| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Find the offset in bytes of the given `$field` of `$Type`. Requires an already initialized `$instance` value to work with."><meta name="keywords" content="rust, rustlang, rust-lang, offset_of"><title>offset_of in bytemuck - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc macro"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../bytemuck/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../bytemuck/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In bytemuck</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Macro <a href="index.html">bytemuck</a>::<wbr><a class="macro" href="#">offset_of</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/bytemuck/offset_of.rs.html#115-135">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules! </span>offset_of { |
| (<span class="macro-nonterminal">$instance</span>:expr, <span class="macro-nonterminal">$Type</span>:path, <span class="macro-nonterminal">$field</span>:tt) => { ... }; |
| (<span class="macro-nonterminal">$Type</span>:path, <span class="macro-nonterminal">$field</span>:tt) => { ... }; |
| }</code></pre></div> |
| </div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Find the offset in bytes of the given <code>$field</code> of <code>$Type</code>. Requires an |
| already initialized <code>$instance</code> value to work with.</p> |
| <p>This is similar to the macro from <a href="https://docs.rs/memoffset"><code>memoffset</code></a>, |
| however it uses no <code>unsafe</code> code.</p> |
| <p>This macro has a 3-argument and 2-argument version.</p> |
| <ul> |
| <li>In the 3-arg version you specify an instance of the type, the type itself, |
| and the field name.</li> |
| <li>In the 2-arg version the macro will call the <a href="https://doc.rust-lang.org/nightly/core/default/trait.Default.html#tymethod.default"><code>default</code></a> |
| method to make a temporary instance of the type for you.</li> |
| </ul> |
| <p>The output of this macro is the byte offset of the field (as a <code>usize</code>). The |
| calculations of the macro are fixed across the entire program, but if the |
| type used is <code>repr(Rust)</code> then they’re <em>not</em> fixed across compilations or |
| compilers.</p> |
| <h3 id="examples"><a href="#examples">Examples</a></h3><h4 id="3-arg-usage"><a href="#3-arg-usage">3-arg Usage</a></h4> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// enums can't derive default, and for this example we don't pick one |
| </span><span class="kw">enum </span>MyExampleEnum { |
| A, |
| B, |
| C, |
| } |
| |
| <span class="comment">// so now our struct here doesn't have Default |
| </span><span class="attribute">#[repr(C)] |
| </span><span class="kw">struct </span>MyNotDefaultType { |
| <span class="kw">pub </span>counter: i32, |
| <span class="kw">pub </span>some_field: MyExampleEnum, |
| } |
| |
| <span class="comment">// but we provide an instance of the type and it's all good. |
| </span><span class="kw">let </span>val = MyNotDefaultType { counter: <span class="number">5</span>, some_field: MyExampleEnum::A }; |
| <span class="macro">assert_eq!</span>(<span class="macro">offset_of!</span>(val, MyNotDefaultType, some_field), <span class="number">4</span>);</code></pre></div> |
| <h4 id="2-arg-usage"><a href="#2-arg-usage">2-arg Usage</a></h4> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[derive(Default)] |
| #[repr(C)] |
| </span><span class="kw">struct </span>Vertex { |
| <span class="kw">pub </span>loc: [f32; <span class="number">3</span>], |
| <span class="kw">pub </span>color: [f32; <span class="number">3</span>], |
| } |
| <span class="comment">// if the type impls Default the macro can make its own default instance. |
| </span><span class="macro">assert_eq!</span>(<span class="macro">offset_of!</span>(Vertex, loc), <span class="number">0</span>); |
| <span class="macro">assert_eq!</span>(<span class="macro">offset_of!</span>(Vertex, color), <span class="number">12</span>);</code></pre></div> |
| <h2 id="usage-with-reprpacked-structs"><a href="#usage-with-reprpacked-structs">Usage with <code>#[repr(packed)]</code> structs</a></h2> |
| <p>Attempting to compute the offset of a <code>#[repr(packed)]</code> struct with |
| <code>bytemuck::offset_of!</code> requires an <code>unsafe</code> block. We hope to relax this in |
| the future, but currently it is required to work around a soundness hole in |
| Rust (See <a href="https://github.com/rust-lang/rust/issues/27060">rust-lang/rust#27060</a>).</p> |
| <p style="background:rgba(255,181,77,0.16);padding:0.75em;"> |
| <strong>Warning:</strong> This is only true for versions of bytemuck > |
| 1.4.0. Previous versions of |
| <code style="background:rgba(41,24,0,0.1);">bytemuck::offset_of!</code> |
| will only emit a warning when used on the field of a packed struct in safe |
| code, which can lead to unsoundness. |
| </p> |
| <p>For example, the following will fail to compile:</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="attribute">#[repr(C, packed)] |
| #[derive(Default)] |
| </span><span class="kw">struct </span>Example { |
| field: u32, |
| } |
| <span class="comment">// Doesn't compile: |
| </span><span class="kw">let </span>_offset = <span class="macro">bytemuck::offset_of!</span>(Example, field);</code></pre></div> |
| <p>While the error message this generates will mention the |
| <code>safe_packed_borrows</code> lint, the macro will still fail to compile even if |
| that lint is <code>#[allow]</code>ed:</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="comment">// Still doesn't compile: |
| </span><span class="attribute">#[allow(safe_packed_borrows)] |
| </span>{ |
| <span class="kw">let </span>_offset = <span class="macro">bytemuck::offset_of!</span>(Example, field); |
| }</code></pre></div> |
| <p>This <em>can</em> be worked around by using <code>unsafe</code>, but it is only sound to do so |
| if you can guarantee that taking a reference to the field is sound.</p> |
| <p>In practice, this means it only works for fields of align(1) types, or if |
| you know the field’s offset in advance (defeating the point of <code>offset_of</code>) |
| and can prove that the struct’s alignment and the field’s offset are enough |
| to prove the field’s alignment.</p> |
| <p>Once the <code>raw_ref</code> macros are available, a future version of this crate will |
| use them to lift the limitations of packed structs. For the duration of the |
| <code>1.x</code> version of this crate that will be behind an on-by-default cargo |
| feature (to maintain minimum rust version support).</p> |
| </div></details></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="bytemuck" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
