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 * `offset_of_union!` for obtaining the offset of a member of a union. 11 * `span_of!` for obtaining the range that a field, or fields, span. 12 13`memoffset` works under `no_std` environments. 14 15If you're using a rustc version greater or equal to 1.77, this crate's `offset_of!()` macro simply forwards to `core::mem::offset_of!()`. 16 17## Usage ## 18Add the following dependency to your `Cargo.toml`: 19 20```toml 21[dependencies] 22memoffset = "0.9" 23``` 24 25These versions will compile fine with rustc versions greater or equal to 1.19. 26 27## Examples ## 28```rust 29use memoffset::{offset_of, span_of}; 30 31#[repr(C, packed)] 32struct Foo { 33 a: u32, 34 b: u32, 35 c: [u8; 5], 36 d: u32, 37} 38 39fn main() { 40 assert_eq!(offset_of!(Foo, b), 4); 41 assert_eq!(offset_of!(Foo, d), 4+4+5); 42 43 assert_eq!(span_of!(Foo, a), 0..4); 44 assert_eq!(span_of!(Foo, a .. c), 0..8); 45 assert_eq!(span_of!(Foo, a ..= c), 0..13); 46 assert_eq!(span_of!(Foo, ..= d), 0..17); 47 assert_eq!(span_of!(Foo, b ..), 4..17); 48} 49``` 50 51## Usage in constants ## 52`memoffset` has support for compile-time `offset_of!` on rust>=1.65. 53 54On versions below 1.77, this is an incomplete implementation with one caveat: 55Due to dependence on [`#![feature(const_refs_to_cell)]`](https://github.com/rust-lang/rust/issues/80384), you cannot get the offset of a `Cell` field in a const-context. 56
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