README.md
1# memoffset #
2
3[](https://crates.io/crates/memoffset)
4
5C-Like `offset_of` functionality for Rust structs.
6
7Introduces the following macros:
8 * `offset_of!` for obtaining the offset of a member of a struct.
9 * `offset_of_tuple!` for obtaining the offset of a member of a tuple. (Requires Rust 1.20+)
10 * `span_of!` for obtaining the range that a field, or fields, span.
11
12`memoffset` works under `no_std` environments.
13
14## Usage ##
15Add the following dependency to your `Cargo.toml`:
16
17```toml
18[dependencies]
19memoffset = "0.6"
20```
21
22These versions will compile fine with rustc versions greater or equal to 1.19.
23
24Add the following lines at the top of your `main.rs` or `lib.rs` files.
25
26```rust,ignore
27#[macro_use]
28extern crate memoffset;
29```
30
31## Examples ##
32```rust
33#[macro_use]
34extern crate memoffset;
35
36#[repr(C, packed)]
37struct Foo {
38 a: u32,
39 b: u32,
40 c: [u8; 5],
41 d: u32,
42}
43
44fn main() {
45 assert_eq!(offset_of!(Foo, b), 4);
46 assert_eq!(offset_of!(Foo, d), 4+4+5);
47
48 assert_eq!(span_of!(Foo, a), 0..4);
49 assert_eq!(span_of!(Foo, a .. c), 0..8);
50 assert_eq!(span_of!(Foo, a ..= c), 0..13);
51 assert_eq!(span_of!(Foo, ..= d), 0..17);
52 assert_eq!(span_of!(Foo, b ..), 4..17);
53}
54```
55
56## Feature flags ##
57
58### Usage in constants ###
59`memoffset` has **experimental** support for compile-time `offset_of!` on a nightly compiler.
60
61In order to use it, you must enable the `unstable_const` crate feature and several compiler features.
62
63Cargo.toml:
64```toml
65[dependencies.memoffset]
66version = "0.6"
67features = ["unstable_const"]
68```
69
70Your crate root: (`lib.rs`/`main.rs`)
71```rust,ignore
72#![feature(ptr_offset_from, const_ptr_offset_from, const_maybe_uninit_as_ptr, const_raw_ptr_deref)]
73```
74
75If you intend to use `offset_of!` inside a `const fn`, also add the `const_fn` compiler feature.
76