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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="API documentation for the Rust `Once` struct in crate `parking_lot`."><meta name="keywords" content="rust, rustlang, rust-lang, Once"><title>parking_lot::Once - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script></head><body class="rustdoc struct"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><p class='location'>Struct Once</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#methods">Methods</a><div class="sidebar-links"><a href="#method.new">new</a><a href="#method.state">state</a><a href="#method.call_once">call_once</a><a href="#method.call_once_force">call_once_force</a></div><a class="sidebar-title" href="#implementations">Trait Implementations</a><div class="sidebar-links"><a href="#impl-Default">Default</a><a href="#impl-Debug">Debug</a></div><a class="sidebar-title" href="#synthetic-implementations">Auto Trait Implementations</a><div class="sidebar-links"><a href="#impl-Send">Send</a><a href="#impl-Sync">Sync</a></div></div><p class='location'><a href='index.html'>parking_lot</a></p><script>window.sidebarCurrent = {name: 'Once', ty: 'struct', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='in-band'>Struct <a href='index.html'>parking_lot</a>::<wbr><a class="struct" href=''>Once</a></span><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/parking_lot/once.rs.html#92' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust struct'>pub struct Once(_);</pre></div><div class='docblock'><p>A synchronization primitive which can be used to run a one-time
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initialization. Useful for one-time initialization for globals, FFI or
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related functionality.</p>
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<h1 id="differences-from-the-standard-library-once" class="section-header"><a href="#differences-from-the-standard-library-once">Differences from the standard library <code>Once</code></a></h1>
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<ul>
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<li>Only requires 1 byte of space, instead of 1 word.</li>
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<li>Not required to be <code>'static</code>.</li>
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<li>Relaxed memory barriers in the fast path, which can significantly improve
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performance on some architectures.</li>
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<li>Efficient handling of micro-contention using adaptive spinning.</li>
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</ul>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">parking_lot</span>::{<span class="ident">Once</span>, <span class="ident">ONCE_INIT</span>};
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<span class="kw">static</span> <span class="ident">START</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">ONCE_INIT</span>;
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<span class="ident">START</span>.<span class="ident">call_once</span>(<span class="op">||</span> {
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<span class="comment">// run initialization here</span>
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});</pre>
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</div><h2 id='methods' class='small-section-header'>Methods<a href='#methods' class='anchor'></a></h2><h3 id='impl' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="struct" href="../parking_lot/struct.Once.html" title="struct parking_lot::Once">Once</a></code><a href='#impl' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#97-337' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.new' class="method"><span id='new.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.new' class='fnname'>new</a>() -> <a class="struct" href="../parking_lot/struct.Once.html" title="struct parking_lot::Once">Once</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#108-110' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Creates a new <code>Once</code> value.</p>
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</div><h4 id='method.state' class="method"><span id='state.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.state' class='fnname'>state</a>(&self) -> <a class="enum" href="../parking_lot/enum.OnceState.html" title="enum parking_lot::OnceState">OnceState</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#114-125' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Returns the current state of this <code>Once</code>.</p>
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</div><h4 id='method.call_once' class="method"><span id='call_once.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.call_once' class='fnname'>call_once</a><F>(&self, f: F) <span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>(), </span></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#177-187' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Performs an initialization routine once and only once. The given closure
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will be executed if this is the first time <code>call_once</code> has been called,
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and otherwise the routine will <em>not</em> be invoked.</p>
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<p>This method will block the calling thread if another initialization
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routine is currently running.</p>
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<p>When this function returns, it is guaranteed that some initialization
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has run and completed (it may not be the closure specified). It is also
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guaranteed that any memory writes performed by the executed closure can
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be reliably observed by other threads at this point (there is a
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happens-before relation between the closure and code executing after the
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return).</p>
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<h1 id="examples-1" class="section-header"><a href="#examples-1">Examples</a></h1>
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<pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">parking_lot</span>::{<span class="ident">Once</span>, <span class="ident">ONCE_INIT</span>};
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<span class="kw">static</span> <span class="kw-2">mut</span> <span class="ident">VAL</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0</span>;
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<span class="kw">static</span> <span class="ident">INIT</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">ONCE_INIT</span>;
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<span class="comment">// Accessing a `static mut` is unsafe much of the time, but if we do so</span>
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<span class="comment">// in a synchronized fashion (e.g. write once or read all) then we're</span>
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<span class="comment">// good to go!</span>
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<span class="comment">//</span>
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<span class="comment">// This function will only call `expensive_computation` once, and will</span>
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<span class="comment">// otherwise always return the value returned from the first invocation.</span>
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<span class="kw">fn</span> <span class="ident">get_cached_val</span>() <span class="op">-></span> <span class="ident">usize</span> {
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<span class="kw">unsafe</span> {
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<span class="ident">INIT</span>.<span class="ident">call_once</span>(<span class="op">||</span> {
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<span class="ident">VAL</span> <span class="op">=</span> <span class="ident">expensive_computation</span>();
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});
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<span class="ident">VAL</span>
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}
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}
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<span class="kw">fn</span> <span class="ident">expensive_computation</span>() <span class="op">-></span> <span class="ident">usize</span> {
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<span class="comment">// ...</span>
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}</pre>
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<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
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<p>The closure <code>f</code> will only be executed once if this is called
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concurrently amongst many threads. If that closure panics, however, then
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it will <em>poison</em> this <code>Once</code> instance, causing all future invocations of
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<code>call_once</code> to also panic.</p>
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</div><h4 id='method.call_once_force' class="method"><span id='call_once_force.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.call_once_force' class='fnname'>call_once_force</a><F>(&self, f: F) <span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>(<a class="enum" href="../parking_lot/enum.OnceState.html" title="enum parking_lot::OnceState">OnceState</a>), </span></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#199-211' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Performs the same function as <code>call_once</code> except ignores poisoning.</p>
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<p>If this <code>Once</code> has been poisoned (some initialization panicked) then
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this function will continue to attempt to call initialization functions
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until one of them doesn't panic.</p>
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<p>The closure <code>f</code> is yielded a structure which can be used to query the
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state of this <code>Once</code> (whether initialization has previously panicked or
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not).</p>
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</div></div><h2 id='implementations' class='small-section-header'>Trait Implementations<a href='#implementations' class='anchor'></a></h2><div id='implementations-list'><h3 id='impl-Default' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="https://doc.rust-lang.org/nightly/core/default/trait.Default.html" title="trait core::default::Default">Default</a> for <a class="struct" href="../parking_lot/struct.Once.html" title="struct parking_lot::Once">Once</a></code><a href='#impl-Default' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#339-344' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.default' class="method"><span id='default.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='https://doc.rust-lang.org/nightly/core/default/trait.Default.html#tymethod.default' class='fnname'>default</a>() -> <a class="struct" href="../parking_lot/struct.Once.html" title="struct parking_lot::Once">Once</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#341-343' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Returns the "default value" for a type. <a href="https://doc.rust-lang.org/nightly/core/default/trait.Default.html#tymethod.default">Read more</a></p>
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</div></div><h3 id='impl-Debug' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a> for <a class="struct" href="../parking_lot/struct.Once.html" title="struct parking_lot::Once">Once</a></code><a href='#impl-Debug' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#346-352' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.fmt' class="method"><span id='fmt.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html#tymethod.fmt' class='fnname'>fmt</a>(&self, f: &mut <a class="struct" href="https://doc.rust-lang.org/nightly/core/fmt/struct.Formatter.html" title="struct core::fmt::Formatter">Formatter</a>) -> <a class="type" href="https://doc.rust-lang.org/nightly/core/fmt/type.Result.html" title="type core::fmt::Result">Result</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/parking_lot/once.rs.html#347-351' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Formats the value using the given formatter. <a href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html#tymethod.fmt">Read more</a></p>
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</div></div></div><h2 id='synthetic-implementations' class='small-section-header'>Auto Trait Implementations<a href='#synthetic-implementations' class='anchor'></a></h2><div id='synthetic-implementations-list'><h3 id='impl-Send' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> for <a class="struct" href="../parking_lot/struct.Once.html" title="struct parking_lot::Once">Once</a></code><a href='#impl-Send' class='anchor'></a></span></td><td><span class='out-of-band'></span></td></tr></tbody></table></h3><div class='impl-items'></div><h3 id='impl-Sync' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> for <a class="struct" href="../parking_lot/struct.Once.html" title="struct parking_lot::Once">Once</a></code><a href='#impl-Sync' class='anchor'></a></span></td><td><span class='out-of-band'></span></td></tr></tbody></table></h3><div class='impl-items'></div></div></section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g. <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../";window.currentCrate = "parking_lot";</script><script src="../aliases.js"></script><script src="../main.js"></script><script defer src="../search-index.js"></script></body></html> |