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<section id='main' class="content"><h1 class='fqn'><span class='in-band'>Function <a href='index.html'>crossbeam_epoch</a>::<wbr><a class="fn" href=''>unprotected</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../src/crossbeam_epoch/guard.rs.html#435-444' title='goto source code'>[src]</a></span></h1><pre class='rust fn'>pub unsafe fn unprotected() -> &'static <a class="struct" href="../crossbeam_epoch/struct.Guard.html" title="struct crossbeam_epoch::Guard">Guard</a></pre><div class='docblock'><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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<p>This guard should be used in special occasions only. Note that it doesn't actually keep any
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thread pinned - it's just a fake guard that allows loading from <a href="struct.Atomic.html"><code>Atomic</code></a>s unsafely.</p>
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<p>Note that calling <a href="struct.Guard.html#method.defer"><code>defer</code></a> with a dummy guard will not defer the function - it will just
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execute the function immediately.</p>
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<p>If necessary, it's possible to create more dummy guards by cloning: <code>unprotected().clone()</code>.</p>
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<h1 id="safety" class="section-header"><a href="#safety">Safety</a></h1>
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<p>Loading and dereferencing data from an <a href="struct.Atomic.html"><code>Atomic</code></a> using this guard is safe only if the
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<a href="struct.Atomic.html"><code>Atomic</code></a> is not being concurrently modified by other threads.</p>
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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">crossbeam_epoch</span>::{<span class="self">self</span> <span class="kw">as</span> <span class="ident">epoch</span>, <span class="ident">Atomic</span>};
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>;
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<span class="kw">let</span> <span class="ident">a</span> <span class="op">=</span> <span class="ident">Atomic</span>::<span class="ident">new</span>(<span class="number">7</span>);
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<span class="kw">unsafe</span> {
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<span class="comment">// Load `a` without pinning the current thread.</span>
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<span class="ident">a</span>.<span class="ident">load</span>(<span class="ident">Relaxed</span>, <span class="ident">epoch</span>::<span class="ident">unprotected</span>());
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<span class="comment">// It's possible to create more dummy guards by calling `clone()`.</span>
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<span class="kw">let</span> <span class="ident">dummy</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">epoch</span>::<span class="ident">unprotected</span>().<span class="ident">clone</span>();
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<span class="ident">dummy</span>.<span class="ident">defer</span>(<span class="kw">move</span> <span class="op">||</span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"This gets executed immediately."</span>);
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});
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<span class="comment">// Dropping `dummy` doesn't affect the current thread - it's just a noop.</span>
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}</pre>
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<p>The most common use of this function is when constructing or destructing a data structure.</p>
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<p>For example, we can use a dummy guard in the destructor of a Treiber stack because at that
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point no other thread could concurrently modify the <a href="struct.Atomic.html"><code>Atomic</code></a>s we are accessing.</p>
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<p>If we were to actually pin the current thread during destruction, that would just unnecessarily
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delay garbage collection and incur some performance cost, so in cases like these <code>unprotected</code>
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is very helpful.</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">crossbeam_epoch</span>::{<span class="self">self</span> <span class="kw">as</span> <span class="ident">epoch</span>, <span class="ident">Atomic</span>};
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">ManuallyDrop</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>;
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<span class="kw">struct</span> <span class="ident">Stack</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">head</span>: <span class="ident">Atomic</span><span class="op"><</span><span class="ident">Node</span><span class="op"><</span><span class="ident">T</span><span class="op">>></span>,
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}
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<span class="kw">struct</span> <span class="ident">Node</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">data</span>: <span class="ident">ManuallyDrop</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
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<span class="ident">next</span>: <span class="ident">Atomic</span><span class="op"><</span><span class="ident">Node</span><span class="op"><</span><span class="ident">T</span><span class="op">>></span>,
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">Stack</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) {
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<span class="kw">unsafe</span> {
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<span class="comment">// Unprotected load.</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">node</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Relaxed</span>, <span class="ident">epoch</span>::<span class="ident">unprotected</span>());
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<span class="kw">while</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">n</span>) <span class="op">=</span> <span class="ident">node</span>.<span class="ident">as_ref</span>() {
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<span class="comment">// Unprotected load.</span>
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<span class="kw">let</span> <span class="ident">next</span> <span class="op">=</span> <span class="ident">n</span>.<span class="ident">next</span>.<span class="ident">load</span>(<span class="ident">Relaxed</span>, <span class="ident">epoch</span>::<span class="ident">unprotected</span>());
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<span class="comment">// Take ownership of the node, then drop its data and deallocate it.</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">o</span> <span class="op">=</span> <span class="ident">node</span>.<span class="ident">into_owned</span>();
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<span class="ident">ManuallyDrop</span>::<span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">o</span>.<span class="ident">data</span>);
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<span class="ident">drop</span>(<span class="ident">o</span>);
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<span class="ident">node</span> <span class="op">=</span> <span class="ident">next</span>;
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}
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}
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}
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}</pre>
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