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new docs and new readme
2023-10-11 17:42:27 +02:00
<!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 compiler memory fence."><title>compiler_fence in atomic_polyfill - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Bold-a2c9cd1067f8b328.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-fa3bb1812debf86c.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="atomic_polyfill" data-themes="" data-resource-suffix="" data-rustdoc-version="1.75.0-nightly (2bbb61989 2023-10-04)" data-channel="nightly" data-search-js="search-8be46b629f5f14a8.js" data-settings-js="settings-74424d7eec62a23e.js" ><script src="../static.files/storage-fec3eaa3851e447d.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-c5bd66d33317d69f.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-5d8b3c7633ad77ba.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.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">&#9776;</button><a class="logo-container" href="../atomic_polyfill/index.html"><img class="rust-logo" src="../static.files/rust-logo-151179464ae7ed46.svg" alt="logo"></a></nav><nav class="sidebar"><a class="logo-container" href="../atomic_polyfill/index.html"><img class="rust-logo" src="../static.files/rust-logo-151179464ae7ed46.svg" alt="logo"></a><div class="sidebar-elems"><h2><a href="index.html">In atomic_polyfill</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><input class="search-input" name="search" aria-label="Run search in the documentation" 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="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Function <a href="index.html">atomic_polyfill</a>::<wbr><a class="fn" href="#">compiler_fence</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><span class="since" title="Stable since Rust version 1.21.0">1.21.0</span> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><pre class="rust item-decl"><code>pub fn compiler_fence(order: <a class="enum" href="enum.Ordering.html" title="enum atomic_polyfill::Ordering">Ordering</a>)</code><
all FX functions are now safe wrapped and I added the crate to the docs also it is added to the prelude
2023-10-04 14:55:54 +02:00
<p><code>compiler_fence</code> does not emit any machine code, but restricts the kinds
of memory re-ordering the compiler is allowed to do. Specifically, depending on
the given <a href="enum.Ordering.html" title="enum atomic_polyfill::Ordering"><code>Ordering</code></a> semantics, the compiler may be disallowed from moving reads
or writes from before or after the call to the other side of the call to
<code>compiler_fence</code>. Note that it does <strong>not</strong> prevent the <em>hardware</em>
from doing such re-ordering. This is not a problem in a single-threaded,
execution context, but when other threads may modify memory at the same
time, stronger synchronization primitives such as <a href="fn.fence.html" title="fn atomic_polyfill::fence"><code>fence</code></a> are required.</p>
<p>The re-ordering prevented by the different ordering semantics are:</p>
<ul>
<li>with <a href="enum.Ordering.html#variant.SeqCst" title="variant atomic_polyfill::Ordering::SeqCst"><code>SeqCst</code></a>, no re-ordering of reads and writes across this point is allowed.</li>
<li>with <a href="enum.Ordering.html#variant.Release" title="variant atomic_polyfill::Ordering::Release"><code>Release</code></a>, preceding reads and writes cannot be moved past subsequent writes.</li>
<li>with <a href="enum.Ordering.html#variant.Acquire" title="variant atomic_polyfill::Ordering::Acquire"><code>Acquire</code></a>, subsequent reads and writes cannot be moved ahead of preceding reads.</li>
<li>with <a href="enum.Ordering.html#variant.AcqRel" title="variant atomic_polyfill::Ordering::AcqRel"><code>AcqRel</code></a>, both of the above rules are enforced.</li>
</ul>
<p><code>compiler_fence</code> is generally only useful for preventing a thread from
racing <em>with itself</em>. That is, if a given thread is executing one piece
of code, and is then interrupted, and starts executing code elsewhere
(while still in the same thread, and conceptually still on the same
core). In traditional programs, this can only occur when a signal
handler is registered. In more low-level code, such situations can also
arise when handling interrupts, when implementing green threads with
pre-emption, etc. Curious readers are encouraged to read the Linux kernel’s
discussion of <a href="https://www.kernel.org/doc/Documentation/memory-barriers.txt">memory barriers</a>.</p>
<h2 id="panics"><a href="#panics">Panics</a></h2>
<p>Panics if <code>order</code> is <a href="enum.Ordering.html#variant.Relaxed" title="variant atomic_polyfill::Ordering::Relaxed"><code>Relaxed</code></a>.</p>
<h2 id="examples"><a href="#examples">Examples</a></h2>
<p>Without <code>compiler_fence</code>, the <code>assert_eq!</code> in following code
is <em>not</em> guaranteed to succeed, despite everything happening in a single thread.
To see why, remember that the compiler is free to swap the stores to
<code>IMPORTANT_VARIABLE</code> and <code>IS_READY</code> since they are both
<code>Ordering::Relaxed</code>. If it does, and the signal handler is invoked right
after <code>IS_READY</code> is updated, then the signal handler will see
<code>IS_READY=1</code>, but <code>IMPORTANT_VARIABLE=0</code>.
Using a <code>compiler_fence</code> remedies this situation.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::atomic::{AtomicBool, AtomicUsize};
<span class="kw">use </span>std::sync::atomic::Ordering;
<span class="kw">use </span>std::sync::atomic::compiler_fence;
<span class="kw">static </span>IMPORTANT_VARIABLE: AtomicUsize = AtomicUsize::new(<span class="number">0</span>);
<span class="kw">static </span>IS_READY: AtomicBool = AtomicBool::new(<span class="bool-val">false</span>);
<span class="kw">fn </span>main() {
IMPORTANT_VARIABLE.store(<span class="number">42</span>, Ordering::Relaxed);
<span class="comment">// prevent earlier writes from being moved beyond this point
</span>compiler_fence(Ordering::Release);
IS_READY.store(<span class="bool-val">true</span>, Ordering::Relaxed);
}
<span class="kw">fn </span>signal_handler() {
<span class="kw">if </span>IS_READY.load(Ordering::Relaxed) {
<span class="macro">assert_eq!</span>(IMPORTANT_VARIABLE.load(Ordering::Relaxed), <span class="number">42</span>);
}
}</code></pre></div>
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