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<!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="Macro to create a mutable reference to a statically allocated value"><title>singleton in cortex_m - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-492a78a4a87dcc01.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="cortex_m" data-themes="" data-resource-suffix="" data-rustdoc-version="1.82.0 (f6e511eec 2024-10-15)" data-channel="1.82.0" data-search-js="search-a99f1315e7cc5121.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-921df33f47b8780c.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-3b12f09e550e0385.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.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" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../cortex_m/index.html">cortex_<wbr>m</a><span class="version">0.7.7</span></h2></div><div class="sidebar-elems"></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1>Macro <a href="index.html">cortex_m</a>::<wbr><a class="macro" href="#">singleton</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/cortex_m/macros.rs.html#63-91">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><span class="macro">macro_rules!</span> singleton {
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(<span class="macro-nonterminal">$name</span>:ident: <span class="macro-nonterminal">$ty</span>:ty = <span class="macro-nonterminal">$expr</span>:expr) => { ... };
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(: <span class="macro-nonterminal">$ty</span>:ty = <span class="macro-nonterminal">$expr</span>:expr) => { ... };
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}</pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Macro to create a mutable reference to a statically allocated value</p>
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<p>This macro returns a value with type <code>Option<&'static mut $ty></code>. <code>Some($expr)</code> will be returned
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the first time the macro is executed; further calls will return <code>None</code>. To avoid <code>unwrap</code>ping a
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<code>None</code> variant the caller must ensure that the macro is called from a function that’s executed
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at most once in the whole lifetime of the program.</p>
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<h2 id="notes"><a class="doc-anchor" href="#notes">§</a>Notes</h2>
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<p>This macro is unsound on multi core systems.</p>
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<p>For debuggability, you can set an explicit name for a singleton. This name only shows up the
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the debugger and is not referencable from other code. See example below.</p>
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<h2 id="example"><a class="doc-anchor" href="#example">§</a>Example</h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>cortex_m::singleton;
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<span class="kw">fn </span>main() {
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<span class="comment">// OK if `main` is executed only once
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</span><span class="kw">let </span>x: <span class="kw-2">&</span><span class="lifetime">'static </span><span class="kw-2">mut </span>bool = <span class="macro">singleton!</span>(: bool = <span class="bool-val">false</span>).unwrap();
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<span class="kw">let </span>y = alias();
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<span class="comment">// BAD this second call to `alias` will definitively `panic!`
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</span><span class="kw">let </span>y_alias = alias();
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}
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<span class="kw">fn </span>alias() -> <span class="kw-2">&</span><span class="lifetime">'static </span><span class="kw-2">mut </span>bool {
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<span class="macro">singleton!</span>(: bool = <span class="bool-val">false</span>).unwrap()
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}
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<span class="kw">fn </span>singleton_with_name() {
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<span class="comment">// A name only for debugging purposes
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</span><span class="macro">singleton!</span>(FOO_BUFFER: [u8; <span class="number">1024</span>] = [<span class="number">0u8</span>; <span class="number">1024</span>]);
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}</code></pre></div>
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