This is by far the best article I've seen on the subject in terms of simplicity and explaining quickly what it means to have a
#[repr(C)] annotation on types.The most eye opening bit are the visuals, with first the C representation. Notice how there's extra padding, to ensure everything is byte-aligned. Consider,
#[repr(C)]
struct Foo {
tiny: bool,
normal: u32,
small: u8,
long: u64,
short: u16,
}- tiny: single byte, padded to 4 bytes so as to be byte-aligned. It's surprising that we are wasting 3-bytes and 7-bits to represent 1 bit!!
- small: This is a single byte, but since the struct's largest value stored is a long (u64), which needs 8-bytes, this is padded as 1 byte + 7 bytes of padding.
See rest of Christopher's article for details on rest of the struct fields.
repr(C) alignment and padding. Photo credit to Christopher Tee
and Rust's default representation
Rust's default alignment and lack of wasted padding. Photo credit to Christopher Tee
This was a great read, and hope you enjoyed it as well.