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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><
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<h1><a class="header" href="#fearless-concurrency" id="fearless-concurrency">Fearless Concurrency</a></h1>
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<p>Handling concurrent programming safely and efficiently is another of Rust’s
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major goals. <em>Concurrent programming</em>, where different parts of a program
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execute independently, and <em>parallel programming</em>, where different parts of a
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program execute at the same time, are becoming increasingly important as more
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computers take advantage of their multiple processors. Historically,
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programming in these contexts has been difficult and error prone: Rust hopes to
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change that.</p>
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<p>Initially, the Rust team thought that ensuring memory safety and preventing
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concurrency problems were two separate challenges to be solved with different
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methods. Over time, the team discovered that the ownership and type systems are
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a powerful set of tools to help manage memory safety <em>and</em> concurrency
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problems! By leveraging ownership and type checking, many concurrency errors
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are compile-time errors in Rust rather than runtime errors. Therefore, rather
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than making you spend lots of time trying to reproduce the exact circumstances
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under which a runtime concurrency bug occurs, incorrect code will refuse to
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compile and present an error explaining the problem. As a result, you can fix
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your code while you’re working on it rather than potentially after it has been
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shipped to production. We’ve nicknamed this aspect of Rust <em>fearless</em>
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<em>concurrency</em>. Fearless concurrency allows you to write code that is free of
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subtle bugs and is easy to refactor without introducing new bugs.</p>
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<blockquote>
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<p>Note: For simplicity’s sake, we’ll refer to many of the problems as
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<em>concurrent</em> rather than being more precise by saying <em>concurrent and/or
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parallel</em>. If this book were about concurrency and/or parallelism, we’d be
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more specific. For this chapter, please mentally substitute <em>concurrent
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and/or parallel</em> whenever we use <em>concurrent</em>.</p>
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</blockquote>
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<p>Many languages are dogmatic about the solutions they offer for handling
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concurrent problems. For example, Erlang has elegant functionality for
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message-passing concurrency but has only obscure ways to share state between
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threads. Supporting only a subset of possible solutions is a reasonable
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strategy for higher-level languages, because a higher-level language promises
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benefits from giving up some control to gain abstractions. However, lower-level
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languages are expected to provide the solution with the best performance in any
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given situation and have fewer abstractions over the hardware. Therefore, Rust
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offers a variety of tools for modeling problems in whatever way is appropriate
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for your situation and requirements.</p>
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<p>Here are the topics we’ll cover in this chapter:</p>
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<ul>
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<li>How to create threads to run multiple pieces of code at the same time</li>
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<li><em>Message-passing</em> concurrency, where channels send messages between threads</li>
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<li><em>Shared-state</em> concurrency, where multiple threads have access to some piece
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of data</li>
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<li>The <code>Sync</code> and <code>Send</code> traits, which extend Rust’s concurrency guarantees to
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user-defined types as well as types provided by the standard library</li>
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</ul>
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