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<title>crossbeam_epoch - Rust</title>
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<p class='location'>Crate crossbeam_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'></p><script>window.sidebarCurrent = {name: 'crossbeam_epoch', ty: 'mod', relpath: '../'};</script></div>
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<section id='main' class="content"><h1 class='fqn'><span class='in-band'>Crate <a class="mod" href=''>crossbeam_epoch</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../src/crossbeam_epoch/lib.rs.html#1-104' title='goto source code'>[src]</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>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Atomic.html"
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title='struct crossbeam_epoch::Atomic'>Atomic</a></td>
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<td class='docblock-short'>
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<p>An atomic pointer that can be safely shared between threads.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Collector.html"
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title='struct crossbeam_epoch::Collector'>Collector</a></td>
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<td class='docblock-short'>
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<p>An epoch-based garbage collector.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.CompareAndSetError.html"
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title='struct crossbeam_epoch::CompareAndSetError'>CompareAndSetError</a></td>
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<td class='docblock-short'>
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<p>The error returned on failed compare-and-set operation.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Guard.html"
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title='struct crossbeam_epoch::Guard'>Guard</a></td>
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<td class='docblock-short'>
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<p>A guard that keeps the current thread pinned.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Handle.html"
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title='struct crossbeam_epoch::Handle'>Handle</a></td>
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<td class='docblock-short'>
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<p>A handle to a garbage collector.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Owned.html"
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title='struct crossbeam_epoch::Owned'>Owned</a></td>
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<td class='docblock-short'>
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<p>An owned heap-allocated object.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Shared.html"
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title='struct crossbeam_epoch::Shared'>Shared</a></td>
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<td class='docblock-short'>
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<p>A pointer to an object protected by the epoch GC.</p>
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</td>
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</tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="trait" href="trait.CompareAndSetOrdering.html"
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title='trait crossbeam_epoch::CompareAndSetOrdering'>CompareAndSetOrdering</a></td>
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<td class='docblock-short'>
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<p>Memory orderings for compare-and-set operations.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="trait" href="trait.Pointer.html"
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title='trait crossbeam_epoch::Pointer'>Pointer</a></td>
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<td class='docblock-short'>
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<p>A trait for either <code>Owned</code> or <code>Shared</code> pointers.</p>
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</td>
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</tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="fn" href="fn.default_collector.html"
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title='fn crossbeam_epoch::default_collector'>default_collector</a></td>
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<td class='docblock-short'>
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<p>Returns the default handle associated with the current thread.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="fn" href="fn.default_handle.html"
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title='fn crossbeam_epoch::default_handle'>default_handle</a></td>
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<td class='docblock-short'>
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<p>Returns the default handle associated with the current thread.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="fn" href="fn.is_pinned.html"
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title='fn crossbeam_epoch::is_pinned'>is_pinned</a></td>
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<td class='docblock-short'>
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<p>Returns <code>true</code> if the current thread is pinned.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="fn" href="fn.pin.html"
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title='fn crossbeam_epoch::pin'>pin</a></td>
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<td class='docblock-short'>
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<p>Pins the current thread.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="fn" href="fn.unprotected.html"
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title='fn crossbeam_epoch::unprotected'>unprotected</a><a title='unsafe function' href='#'><sup>⚠</sup></a></td>
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<td class='docblock-short'>
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<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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