le_mut();

    println!("{:#X}", myStruct.field_a); //"0xEFCDAB01"
}

It is up to the user to determine if they want to switch the endianness before they operate on a transmuted struct (e.g. manipulating a big-endian struct on a little-endian machine).

Bitfields

The zorua crate also supports adding virtual bitfields to your structs:

bitfields! {
    #[repr(C)]
    #[derive(ZoruaField)]
    pub struct MyStruct {
        pub field_a: u16,
        field_b: u16, // Backing field (no `pub` so only bitfields are exposed)
        |pub bitfield_a: u5@0, // 5 bit uint @ index 0
        |pub bitfield_b: bool@5, // 1 bit bool @ index 5
        |bitfield_c: u7@6, // Bitfields can be private too
        |pub padding: u3@13, // The rest of the bits
    };
}

fn main() {
    let myStruct = MyStruct {
        field_a: 12,
        field_b: 367, //0b000-0000101-1-01111
    }

    println!("{:#b}", myStruct.bitfield_a()); // "0b01111"
    myStruct.set_bitfield_a(2u8.try_into().unwrap()); //casts u8->u5
    println!("{:#b}", myStruct.bitfield_a()); // "0b01111"
}

Declaring a bitfield

field_b: u16, <------------ Backing field: field_b
|pub bitfield_a: bool@0,
|pub bitfield_b: u5@1,
 ^   ^           ^  ^
 |   |           |  |
 |   |           |  +- Type: u5
 |   |           +---- Index: 1
 |   +---------------- Name: bitfield_b
 +-------------------- (Optional) Vis keyword: pub

Bitfield types

Only certain types can be used as bitfield types in the bitfield! macro.

uX Types

Importing zorua::prelude::* makes the types u1-u15 available. These types are exactly what they seem, they are unsigned integers of varying bitsizes. Just like with the built-in uX's, if a type is smaller than the data trying to be fit into it (e.g. u16 into a u12), a try_into() is required instead of an into().

Custom Bitfields

Just like the ZoruaField trait allows one to create structs capable of being transmuted, implementing the ZoruaBitField trait allows a struct to be used as a bitfield type in the bitfield! macro.

Enums

While we've covered implementing ZoruaField and ZoruaBitField w.r.t to structs, the zorua crate also supports implementing these traits on c-like enums.

Exhaustive enums

Since every possible bit-pattern must be valid for a ZoruaField/ZoruaBitField, implementing them on enums directly requires that type has to have a variant for for each possible value it could hold.

ZoruaField enums must have exactly 256 (2^8) variants:

#[repr(u8)] //requires some `uX` repr
#[derive(ZoruaField)]
enum MyExhaustiveEnum {
    Variant0,
    Variant1,
    //...,
    Variant255,
}

ZoruaBitField enums must have some power of 2 (2^n) variants:

#[repr(u8)] //requires some `uX` repr
#[derive(ZoruaBitField)]
enum MyExhaustive2BitEnum {
    Variant1,
    Variant2,
    Variant3,
    Variant4, //2^2 = 4
}

Fallible enums

This is great, but its unlikely that all your enums will have exactly 2^n variants. What do you do in those cases? This is where ZoruaFallible comes in. It allows you to define valid and invalid bitpatterns that are checked before every access.

#[repr(u8)]
#[derive(ZoruaFallible)]
#[targets(u8, u2)]
enum MyEnum {
    Variant0,
    Variant1,
    Variant2, //2^1 < 3 < 2^2
}

struct MyStruct {
    field_a: Fallible<MyEnum, u8>,
    field_b: u8,
    |pub test: Fallible<MyEnum, u2>@0
}

fn main() {
    let myStruct = MyStruct {
      field_b: 0,
    }
    
    //valid value
    myStruct.set_test(MyEnum::Variant1.into())
    let val: Fallible<MyEnum, u2> = myStruct.test();
    let res: Result<MyEnum, u2> = val.try_into();
    assert!(res.is_ok());

    //invalid value
    myStruct.set_test(Fallible::from_raw(u2::new(3)))
    let val: Fallible<MyEnum, u2> = myStruct.test();
    let res: Result<MyEnum, u2> = val.try_into();
    assert!(res.is_err()); //if value is invalid, raw value is error
}

Note that a Fallible type can be used either as a ZoruaField or ZoruaBitField depending on the given target (e.g. targets of u8, u16, etc. can be used as both fields and bitfields, while u2, u12, etc. can only be used as bitfields).

Also note that while ZoruaFallible can be derived for enums, you can manually implement it for structs as well.