1 # memoffset # 2 3 [](https://crates.io/crates/memoffset) 4 5 C-Like `offset_of` functionality for Rust structs. 6 7 Introduces 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 15 ## Usage ## 16 Add the following dependency to your `Cargo.toml`: 17 18 ```toml 19 [dependencies] 20 memoffset = "0.8" 21 ``` 22 23 These versions will compile fine with rustc versions greater or equal to 1.19. 24 25 ## Examples ## 26 ```rust 27 use memoffset::{offset_of, span_of}; 28 29 #[repr(C, packed)] 30 struct Foo { 31 a: u32, 32 b: u32, 33 c: [u8; 5], 34 d: u32, 35 } 36 37 fn main() { 38 assert_eq!(offset_of!(Foo, b), 4); 39 assert_eq!(offset_of!(Foo, d), 4+4+5); 40 41 assert_eq!(span_of!(Foo, a), 0..4); 42 assert_eq!(span_of!(Foo, a .. c), 0..8); 43 assert_eq!(span_of!(Foo, a ..= c), 0..13); 44 assert_eq!(span_of!(Foo, ..= d), 0..17); 45 assert_eq!(span_of!(Foo, b ..), 4..17); 46 } 47 ``` 48 49 ## Usage in constants ## 50 `memoffset` has support for compile-time `offset_of!` on rust>=1.65, or on older nightly compilers. 51 52 ### Usage on stable Rust ### 53 Constant evaluation is automatically enabled and avilable on stable compilers starting with rustc 1.65. 54 55 This is an incomplete implementation with one caveat: 56 Due 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. 57 58 This means that if need to get the offset of a cell, you'll have to remain on nightly for now. 59 60 ### Usage on recent nightlies ### 61 62 If you're using a new-enough nightly and you require the ability to get the offset of a `Cell`, 63 you'll have to enable the `unstable_const` cargo feature, as well as enabling `const_refs_to_cell` in your crate root. 64 65 Do note that `unstable_const` is an unstable feature that is set to be removed in a future version of `memoffset`. 66 67 Cargo.toml: 68 ```toml 69 [dependencies.memoffset] 70 version = "0.8" 71 features = ["unstable_const"] 72 ``` 73 74 Your crate root: (`lib.rs`/`main.rs`) 75 ```rust,ignore 76 #![feature(const_refs_to_cell)] 77 ``` 78 79 ### Usage on older nightlies ### 80 In order to use it on an older nightly compiler, you must enable the `unstable_const` crate feature and several compiler features. 81 82 Your crate root: (`lib.rs`/`main.rs`) 83 ```rust,ignore 84 #![feature(const_ptr_offset_from, const_refs_to_cell)] 85 ``` 86