81 lines
2.1 KiB
Markdown
81 lines
2.1 KiB
Markdown
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# memoffset #
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[](https://crates.io/crates/memoffset)
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C-Like `offset_of` functionality for Rust structs.
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Introduces the following macros:
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* `offset_of!` for obtaining the offset of a member of a struct.
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* `offset_of_tuple!` for obtaining the offset of a member of a tuple. (Requires Rust 1.20+)
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* `span_of!` for obtaining the range that a field, or fields, span.
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`memoffset` works under `no_std` environments.
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## Usage ##
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Add the following dependency to your `Cargo.toml`:
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```toml
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[dependencies]
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memoffset = "0.6"
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```
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These versions will compile fine with rustc versions greater or equal to 1.19.
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Add the following lines at the top of your `main.rs` or `lib.rs` files.
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```rust,ignore
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#[macro_use]
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extern crate memoffset;
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```
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## Examples ##
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```rust
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#[macro_use]
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extern crate memoffset;
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#[repr(C, packed)]
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struct Foo {
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a: u32,
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b: u32,
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c: [u8; 5],
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d: u32,
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}
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fn main() {
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assert_eq!(offset_of!(Foo, b), 4);
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assert_eq!(offset_of!(Foo, d), 4+4+5);
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assert_eq!(span_of!(Foo, a), 0..4);
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assert_eq!(span_of!(Foo, a .. c), 0..8);
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assert_eq!(span_of!(Foo, a ..= c), 0..13);
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assert_eq!(span_of!(Foo, ..= d), 0..17);
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assert_eq!(span_of!(Foo, b ..), 4..17);
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}
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```
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## Feature flags ##
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### Usage in constants ###
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`memoffset` has **experimental** support for compile-time `offset_of!` on a nightly compiler.
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In order to use it, you must enable the `unstable_const` crate feature and several compiler features.
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Cargo.toml:
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```toml
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[dependencies.memoffset]
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version = "0.6"
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features = ["unstable_const"]
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```
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Your crate root: (`lib.rs`/`main.rs`)
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```rust,ignore
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#![feature(ptr_offset_from, const_ptr_offset_from, const_maybe_uninit_as_ptr, const_raw_ptr_deref)]
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```
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If you intend to use `offset_of!` inside a `const fn`, also add the `const_fn` compiler feature.
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### Raw references ###
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Recent nightlies support [a way to create raw pointers](https://github.com/rust-lang/rust/issues/73394) that avoids creating intermediate safe references.
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`memoffset` can make use of that feature to avoid what is technically Undefined Behavior.
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Use the `unstable_raw` feature to enable this.
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