structview/

lib.rs

#![deny(missing_docs)]
#![cfg_attr(not(feature = "std"), no_std)]

//! This crate enables casting references to binary data into references to
//! higher-level data structures, such as structs, unions, and arrays.
//!
//! The implemented approach is similar to a common pattern used when parsing
//! binary data formats in C, where `char *`s representing the raw data are
//! directly cast to, for example, struct pointers. This technique has the
//! benefits of being simple and highly efficient. Unfortunately, it is also
//! unsafe, as issues with alignment and integer endianess are usually ignored.
//!
//! `structview` avoids these issues by providing a safe and convenient
//! interface to its users: an (automatically derivable) trait [`View`],
//! as well as types for safely viewing integer fields.
//!
//! # Example
//!
//! ```
//! use structview::{u32_le, View};
//!
//! #[derive(Clone, Copy, View)]
//! #[repr(C)]
//! struct Animal {
//!     name: [u8; 4],
//!     number_of_heads: u8,
//!     number_of_legs: u32_le,
//! }
//!
//! fn main() -> Result<(), structview::Error> {
//!     let data = [0x43, 0x61, 0x74, 0x00, 0x01, 0x04, 0x00, 0x00, 0x00];
//!     let animal = Animal::view(&data)?;
//!
//!     assert_eq!(animal.name, *b"Cat\x00");
//!     assert_eq!(animal.number_of_heads, 1);
//!     assert_eq!(animal.number_of_legs.to_int(), 4);
//!
//!     Ok(())
//! }
//! ```
//!
//! [`View`]: trait.View.html

use byteorder::ByteOrder;
use core::{fmt, marker::PhantomData, mem, slice};

pub use structview_derive::*;

/// Error type returned when creating a view fails.
#[derive(Debug, Clone, Copy, Eq, Hash, PartialEq)]
pub enum Error {
    /// Creating a view failed because there was not enough data to fill it.
    NotEnoughData,
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::NotEnoughData => f.write_str("not enough data"),
        }
    }
}

#[cfg(feature = "std")]
impl std::error::Error for Error {}

/// Trait for viewing byte data as a higher-level representation.
///
/// By implementing [`View`] a type declares that it is safe to be cast from
/// raw byte data.
///
/// # Safety
///
/// Implementing this trait is unsafe since implementing types must fulfill
/// some requirements that cannot be checked through the type system only:
///
///   - There must be no raw byte values that constitute invalid data for the
///     implementing type. For example, implementing [`View`] for
///     `std::num::NonZeroI32` would be unsafe.
///   - The implementing type must be 1-byte aligned.
///   - If the implementing type is a compound type, it must be ensured that
///     the compiler doesn't change the order of fields. This can be achieved
///     through `#[repr(C)]`.
///
/// It is recommended to use the custom `View` derive also provided by this
/// crate instead of implementing this trait manually.
///
/// [`View`]: #trait.View
pub unsafe trait View: Copy {
    /// View `data` as a value of the implementing type.
    ///
    /// This simply casts the `&[u8]` reference to a reference of the
    /// implementing type.
    ///
    /// # Errors
    ///
    /// If `data` is too short to fill the whole view,
    /// [`Error::NotEnoughData`] will be returned.
    ///
    /// [`Error::NotEnoughData`]: enum.Error.html
    fn view(data: &[u8]) -> Result<&Self, Error> {
        if data.len() < mem::size_of::<Self>() {
            return Err(Error::NotEnoughData);
        }

        let data_ptr = data.as_ptr();
        let struct_ptr = data_ptr as *const Self;
        let struct_ref = unsafe { &*struct_ptr };

        Ok(struct_ref)
    }

    /// View boxed slice `data` as a boxed slice of the implementing type.
    ///
    /// This first reinterprets the data as a slice of the implementing type
    /// and then as a boxed slice
    ///
    ///  # Errors
    ///
    /// If the length of the u8 boxed slice is not a multiple of the struct's
    /// size, [`Error::NotEnoughData`] will be returned.
    ///
    /// # Panics
    ///
    /// If the implementing type has a size of 0.
    ///
    /// [`Error::NotEnoughData`]: enum.Error.html
    #[cfg(feature = "std")]
    fn view_boxed_slice(data: Box<[u8]>) -> Result<Box<[Self]>, Error> {
        let size = mem::size_of::<Self>();
        assert_ne!(size, 0);
        let len = data.len();
        if len % size != 0 {
            return Err(Error::NotEnoughData);
        }
        let ptr = Box::into_raw(data);
        let adj_len = len / size;
        unsafe {
            let slice = slice::from_raw_parts_mut(ptr as *mut Self, adj_len);
            Ok(Box::from_raw(slice))
        }
    }

    /// View slice `data` as a slice of the implementing type.
    ///
    /// This simply reinterprets the data as a slice of the implementing type.
    ///
    ///  # Errors
    ///
    /// If the length of the u8 slice is not a multiple of the struct's size,
    /// [`Error::NotEnoughData`] will be returned.
    ///
    /// # Panics
    ///
    /// If the implementing type has a size of 0.
    ///
    /// [`Error::NotEnoughData`]: enum.Error.html
    fn view_slice(data: &[u8]) -> Result<&[Self], Error> {
        let size = mem::size_of::<Self>();
        assert_ne!(size, 0);
        let len = data.len();
        if len % size != 0 {
            return Err(Error::NotEnoughData);
        }
        let ptr = data.as_ptr();
        let adj_len = len / size;
        unsafe {
            Ok(slice::from_raw_parts(ptr as *const Self, adj_len))
        }
    }
}

unsafe impl View for i8 {}
unsafe impl View for u8 {}

// We implement `View` for arrays of `View` elements of sizes 0 to 256.
// Hopefully thats enough for most binary parsing needs. Once const generics
// land (https://github.com/rust-lang/rust/issues/44580), we can get rid of
// this workaround.

macro_rules! array_view_impls {
    ( $($N:expr)+ ) => {
        $( unsafe impl<V: View> View for [V; $N] {} )+
    };
}

array_view_impls! {
      0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19
     20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39
     40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59
     60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79
     80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99

    100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119
    120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139
    140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159
    160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179
    180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199

    200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219
    220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239
    240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256
}

macro_rules! int_view {
    ( $name:ident, $int:ty, $read:ident, $le:ident, $be:ident ) => {
        int_view!($name, $int, stringify!($int), $read, $le, $be);
    };

    ( $name:ident, $int:ty, $int_name:expr, $read:ident, $le:ident, $be:ident ) => {
        #[doc = "View of an `"]
        #[doc = $int_name]
        #[doc = "` value."]
        #[derive(Debug, Clone, Copy, Eq, Hash, PartialEq)]
        pub struct $name<BO>([u8; mem::size_of::<$int>()], PhantomData<BO>);

        impl<BO: ByteOrder> $name<BO> {
            /// Return the viewed integer value.
            ///
            /// Calling this function incurs some runtime cost as the raw
            /// bytes have to be parsed according to the view's endianess.
            pub fn to_int(&self) -> $int {
                BO::$read(&self.0)
            }
        }

        impl<BO: ByteOrder> fmt::Display for $name<BO> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                write!(f, "[{}]", self.to_int())
            }
        }

        impl<BO: ByteOrder> From<$name<BO>> for $int {
            fn from(view: $name<BO>) -> Self {
                view.to_int()
            }
        }

        unsafe impl<BO: Copy> View for $name<BO> {}

        #[doc = "View of a little-endian `"]
        #[doc = $int_name]
        #[doc = "` value."]
        #[allow(non_camel_case_types)]
        pub type $le = $name<byteorder::LE>;

        #[doc = "View of a big-endian `"]
        #[doc = $int_name]
        #[doc = "` value."]
        #[allow(non_camel_case_types)]
        pub type $be = $name<byteorder::BE>;
    };
}

int_view!(I16, i16, read_i16, i16_le, i16_be);
int_view!(U16, u16, read_u16, u16_le, u16_be);

int_view!(I32, i32, read_i32, i32_le, i32_be);
int_view!(U32, u32, read_u32, u32_le, u32_be);

int_view!(I64, i64, read_i64, i64_le, i64_be);
int_view!(U64, u64, read_u64, u64_le, u64_be);