2.1 KiB
memoffset
C-Like offset_of functionality for Rust structs.
Introduces the following macros:
offset_of!for obtaining the offset of a member of a struct.offset_of_tuple!for obtaining the offset of a member of a tuple. (Requires Rust 1.20+)span_of!for obtaining the range that a field, or fields, span.
memoffset works under no_std environments.
Usage
Add the following dependency to your Cargo.toml:
[dependencies]
memoffset = "0.5"
These versions will compile fine with rustc versions greater or equal to 1.19.
Add the following lines at the top of your main.rs or lib.rs files.
#[macro_use]
extern crate memoffset;
Examples
#[macro_use]
extern crate memoffset;
#[repr(C, packed)]
struct Foo {
a: u32,
b: u32,
c: [u8; 5],
d: u32,
}
fn main() {
assert_eq!(offset_of!(Foo, b), 4);
assert_eq!(offset_of!(Foo, d), 4+4+5);
assert_eq!(span_of!(Foo, a), 0..4);
assert_eq!(span_of!(Foo, a .. c), 0..8);
assert_eq!(span_of!(Foo, a ..= c), 0..13);
assert_eq!(span_of!(Foo, ..= d), 0..17);
assert_eq!(span_of!(Foo, b ..), 4..17);
}
Feature flags
Usage in constants
memoffset has experimental support for compile-time offset_of! on a nightly compiler.
In order to use it, you must enable the unstable_const crate feature and several compiler features.
Cargo.toml:
[dependencies.memoffset]
version = "0.5"
features = ["unstable_const"]
Your crate root: (lib.rs/main.rs)
#![feature(ptr_offset_from, const_ptr_offset_from, const_maybe_uninit_as_ptr, const_raw_ptr_deref)]
If you intend to use offset_of! inside a const fn, also add the const_fn compiler feature.
Raw references
Recent nightlies support a way to create raw pointers that avoids creating intermediate safe references.
memoffset can make use of that feature to avoid what is technically Undefined Behavior.
Use the unstable_raw feature to enable this.