| <!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="Creates a “fork-join” scope `s` and invokes the closure with a reference to `s`. This closure can then spawn asynchronous tasks into `s`. Those tasks may run asynchronously with respect to the closure; they may themselves spawn additional tasks into `s`. When the closure returns, it will block until all tasks that have been spawned into `s` complete."><meta name="keywords" content="rust, rustlang, rust-lang, scope"><title>scope in rayon_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 fn"><!--[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="../rayon_core/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="../rayon_core/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 rayon_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">Function <a href="index.html">rayon_core</a>::<wbr><a class="fn" href="#">scope</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/rayon_core/scope/mod.rs.html#304-313">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"><pre class="rust fn"><code>pub fn scope<'scope, OP, R>(op: OP) -> R<span class="where fmt-newline">where<br> OP: FnOnce(&<a class="struct" href="struct.Scope.html" title="struct rayon_core::Scope">Scope</a><'scope>) -> R + Send,<br> R: Send,</span></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Creates a “fork-join” scope <code>s</code> and invokes the closure with a |
| reference to <code>s</code>. This closure can then spawn asynchronous tasks |
| into <code>s</code>. Those tasks may run asynchronously with respect to the |
| closure; they may themselves spawn additional tasks into <code>s</code>. When |
| the closure returns, it will block until all tasks that have been |
| spawned into <code>s</code> complete.</p> |
| <p><code>scope()</code> is a more flexible building block compared to <code>join()</code>, |
| since a loop can be used to spawn any number of tasks without |
| recursing. However, that flexibility comes at a performance price: |
| tasks spawned using <code>scope()</code> must be allocated onto the heap, |
| whereas <code>join()</code> can make exclusive use of the stack. <strong>Prefer |
| <code>join()</code> (or, even better, parallel iterators) where possible.</strong></p> |
| <h2 id="example"><a href="#example">Example</a></h2> |
| <p>The Rayon <code>join()</code> function launches two closures and waits for them |
| to stop. One could implement <code>join()</code> using a scope like so, although |
| it would be less efficient than the real implementation:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">pub fn </span>join<A,B,RA,RB>(oper_a: A, oper_b: B) -> (RA, RB) |
| <span class="kw">where </span>A: FnOnce() -> RA + Send, |
| B: FnOnce() -> RB + Send, |
| RA: Send, |
| RB: Send, |
| { |
| <span class="kw">let </span><span class="kw-2">mut </span>result_a: <span class="prelude-ty">Option</span><RA> = <span class="prelude-val">None</span>; |
| <span class="kw">let </span><span class="kw-2">mut </span>result_b: <span class="prelude-ty">Option</span><RB> = <span class="prelude-val">None</span>; |
| rayon::scope(|s| { |
| s.spawn(|<span class="kw">_</span>| result_a = <span class="prelude-val">Some</span>(oper_a())); |
| s.spawn(|<span class="kw">_</span>| result_b = <span class="prelude-val">Some</span>(oper_b())); |
| }); |
| (result_a.unwrap(), result_b.unwrap()) |
| }</code></pre></div> |
| <h2 id="a-note-on-threading"><a href="#a-note-on-threading">A note on threading</a></h2> |
| <p>The closure given to <code>scope()</code> executes in the Rayon thread-pool, |
| as do those given to <code>spawn()</code>. This means that you can’t access |
| thread-local variables (well, you can, but they may have |
| unexpected values).</p> |
| <h2 id="task-execution"><a href="#task-execution">Task execution</a></h2> |
| <p>Task execution potentially starts as soon as <code>spawn()</code> is called. |
| The task will end sometime before <code>scope()</code> returns. Note that the |
| <em>closure</em> given to scope may return much earlier. In general |
| the lifetime of a scope created like <code>scope(body)</code> goes something like this:</p> |
| <ul> |
| <li>Scope begins when <code>scope(body)</code> is called</li> |
| <li>Scope body <code>body()</code> is invoked |
| <ul> |
| <li>Scope tasks may be spawned</li> |
| </ul> |
| </li> |
| <li>Scope body returns</li> |
| <li>Scope tasks execute, possibly spawning more tasks</li> |
| <li>Once all tasks are done, scope ends and <code>scope()</code> returns</li> |
| </ul> |
| <p>To see how and when tasks are joined, consider this example:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// point start |
| </span>rayon::scope(|s| { |
| s.spawn(|s| { <span class="comment">// task s.1 |
| </span>s.spawn(|s| { <span class="comment">// task s.1.1 |
| </span>rayon::scope(|t| { |
| t.spawn(|<span class="kw">_</span>| ()); <span class="comment">// task t.1 |
| </span>t.spawn(|<span class="kw">_</span>| ()); <span class="comment">// task t.2 |
| </span>}); |
| }); |
| }); |
| s.spawn(|s| { <span class="comment">// task s.2 |
| </span>}); |
| <span class="comment">// point mid |
| </span>}); |
| <span class="comment">// point end</span></code></pre></div> |
| <p>The various tasks that are run will execute roughly like so:</p> |
| <div class="example-wrap"><pre class="language-notrust"><code>| (start) |
| | |
| | (scope `s` created) |
| +-----------------------------------------------+ (task s.2) |
| +-------+ (task s.1) | |
| | | | |
| | +---+ (task s.1.1) | |
| | | | | |
| | | | (scope `t` created) | |
| | | +----------------+ (task t.2) | |
| | | +---+ (task t.1) | | |
| | (mid) | | | | | |
| : | + <-+------------+ (scope `t` ends) | |
| : | | | |
| |<------+---+-----------------------------------+ (scope `s` ends) |
| | |
| | (end)</code></pre></div> |
| <p>The point here is that everything spawned into scope <code>s</code> will |
| terminate (at latest) at the same point – right before the |
| original call to <code>rayon::scope</code> returns. This includes new |
| subtasks created by other subtasks (e.g., task <code>s.1.1</code>). If a new |
| scope is created (such as <code>t</code>), the things spawned into that scope |
| will be joined before that scope returns, which in turn occurs |
| before the creating task (task <code>s.1.1</code> in this case) finishes.</p> |
| <p>There is no guaranteed order of execution for spawns in a scope, |
| given that other threads may steal tasks at any time. However, they |
| are generally prioritized in a LIFO order on the thread from which |
| they were spawned. So in this example, absent any stealing, we can |
| expect <code>s.2</code> to execute before <code>s.1</code>, and <code>t.2</code> before <code>t.1</code>. Other |
| threads always steal from the other end of the deque, like FIFO |
| order. The idea is that “recent” tasks are most likely to be fresh |
| in the local CPU’s cache, while other threads can steal older |
| “stale” tasks. For an alternate approach, consider |
| <a href="fn.scope_fifo.html"><code>scope_fifo()</code></a> instead.</p> |
| <h2 id="accessing-stack-data"><a href="#accessing-stack-data">Accessing stack data</a></h2> |
| <p>In general, spawned tasks may access stack data in place that |
| outlives the scope itself. Other data must be fully owned by the |
| spawned task.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>ok: Vec<i32> = <span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]; |
| rayon::scope(|s| { |
| <span class="kw">let </span>bad: Vec<i32> = <span class="macro">vec!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>]; |
| s.spawn(|<span class="kw">_</span>| { |
| <span class="comment">// We can access `ok` because outlives the scope `s`. |
| </span><span class="macro">println!</span>(<span class="string">"ok: {:?}"</span>, ok); |
| |
| <span class="comment">// If we just try to use `bad` here, the closure will borrow `bad` |
| // (because we are just printing it out, and that only requires a |
| // borrow), which will result in a compilation error. Read on |
| // for options. |
| // println!("bad: {:?}", bad); |
| </span>}); |
| });</code></pre></div> |
| <p>As the comments example above suggest, to reference <code>bad</code> we must |
| take ownership of it. One way to do this is to detach the closure |
| from the surrounding stack frame, using the <code>move</code> keyword. This |
| will cause it to take ownership of <em>all</em> the variables it touches, |
| in this case including both <code>ok</code> <em>and</em> <code>bad</code>:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>ok: Vec<i32> = <span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]; |
| rayon::scope(|s| { |
| <span class="kw">let </span>bad: Vec<i32> = <span class="macro">vec!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>]; |
| s.spawn(<span class="kw">move </span>|<span class="kw">_</span>| { |
| <span class="macro">println!</span>(<span class="string">"ok: {:?}"</span>, ok); |
| <span class="macro">println!</span>(<span class="string">"bad: {:?}"</span>, bad); |
| }); |
| |
| <span class="comment">// That closure is fine, but now we can't use `ok` anywhere else, |
| // since it is owned by the previous task: |
| // s.spawn(|_| println!("ok: {:?}", ok)); |
| </span>});</code></pre></div> |
| <p>While this works, it could be a problem if we want to use <code>ok</code> elsewhere. |
| There are two choices. We can keep the closure as a <code>move</code> closure, but |
| instead of referencing the variable <code>ok</code>, we create a shadowed variable that |
| is a borrow of <code>ok</code> and capture <em>that</em>:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>ok: Vec<i32> = <span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]; |
| rayon::scope(|s| { |
| <span class="kw">let </span>bad: Vec<i32> = <span class="macro">vec!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>]; |
| <span class="kw">let </span>ok: <span class="kw-2">&</span>Vec<i32> = <span class="kw-2">&</span>ok; <span class="comment">// shadow the original `ok` |
| </span>s.spawn(<span class="kw">move </span>|<span class="kw">_</span>| { |
| <span class="macro">println!</span>(<span class="string">"ok: {:?}"</span>, ok); <span class="comment">// captures the shadowed version |
| </span><span class="macro">println!</span>(<span class="string">"bad: {:?}"</span>, bad); |
| }); |
| |
| <span class="comment">// Now we too can use the shadowed `ok`, since `&Vec<i32>` references |
| // can be shared freely. Note that we need a `move` closure here though, |
| // because otherwise we'd be trying to borrow the shadowed `ok`, |
| // and that doesn't outlive `scope`. |
| </span>s.spawn(<span class="kw">move </span>|<span class="kw">_</span>| <span class="macro">println!</span>(<span class="string">"ok: {:?}"</span>, ok)); |
| });</code></pre></div> |
| <p>Another option is not to use the <code>move</code> keyword but instead to take ownership |
| of individual variables:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>ok: Vec<i32> = <span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]; |
| rayon::scope(|s| { |
| <span class="kw">let </span>bad: Vec<i32> = <span class="macro">vec!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>]; |
| s.spawn(|<span class="kw">_</span>| { |
| <span class="comment">// Transfer ownership of `bad` into a local variable (also named `bad`). |
| // This will force the closure to take ownership of `bad` from the environment. |
| </span><span class="kw">let </span>bad = bad; |
| <span class="macro">println!</span>(<span class="string">"ok: {:?}"</span>, ok); <span class="comment">// `ok` is only borrowed. |
| </span><span class="macro">println!</span>(<span class="string">"bad: {:?}"</span>, bad); <span class="comment">// refers to our local variable, above. |
| </span>}); |
| |
| s.spawn(|<span class="kw">_</span>| <span class="macro">println!</span>(<span class="string">"ok: {:?}"</span>, ok)); <span class="comment">// we too can borrow `ok` |
| </span>});</code></pre></div> |
| <h2 id="panics"><a href="#panics">Panics</a></h2> |
| <p>If a panic occurs, either in the closure given to <code>scope()</code> or in |
| any of the spawned jobs, that panic will be propagated and the |
| call to <code>scope()</code> will panic. If multiple panics occurs, it is |
| non-deterministic which of their panic values will propagate. |
| Regardless, once a task is spawned using <code>scope.spawn()</code>, it will |
| execute, even if the spawning task should later panic. <code>scope()</code> |
| returns once all spawned jobs have completed, and any panics are |
| propagated at that point.</p> |
| </div></details></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="rayon_core" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
