Endianness is the convention that determines how the bytes of a multibyte value are arranged in memory. Big-endian order places the most significant byte first; little-endian order places it last. The same bytes can therefore produce different integer values on different CPUs. section 5.2.1 section 5.8
A four-byte pattern makes the difference visible:
- Big-endian order stores AA BB CC DD. section 5.2.1
- Little-endian order stores DD CC BB AA. section 5.2.1
- Byte significance follows powers of two, just as decimal significance follows powers of ten. section 5.2.1
- Bit numbering is a related layout question inside an individual byte, but it is distinct from byte endianness. section 5.2.1
Inspecting endianness in Rust
A Rust program can reinterpret [0xAA, 0xBB, 0xCC, 0xDD] and its reversed sequence as i32 values with unsafe std::mem::transmute. The printed pair changes with the machine's byte order, making the portability problem concrete. section 5.2.1
Listing 5.6 Inspecting endianness
use std::mem::transmute;
fn main() {
let big_endian: [u8; 4] = [0xAA, 0xBB, 0xCC, 0xDD];
let little_endian: [u8; 4] = [0xDD, 0xCC, 0xBB, 0xAA];
let a: i32 = unsafe { transmute(big_endian) }; #1
let b: i32 = unsafe { transmute(little_endian) }; #1
println!("{} vs {}", a, b);
}
Run the four-byte Rust inspection to compare the integer values produced by big-endian and little-endian layouts. section 5.2.1
