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//! Tools for converting file format structures to and from bytes.
//!
//! This module should be replaced once rust provides safe transmutes.
// This module provides functions for both read and write features.
#![cfg_attr(
not(all(feature = "read_core", feature = "write_core")),
allow(dead_code)
)]
use core::{mem, result, slice};
type Result<T> = result::Result<T, ()>;
/// A trait for types that can safely be converted from and to byte slices.
///
/// # Safety
/// A type that is `Pod` must:
/// - be `#[repr(C)]` or `#[repr(transparent)]`
/// - have no invalid byte values
/// - have no padding
pub unsafe trait Pod: Copy + 'static {}
/// Cast a byte slice to a `Pod` type.
///
/// Returns the type and the tail of the slice.
#[inline]
pub fn from_bytes<T: Pod>(data: &[u8]) -> Result<(&T, &[u8])> {
let size = mem::size_of::<T>();
let tail = data.get(size..).ok_or(())?;
let ptr = data.as_ptr();
if (ptr as usize) % mem::align_of::<T>() != 0 {
return Err(());
}
// Safety:
// The alignment and size are checked by this function.
// The Pod trait ensures the type is valid to cast from bytes.
let val = unsafe { &*ptr.cast() };
Ok((val, tail))
}
/// Cast a mutable byte slice to a `Pod` type.
///
/// Returns the type and the tail of the slice.
#[inline]
pub fn from_bytes_mut<T: Pod>(data: &mut [u8]) -> Result<(&mut T, &mut [u8])> {
let size = mem::size_of::<T>();
if size > data.len() {
return Err(());
}
let (data, tail) = data.split_at_mut(size);
let ptr = data.as_mut_ptr();
if (ptr as usize) % mem::align_of::<T>() != 0 {
return Err(());
}
// Safety:
// The alignment and size are checked by this function.
// The Pod trait ensures the type is valid to cast from bytes.
let val = unsafe { &mut *ptr.cast() };
Ok((val, tail))
}
/// Cast a byte slice to a slice of a `Pod` type.
///
/// Returns the type slice and the tail of the byte slice.
#[inline]
pub fn slice_from_bytes<T: Pod>(data: &[u8], count: usize) -> Result<(&[T], &[u8])> {
let size = count.checked_mul(mem::size_of::<T>()).ok_or(())?;
let tail = data.get(size..).ok_or(())?;
let ptr = data.as_ptr();
if (ptr as usize) % mem::align_of::<T>() != 0 {
return Err(());
}
// Safety:
// The alignment and size are checked by this function.
// The Pod trait ensures the type is valid to cast from bytes.
let slice = unsafe { slice::from_raw_parts(ptr.cast(), count) };
Ok((slice, tail))
}
/// Cast a mutable byte slice to a slice of a `Pod` type.
///
/// Returns the type slice and the tail of the byte slice.
#[inline]
pub fn slice_from_bytes_mut<T: Pod>(
data: &mut [u8],
count: usize,
) -> Result<(&mut [T], &mut [u8])> {
let size = count.checked_mul(mem::size_of::<T>()).ok_or(())?;
if size > data.len() {
return Err(());
}
let (data, tail) = data.split_at_mut(size);
let ptr = data.as_mut_ptr();
if (ptr as usize) % mem::align_of::<T>() != 0 {
return Err(());
}
// Safety:
// The alignment and size are checked by this function.
// The Pod trait ensures the type is valid to cast from bytes.
let slice = unsafe { slice::from_raw_parts_mut(ptr.cast(), count) };
Ok((slice, tail))
}
/// Cast a `Pod` type to a byte slice.
#[inline]
pub fn bytes_of<T: Pod>(val: &T) -> &[u8] {
let size = mem::size_of::<T>();
// Safety:
// Any alignment is allowed.
// The size is determined in this function.
// The Pod trait ensures the type is valid to cast to bytes.
unsafe { slice::from_raw_parts(slice::from_ref(val).as_ptr().cast(), size) }
}
/// Cast a `Pod` type to a mutable byte slice.
#[inline]
pub fn bytes_of_mut<T: Pod>(val: &mut T) -> &mut [u8] {
let size = mem::size_of::<T>();
// Safety:
// Any alignment is allowed.
// The size is determined in this function.
// The Pod trait ensures the type is valid to cast to bytes.
unsafe { slice::from_raw_parts_mut(slice::from_mut(val).as_mut_ptr().cast(), size) }
}
/// Cast a slice of a `Pod` type to a byte slice.
#[inline]
pub fn bytes_of_slice<T: Pod>(val: &[T]) -> &[u8] {
let size = val.len().wrapping_mul(mem::size_of::<T>());
// Safety:
// Any alignment is allowed.
// The size is determined in this function.
// The Pod trait ensures the type is valid to cast to bytes.
unsafe { slice::from_raw_parts(val.as_ptr().cast(), size) }
}
/// Cast a slice of a `Pod` type to a mutable byte slice.
#[inline]
pub fn bytes_of_slice_mut<T: Pod>(val: &mut [T]) -> &mut [u8] {
let size = val.len().wrapping_mul(mem::size_of::<T>());
// Safety:
// Any alignment is allowed.
// The size is determined in this function.
// The Pod trait ensures the type is valid to cast to bytes.
unsafe { slice::from_raw_parts_mut(val.as_mut_ptr().cast(), size) }
}
macro_rules! unsafe_impl_pod {
($($struct_name:ident),+ $(,)?) => {
$(
unsafe impl Pod for $struct_name { }
)+
}
}
unsafe_impl_pod!(u8, u16, u32, u64);
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn single() {
let x = u32::to_be(0x0123_4567);
let mut x_mut = x;
let bytes = bytes_of(&x);
let bytes_mut = bytes_of_mut(&mut x_mut);
assert_eq!(bytes, [0x01, 0x23, 0x45, 0x67]);
assert_eq!(bytes, bytes_mut);
let x16 = [u16::to_be(0x0123), u16::to_be(0x4567)];
let (y, tail) = from_bytes::<u32>(bytes).unwrap();
let (y_mut, tail_mut) = from_bytes_mut::<u32>(bytes_mut).unwrap();
assert_eq!(*y, x);
assert_eq!(y, y_mut);
assert_eq!(tail, &[]);
assert_eq!(tail, tail_mut);
let (y, tail) = from_bytes::<u16>(bytes).unwrap();
let (y_mut, tail_mut) = from_bytes_mut::<u16>(bytes_mut).unwrap();
assert_eq!(*y, x16[0]);
assert_eq!(y, y_mut);
assert_eq!(tail, &bytes[2..]);
assert_eq!(tail, tail_mut);
let (y, tail) = from_bytes::<u16>(&bytes[2..]).unwrap();
let (y_mut, tail_mut) = from_bytes_mut::<u16>(&mut bytes_mut[2..]).unwrap();
assert_eq!(*y, x16[1]);
assert_eq!(y, y_mut);
assert_eq!(tail, &[]);
assert_eq!(tail, tail_mut);
assert_eq!(from_bytes::<u16>(&bytes[1..]), Err(()));
assert_eq!(from_bytes::<u16>(&bytes[3..]), Err(()));
assert_eq!(from_bytes::<u16>(&bytes[4..]), Err(()));
assert_eq!(from_bytes_mut::<u16>(&mut bytes_mut[1..]), Err(()));
assert_eq!(from_bytes_mut::<u16>(&mut bytes_mut[3..]), Err(()));
assert_eq!(from_bytes_mut::<u16>(&mut bytes_mut[4..]), Err(()));
}
#[test]
fn slice() {
let x = [
u16::to_be(0x0123),
u16::to_be(0x4567),
u16::to_be(0x89ab),
u16::to_be(0xcdef),
];
let mut x_mut = x;
let bytes = bytes_of_slice(&x);
let bytes_mut = bytes_of_slice_mut(&mut x_mut);
assert_eq!(bytes, [0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef]);
assert_eq!(bytes, bytes_mut);
let (y, tail) = slice_from_bytes::<u16>(bytes, 4).unwrap();
let (y_mut, tail_mut) = slice_from_bytes_mut::<u16>(bytes_mut, 4).unwrap();
assert_eq!(y, x);
assert_eq!(y, y_mut);
assert_eq!(tail, &[]);
assert_eq!(tail, tail_mut);
let (y, tail) = slice_from_bytes::<u16>(&bytes[2..], 2).unwrap();
let (y_mut, tail_mut) = slice_from_bytes::<u16>(&mut bytes_mut[2..], 2).unwrap();
assert_eq!(y, &x[1..3]);
assert_eq!(y, y_mut);
assert_eq!(tail, &bytes[6..]);
assert_eq!(tail, tail_mut);
assert_eq!(slice_from_bytes::<u16>(bytes, 5), Err(()));
assert_eq!(slice_from_bytes::<u16>(&bytes[2..], 4), Err(()));
assert_eq!(slice_from_bytes::<u16>(&bytes[1..], 2), Err(()));
assert_eq!(slice_from_bytes_mut::<u16>(bytes_mut, 5), Err(()));
assert_eq!(slice_from_bytes_mut::<u16>(&mut bytes_mut[2..], 4), Err(()));
assert_eq!(slice_from_bytes_mut::<u16>(&mut bytes_mut[1..], 2), Err(()));
}
}
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