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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 `epoch` mod in crate `crossbeam`."><meta name="keywords" content="rust, rustlang, rust-lang, epoch"><title>crossbeam::epoch - 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><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="shortcut icon" href="../../favicon.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[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><a href='../../crossbeam/index.html'><div class='logo-container'><img src='../../rust-logo.png' alt='logo'></div></a><p class='location'>Module epoch</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li></ul></div><p class='location'><a href='../index.html'>crossbeam</a></p><script>window.sidebarCurrent = {name: 'epoch', ty: 'mod', 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"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><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='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/crossbeam_epoch/lib.rs.html#1-106' title='goto source code'>[src]</a></span><span class='in-band'>Module <a href='../index.html'>crossbeam</a>::<wbr><a class="mod" href=''>epoch</a></span></h1><div class='docblock'><p>Epoch-based memory reclamation.</p>
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<p>An interesting problem concurrent collections deal with comes from the remove operation.
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Suppose that a thread removes an element from a lock-free map, while another thread is reading
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that same element at the same time. The first thread must wait until the second thread stops
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reading the element. Only then it is safe to destruct it.</p>
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<p>Programming languages that come with garbage collectors solve this problem trivially. The
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garbage collector will destruct the removed element when no thread can hold a reference to it
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anymore.</p>
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<p>This crate implements a basic memory reclamation mechanism, which is based on epochs. When an
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element gets removed from a concurrent collection, it is inserted into a pile of garbage and
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marked with the current epoch. Every time a thread accesses a collection, it checks the current
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epoch, attempts to increment it, and destructs some garbage that became so old that no thread
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can be referencing it anymore.</p>
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<p>That is the general mechanism behind epoch-based memory reclamation, but the details are a bit
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more complicated. Anyhow, memory reclamation is designed to be fully automatic and something
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users of concurrent collections don't have to worry much about.</p>
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<h1 id="pointers" class="section-header"><a href="#pointers">Pointers</a></h1>
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<p>Concurrent collections are built using atomic pointers. This module provides <a href="struct.Atomic.html"><code>Atomic</code></a>, which
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is just a shared atomic pointer to a heap-allocated object. Loading an <a href="struct.Atomic.html"><code>Atomic</code></a> yields a
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<a href="struct.Shared.html"><code>Shared</code></a>, which is an epoch-protected pointer through which the loaded object can be safely
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read.</p>
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<h1 id="pinning" class="section-header"><a href="#pinning">Pinning</a></h1>
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<p>Before an <a href="struct.Atomic.html"><code>Atomic</code></a> can be loaded, a participant must be <a href="fn.pin.html"><code>pin</code></a>ned. By pinning a participant
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we declare that any object that gets removed from now on must not be destructed just
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yet. Garbage collection of newly removed objects is suspended until the participant gets
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unpinned.</p>
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<h1 id="garbage" class="section-header"><a href="#garbage">Garbage</a></h1>
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<p>Objects that get removed from concurrent collections must be stashed away until all currently
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pinned participants get unpinned. Such objects can be stored into a thread-local or global
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storage, where they are kept until the right time for their destruction comes.</p>
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<p>There is a global shared instance of garbage queue. You can <a href="fn.defer.html"><code>defer</code></a> the execution of an
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arbitrary function until the global epoch is advanced enough. Most notably, concurrent data
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structures may defer the deallocation of an object.</p>
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<h1 id="apis" class="section-header"><a href="#apis">APIs</a></h1>
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<p>For majority of use cases, just use the default garbage collector by invoking <a href="fn.pin.html"><code>pin</code></a>. If you
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want to create your own garbage collector, use the <a href="struct.Collector.html"><code>Collector</code></a> API.</p>
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</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table><tr class='module-item'><td><a class="struct" href="struct.Atomic.html" title='crossbeam::epoch::Atomic struct'>Atomic</a></td><td class='docblock-short'><p>An atomic pointer that can be safely shared between threads.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.Collector.html" title='crossbeam::epoch::Collector struct'>Collector</a></td><td class='docblock-short'><p>An epoch-based garbage collector.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.CompareAndSetError.html" title='crossbeam::epoch::CompareAndSetError struct'>CompareAndSetError</a></td><td class='docblock-short'><p>The error returned on failed compare-and-set operation.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.Guard.html" title='crossbeam::epoch::Guard struct'>Guard</a></td><td class='docblock-short'><p>A guard that keeps the current thread pinned.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.LocalHandle.html" title='crossbeam::epoch::LocalHandle struct'>LocalHandle</a></td><td class='docblock-short'><p>A handle to a garbage collector.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.Owned.html" title='crossbeam::epoch::Owned struct'>Owned</a></td><td class='docblock-short'><p>An owned heap-allocated object.</p>
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</td></tr><tr class='module-item'><td><a class="struct" href="struct.Shared.html" title='crossbeam::epoch::Shared struct'>Shared</a></td><td class='docblock-short'><p>A pointer to an object protected by the epoch GC.</p>
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</td></tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table><tr class='module-item'><td><a class="trait" href="trait.CompareAndSetOrdering.html" title='crossbeam::epoch::CompareAndSetOrdering trait'>CompareAndSetOrdering</a></td><td class='docblock-short'><p>Memory orderings for compare-and-set operations.</p>
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</td></tr><tr class='module-item'><td><a class="trait" href="trait.Pointer.html" title='crossbeam::epoch::Pointer trait'>Pointer</a></td><td class='docblock-short'><p>A trait for either <code>Owned</code> or <code>Shared</code> pointers.</p>
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</td></tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table><tr class='module-item'><td><a class="fn" href="fn.default_collector.html" title='crossbeam::epoch::default_collector fn'>default_collector</a></td><td class='docblock-short'><p>Returns the default global collector.</p>
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</td></tr><tr class='module-item'><td><a class="fn" href="fn.is_pinned.html" title='crossbeam::epoch::is_pinned fn'>is_pinned</a></td><td class='docblock-short'><p>Returns <code>true</code> if the current thread is pinned.</p>
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</td></tr><tr class='module-item'><td><a class="fn" href="fn.pin.html" title='crossbeam::epoch::pin fn'>pin</a></td><td class='docblock-short'><p>Pins the current thread.</p>
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</td></tr><tr class='module-item'><td><a class="fn" href="fn.unprotected.html" title='crossbeam::epoch::unprotected fn'>unprotected</a><a title='unsafe function' href='#'><sup>⚠</sup></a></td><td class='docblock-short'><p>Returns a reference to a dummy guard that allows unprotected access to <a href="struct.Atomic.html"><code>Atomic</code></a>s.</p>
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</td></tr></table></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 = "crossbeam";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>
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