| <!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="A synchronization primitive which can be used to run a one-time global initialization. Useful for one-time initialization for FFI or related functionality. This type can only be constructed with [`Once::new()`]."><meta name="keywords" content="rust, rustlang, rust-lang, Once"><title>Once in tracing_core - 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 struct"><!--[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="../tracing_core/index.html"><div class="logo-container"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../tracing_core/index.html"><div class="logo-container"> |
| <img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></div></a><h2 class="location"><a href="#">Once</a></h2><div class="sidebar-elems"><section><h3><a href="#implementations">Methods</a></h3><ul class="block"><li><a href="#method.call_once">call_once</a></li><li><a href="#method.call_once_force">call_once_force</a></li><li><a href="#method.is_completed">is_completed</a></li><li><a href="#method.new">new</a></li></ul><h3><a href="#trait-implementations">Trait Implementations</a></h3><ul class="block"><li><a href="#impl-Debug-for-Once">Debug</a></li><li><a href="#impl-RefUnwindSafe-for-Once">RefUnwindSafe</a></li><li><a href="#impl-UnwindSafe-for-Once">UnwindSafe</a></li></ul><h3><a href="#synthetic-implementations">Auto Trait Implementations</a></h3><ul class="block"><li><a href="#impl-Send-for-Once">Send</a></li><li><a href="#impl-Sync-for-Once">Sync</a></li><li><a href="#impl-Unpin-for-Once">Unpin</a></li></ul><h3><a href="#blanket-implementations">Blanket Implementations</a></h3><ul class="block"><li><a href="#impl-Any-for-Once">Any</a></li><li><a href="#impl-Borrow%3CT%3E-for-Once">Borrow<T></a></li><li><a href="#impl-BorrowMut%3CT%3E-for-Once">BorrowMut<T></a></li><li><a href="#impl-From%3CT%3E-for-Once">From<T></a></li><li><a href="#impl-Into%3CU%3E-for-Once">Into<U></a></li><li><a href="#impl-TryFrom%3CU%3E-for-Once">TryFrom<U></a></li><li><a href="#impl-TryInto%3CU%3E-for-Once">TryInto<U></a></li></ul></section><h2><a href="index.html">In tracing_core</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">Struct <a href="index.html">tracing_core</a>::<wbr><a class="struct" href="#">Once</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 id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust struct"><code>pub struct Once { /* private fields */ }</code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A synchronization primitive which can be used to run a one-time global |
| initialization. Useful for one-time initialization for FFI or related |
| functionality. This type can only be constructed with <a href="struct.Once.html#method.new" title="Once::new()"><code>Once::new()</code></a>.</p> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Once; |
| |
| <span class="kw">static </span>START: Once = Once::new(); |
| |
| START.call_once(|| { |
| <span class="comment">// run initialization here |
| </span>});</code></pre></div> |
| </div></details><h2 id="implementations" class="small-section-header">Implementations<a href="#implementations" class="anchor"></a></h2><div id="implementations-list"><details class="rustdoc-toggle implementors-toggle" open><summary><section id="impl-Once" class="impl has-srclink"><a href="#impl-Once" class="anchor"></a><h3 class="code-header">impl <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle method-toggle" open><summary><section id="method.new" class="method has-srclink"><h4 class="code-header">pub const fn <a href="#method.new" class="fnname">new</a>() -> <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h4></section></summary><div class="docblock"><p>Creates a new <code>Once</code> value.</p> |
| </div></details><details class="rustdoc-toggle method-toggle" open><summary><section id="method.call_once" class="method has-srclink"><h4 class="code-header">pub fn <a href="#method.call_once" class="fnname">call_once</a><F>(&self, f: F)<span class="where fmt-newline">where<br> F: FnOnce(),</span></h4></section></summary><div class="docblock"><p>Performs an initialization routine once and only once. The given closure |
| will be executed if this is the first time <code>call_once</code> has been called, |
| and otherwise the routine will <em>not</em> be invoked.</p> |
| <p>This method will block the calling thread if another initialization |
| routine is currently running.</p> |
| <p>When this function returns, it is guaranteed that some initialization |
| has run and completed (it might not be the closure specified). It is also |
| guaranteed that any memory writes performed by the executed closure can |
| be reliably observed by other threads at this point (there is a |
| happens-before relation between the closure and code executing after the |
| return).</p> |
| <p>If the given closure recursively invokes <code>call_once</code> on the same <a href="struct.Once.html" title="Once"><code>Once</code></a> |
| instance the exact behavior is not specified, allowed outcomes are |
| a panic or a deadlock.</p> |
| <h5 id="examples-1"><a href="#examples-1">Examples</a></h5> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Once; |
| |
| <span class="kw">static </span><span class="kw-2">mut </span>VAL: usize = <span class="number">0</span>; |
| <span class="kw">static </span>INIT: Once = Once::new(); |
| |
| <span class="comment">// Accessing a `static mut` is unsafe much of the time, but if we do so |
| // in a synchronized fashion (e.g., write once or read all) then we're |
| // good to go! |
| // |
| // This function will only call `expensive_computation` once, and will |
| // otherwise always return the value returned from the first invocation. |
| </span><span class="kw">fn </span>get_cached_val() -> usize { |
| <span class="kw">unsafe </span>{ |
| INIT.call_once(|| { |
| VAL = expensive_computation(); |
| }); |
| VAL |
| } |
| } |
| |
| <span class="kw">fn </span>expensive_computation() -> usize { |
| <span class="comment">// ... |
| </span>}</code></pre></div> |
| <h5 id="panics"><a href="#panics">Panics</a></h5> |
| <p>The closure <code>f</code> will only be executed once if this is called |
| concurrently amongst many threads. If that closure panics, however, then |
| it will <em>poison</em> this <a href="struct.Once.html" title="Once"><code>Once</code></a> instance, causing all future invocations of |
| <code>call_once</code> to also panic.</p> |
| <p>This is similar to <a href="struct.Mutex.html#poisoning">poisoning with mutexes</a>.</p> |
| </div></details><details class="rustdoc-toggle method-toggle" open><summary><section id="method.call_once_force" class="method has-srclink"><h4 class="code-header">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: FnOnce(&OnceState),</span></h4></section></summary><div class="docblock"><p>Performs the same function as <a href="struct.Once.html#method.call_once"><code>call_once()</code></a> except ignores poisoning.</p> |
| <p>Unlike <a href="struct.Once.html#method.call_once"><code>call_once()</code></a>, if this <a href="struct.Once.html" title="Once"><code>Once</code></a> has been poisoned (i.e., a previous |
| call to <a href="struct.Once.html#method.call_once"><code>call_once()</code></a> or <a href="struct.Once.html#method.call_once_force"><code>call_once_force()</code></a> caused a panic), calling |
| <a href="struct.Once.html#method.call_once_force"><code>call_once_force()</code></a> will still invoke the closure <code>f</code> and will <em>not</em> |
| result in an immediate panic. If <code>f</code> panics, the <a href="struct.Once.html" title="Once"><code>Once</code></a> will remain |
| in a poison state. If <code>f</code> does <em>not</em> panic, the <a href="struct.Once.html" title="Once"><code>Once</code></a> will no |
| longer be in a poison state and all future calls to <a href="struct.Once.html#method.call_once"><code>call_once()</code></a> or |
| <a href="struct.Once.html#method.call_once_force"><code>call_once_force()</code></a> will be no-ops.</p> |
| <p>The closure <code>f</code> is yielded a [<code>OnceState</code>] structure which can be used |
| to query the poison status of the <a href="struct.Once.html" title="Once"><code>Once</code></a>.</p> |
| <h5 id="examples-2"><a href="#examples-2">Examples</a></h5> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Once; |
| <span class="kw">use </span>std::thread; |
| |
| <span class="kw">static </span>INIT: Once = Once::new(); |
| |
| <span class="comment">// poison the once |
| </span><span class="kw">let </span>handle = thread::spawn(|| { |
| INIT.call_once(|| <span class="macro">panic!</span>()); |
| }); |
| <span class="macro">assert!</span>(handle.join().is_err()); |
| |
| <span class="comment">// poisoning propagates |
| </span><span class="kw">let </span>handle = thread::spawn(|| { |
| INIT.call_once(|| {}); |
| }); |
| <span class="macro">assert!</span>(handle.join().is_err()); |
| |
| <span class="comment">// call_once_force will still run and reset the poisoned state |
| </span>INIT.call_once_force(|state| { |
| <span class="macro">assert!</span>(state.is_poisoned()); |
| }); |
| |
| <span class="comment">// once any success happens, we stop propagating the poison |
| </span>INIT.call_once(|| {});</code></pre></div> |
| </div></details><details class="rustdoc-toggle method-toggle" open><summary><section id="method.is_completed" class="method has-srclink"><h4 class="code-header">pub fn <a href="#method.is_completed" class="fnname">is_completed</a>(&self) -> bool</h4></section></summary><div class="docblock"><p>Returns <code>true</code> if some <a href="struct.Once.html#method.call_once"><code>call_once()</code></a> call has completed |
| successfully. Specifically, <code>is_completed</code> will return false in |
| the following situations:</p> |
| <ul> |
| <li><a href="struct.Once.html#method.call_once"><code>call_once()</code></a> was not called at all,</li> |
| <li><a href="struct.Once.html#method.call_once"><code>call_once()</code></a> was called, but has not yet completed,</li> |
| <li>the <a href="struct.Once.html" title="Once"><code>Once</code></a> instance is poisoned</li> |
| </ul> |
| <p>This function returning <code>false</code> does not mean that <a href="struct.Once.html" title="Once"><code>Once</code></a> has not been |
| executed. For example, it may have been executed in the time between |
| when <code>is_completed</code> starts executing and when it returns, in which case |
| the <code>false</code> return value would be stale (but still permissible).</p> |
| <h5 id="examples-3"><a href="#examples-3">Examples</a></h5> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Once; |
| |
| <span class="kw">static </span>INIT: Once = Once::new(); |
| |
| <span class="macro">assert_eq!</span>(INIT.is_completed(), <span class="bool-val">false</span>); |
| INIT.call_once(|| { |
| <span class="macro">assert_eq!</span>(INIT.is_completed(), <span class="bool-val">false</span>); |
| }); |
| <span class="macro">assert_eq!</span>(INIT.is_completed(), <span class="bool-val">true</span>);</code></pre></div> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Once; |
| <span class="kw">use </span>std::thread; |
| |
| <span class="kw">static </span>INIT: Once = Once::new(); |
| |
| <span class="macro">assert_eq!</span>(INIT.is_completed(), <span class="bool-val">false</span>); |
| <span class="kw">let </span>handle = thread::spawn(|| { |
| INIT.call_once(|| <span class="macro">panic!</span>()); |
| }); |
| <span class="macro">assert!</span>(handle.join().is_err()); |
| <span class="macro">assert_eq!</span>(INIT.is_completed(), <span class="bool-val">false</span>);</code></pre></div> |
| </div></details></div></details></div><h2 id="trait-implementations" class="small-section-header">Trait Implementations<a href="#trait-implementations" class="anchor"></a></h2><div id="trait-implementations-list"><details class="rustdoc-toggle implementors-toggle" open><summary><section id="impl-Debug-for-Once" class="impl has-srclink"><a href="#impl-Debug-for-Once" class="anchor"></a><h3 class="code-header">impl Debug for <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle method-toggle" open><summary><section id="method.fmt" class="method trait-impl has-srclink"><a href="#method.fmt" class="anchor"></a><h4 class="code-header">fn <a class="fnname">fmt</a>(&self, f: &mut Formatter<'_>) -> Result<(), Error></h4></section></summary><div class='docblock'>Formats the value using the given formatter. <a>Read more</a></div></details></div></details><section id="impl-RefUnwindSafe-for-Once" class="impl has-srclink"><a href="#impl-RefUnwindSafe-for-Once" class="anchor"></a><h3 class="code-header">impl RefUnwindSafe for <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h3></section><section id="impl-UnwindSafe-for-Once" class="impl has-srclink"><a href="#impl-UnwindSafe-for-Once" class="anchor"></a><h3 class="code-header">impl UnwindSafe for <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h3></section></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"><section id="impl-Send-for-Once" class="impl has-srclink"><a href="#impl-Send-for-Once" class="anchor"></a><h3 class="code-header">impl Send for <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h3></section><section id="impl-Sync-for-Once" class="impl has-srclink"><a href="#impl-Sync-for-Once" class="anchor"></a><h3 class="code-header">impl Sync for <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h3></section><section id="impl-Unpin-for-Once" class="impl has-srclink"><a href="#impl-Unpin-for-Once" class="anchor"></a><h3 class="code-header">impl Unpin for <a class="struct" href="struct.Once.html" title="struct tracing_core::Once">Once</a></h3></section></div><h2 id="blanket-implementations" class="small-section-header">Blanket Implementations<a href="#blanket-implementations" class="anchor"></a></h2><div id="blanket-implementations-list"><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-Any-for-Once" class="impl has-srclink"><a href="#impl-Any-for-Once" class="anchor"></a><h3 class="code-header">impl<T> Any for T<span class="where fmt-newline">where<br> T: 'static + ?Sized,</span></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle method-toggle" open><summary><section id="method.type_id" class="method trait-impl has-srclink"><a href="#method.type_id" class="anchor"></a><h4 class="code-header">fn <a class="fnname">type_id</a>(&self) -> TypeId</h4></section></summary><div class='docblock'>Gets the <code>TypeId</code> of <code>self</code>. <a>Read more</a></div></details></div></details><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-Borrow%3CT%3E-for-Once" class="impl has-srclink"><a href="#impl-Borrow%3CT%3E-for-Once" class="anchor"></a><h3 class="code-header">impl<T> Borrow<T> for T<span class="where fmt-newline">where<br> T: ?Sized,</span></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle method-toggle" open><summary><section id="method.borrow" class="method trait-impl has-srclink"><span class="since rightside" title="const unstable">const: <a href="https://github.com/rust-lang/rust/issues/91522" title="Tracking issue for const_borrow">unstable</a></span><a href="#method.borrow" class="anchor"></a><h4 class="code-header">fn <a class="fnname">borrow</a>(&self) -> &T</h4></section></summary><div class='docblock'>Immutably borrows from an owned value. <a>Read more</a></div></details></div></details><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-BorrowMut%3CT%3E-for-Once" class="impl has-srclink"><a href="#impl-BorrowMut%3CT%3E-for-Once" class="anchor"></a><h3 class="code-header">impl<T> BorrowMut<T> for T<span class="where fmt-newline">where<br> T: ?Sized,</span></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle method-toggle" open><summary><section id="method.borrow_mut" class="method trait-impl has-srclink"><span class="since rightside" title="const unstable">const: <a href="https://github.com/rust-lang/rust/issues/91522" title="Tracking issue for const_borrow">unstable</a></span><a href="#method.borrow_mut" class="anchor"></a><h4 class="code-header">fn <a class="fnname">borrow_mut</a>(&mut self) -> &mut T</h4></section></summary><div class='docblock'>Mutably borrows from an owned value. <a>Read more</a></div></details></div></details><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-From%3CT%3E-for-Once" class="impl has-srclink"><a href="#impl-From%3CT%3E-for-Once" class="anchor"></a><h3 class="code-header">impl<T> From<T> for T</h3></section></summary><div class="impl-items"><details class="rustdoc-toggle method-toggle" open><summary><section id="method.from" class="method trait-impl has-srclink"><span class="since rightside" title="const unstable">const: <a href="https://github.com/rust-lang/rust/issues/88674" title="Tracking issue for const_convert">unstable</a></span><a href="#method.from" class="anchor"></a><h4 class="code-header">fn <a class="fnname">from</a>(t: T) -> T</h4></section></summary><div class="docblock"><p>Returns the argument unchanged.</p> |
| </div></details></div></details><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-Into%3CU%3E-for-Once" class="impl has-srclink"><a href="#impl-Into%3CU%3E-for-Once" class="anchor"></a><h3 class="code-header">impl<T, U> Into<U> for T<span class="where fmt-newline">where<br> U: From<T>,</span></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle method-toggle" open><summary><section id="method.into" class="method trait-impl has-srclink"><span class="since rightside" title="const unstable">const: <a href="https://github.com/rust-lang/rust/issues/88674" title="Tracking issue for const_convert">unstable</a></span><a href="#method.into" class="anchor"></a><h4 class="code-header">fn <a class="fnname">into</a>(self) -> U</h4></section></summary><div class="docblock"><p>Calls <code>U::from(self)</code>.</p> |
| <p>That is, this conversion is whatever the implementation of |
| <code>[From]<T> for U</code> chooses to do.</p> |
| </div></details></div></details><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-TryFrom%3CU%3E-for-Once" class="impl has-srclink"><a href="#impl-TryFrom%3CU%3E-for-Once" class="anchor"></a><h3 class="code-header">impl<T, U> TryFrom<U> for T<span class="where fmt-newline">where<br> U: Into<T>,</span></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle" open><summary><section id="associatedtype.Error-1" class="associatedtype trait-impl has-srclink"><a href="#associatedtype.Error-1" class="anchor"></a><h4 class="code-header">type <a class="associatedtype">Error</a> = Infallible</h4></section></summary><div class='docblock'>The type returned in the event of a conversion error.</div></details><details class="rustdoc-toggle method-toggle" open><summary><section id="method.try_from" class="method trait-impl has-srclink"><span class="since rightside" title="const unstable">const: <a href="https://github.com/rust-lang/rust/issues/88674" title="Tracking issue for const_convert">unstable</a></span><a href="#method.try_from" class="anchor"></a><h4 class="code-header">fn <a class="fnname">try_from</a>(value: U) -> Result<T, <T as TryFrom<U>>::Error></h4></section></summary><div class='docblock'>Performs the conversion.</div></details></div></details><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-TryInto%3CU%3E-for-Once" class="impl has-srclink"><a href="#impl-TryInto%3CU%3E-for-Once" class="anchor"></a><h3 class="code-header">impl<T, U> TryInto<U> for T<span class="where fmt-newline">where<br> U: TryFrom<T>,</span></h3></section></summary><div class="impl-items"><details class="rustdoc-toggle" open><summary><section id="associatedtype.Error" class="associatedtype trait-impl has-srclink"><a href="#associatedtype.Error" class="anchor"></a><h4 class="code-header">type <a class="associatedtype">Error</a> = <U as TryFrom<T>>::Error</h4></section></summary><div class='docblock'>The type returned in the event of a conversion error.</div></details><details class="rustdoc-toggle method-toggle" open><summary><section id="method.try_into" class="method trait-impl has-srclink"><span class="since rightside" title="const unstable">const: <a href="https://github.com/rust-lang/rust/issues/88674" title="Tracking issue for const_convert">unstable</a></span><a href="#method.try_into" class="anchor"></a><h4 class="code-header">fn <a class="fnname">try_into</a>(self) -> Result<U, <U as TryFrom<T>>::Error></h4></section></summary><div class='docblock'>Performs the conversion.</div></details></div></details></div></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="tracing_core" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
