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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">Rust 程序设计语言</a></li><li class="chapter-item expanded affix "><a href="foreword.html">前言</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">简介</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> 入门指南</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> 安装</a></li><li class="chapter-item expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="chapter-item expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> 写个猜数字游戏</a></li><li class="chapter-item expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> 常见编程概念</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> 变量与可变性</a></li><li class="chapter-item expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> 数据类型</a></li><li class="chapter-item expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> 函数</a></li><li class="chapter-item expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> 控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> 认识所有权</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> 什么是所有权？</a></li><li class="chapter-item expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> 引用与借用</a></li><li class="chapter-item expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slice 类型</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> 使用结构体组织相关联的数据</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> 结构体的定义和实例化</a></li><li class="chapter-item expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> 结构体示例程序</a></li><li class="chapter-item expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> 方法语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> 枚举和模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> 枚举的定义</a></li><li class="chapter-item expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> match 控制流结构</a></li><li class="chapter-item expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> if let 简洁控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> 使用包、Crate 和模块管理不断增长的项目</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> 包和 Crate</a></li><li class="chapter-item expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> 定义模块来控制作用域与私有性</a></li><li class="chapter-item expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> 引用模块项目的路径</a></li><li class="chapter-item expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> 使用 use 关键字将路径引入作用域</a></li><li class="chapter-item expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> 将模块拆分成多个文件</a></li></ol></li><li class="chapter-item expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> 常见集合</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> 使用 Vector 储存列表</a></li><li class="chapter-item expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> 使用字符串储存 UTF-8 编码的文本</a></li><li class="chapter-item expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> 使用 Hash Map 储存键值对</a></li></ol></li><li class="chapter-item expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> 错误处理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> 用 panic! 处理不可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> 用 Result 处理可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> 要不要 panic!</a></li></ol></li><li class="chapter-item expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> 泛型、Trait 和生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> 泛型数据类型</a></li><li class="chapter-item expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Trait：定义共同行为</a></li><li class="chapter-item expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> 生命周期确保引用有效</a></li></ol></li><li class="chapter-item expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> 编写自动化测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> 如何编写测试</a></li><li class="chapter-item expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> 控制测试如何运行</a></li><li class="chapter-item expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> 测试的组织结构</a></li></ol></li><li class="chapter-item expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> 一个 I/O 项目：构建命令行程序</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> 接受命令行参数</a></li><li class="chapter-item expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> 读取文件</a></li><li class="chapter-item expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> 重构以改进模块化与错误处理</a></li><li class="chapter-item expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> 采用测试驱动开发完善库的功能</a></li><li class="chapter-item expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> 处理环境变量</a></li><li class="chapter-item expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> 将错误信息输出到标准错误而不是标准输出</a></li></ol></li><li class="chapter-item expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Rust 中的函数式语言功能：迭代器与闭包</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> 闭包：可以捕获其环境的匿名函数</a></li><li class="chapter-item expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> 使用迭代器处理元素序列</a></li><li class="chapter-item expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> 改进之前的 I/O 项目</a></li><li class="chapter-item expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> 性能比较：循环对迭代器</a></li></ol></li><li class="chapter-item expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> 更多关于 Cargo 和 Crates.io 的内容</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> 采用发布配置自定义构建</a></li><li class="chapter-item expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> 将 crate 发布到 Crates.io</a></li><li class="chapter-item expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo 工作空间</a></li><li class="chapter-item expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> 使用 cargo install 安装二进制文件</a></li><li class="chapter-item expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Cargo 自定义扩展命令</a></li></ol></li><li class="chapter-item expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> 智能指针</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> 使用 Box<T> 指向堆上数据</a></li><li class="chapter-item expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> 使用 Deref Trait 将智能指针当作常规引用处理</a></li><li class="chapter-item expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> 使用 Drop Trait 运行清理代码</a></li><li class="chapter-item expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T> 引用计数智能指针</a></li><li class="chapter-item expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> 与内部可变性模式</a></li><li class="chapter-item expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> 引用循环会导致内存泄漏</a></li></ol></li><li class="chapter-item expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> 使用线程同时地运行代码</a></li><li class="chapter-item expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> 使用消息传递在线程间通信</a></li><li class="chapter-item expanded "><a href="ch16-03-shared-state.html" class="active"><strong aria-hidden="true">16.3.</strong> 共享状态并发</a></li><li class="chapter-item expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> 使用 Sync 与 Send Traits 的可扩展并发</a></li></ol></li><li class="chapter-item expanded "><a href="ch17-00-async-await.html"><strong aria-hidden="true">17.</strong> Async 和 await</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch17-01-futures-and-syntax.html"><strong aria-hidden="true">17.1.</strong> Futures 和 async 语法</a></li><li class="chapter-item expanded "><a href="ch17-02-concurrency-with-async.html"><strong aria-hidden="true">17.2.</strong> 并发与 async</a></li><li class="chapter-item expanded "><a href="ch17-03-more-futures.html"><strong aria-hidden="true">17.3.</strong> 使用任意数量的 futures</a></li><li class="chapter-item expanded "><a href="ch17-04-streams.html"><strong aria-hidden="true">17.4.</strong> 流（Streams）</a></li><li class="chapter-item expanded "><a href="ch17-05-traits-for-async.html"><strong aria-hidden="true">17.5.</strong> 深入理解 async 相关的 traits</a></li><li class="chapter-item expanded "><a href="ch17-06-futures-tasks-threads.html"><strong aria-hidden="true">17.6.</strong> Futures，任务（tasks）和线程（threads）</a></li></ol></li><li class="chapter-item expanded "><a href="ch18-00-oop.html"><strong aria-hidden="true">18.</strong> Rust 的面向对象编程特性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-what-is-oo.html"><strong aria-hidden="true">18.1.</strong> 面向对象语言的特点</a></li><li class="chapter-item expanded "><a href="ch18-02-trait-objects.html"><strong aria-hidden="true">18.2.</strong> 顾及不同类型值的 trait 对象</a></li><li class="chapter-item expanded "><a href="ch18-03-oo-design-patterns.html"><strong aria-hidden="true">18.3.</strong> 面向对象设计模式的实现</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-patterns.html"><strong aria-hidden="true">19.</strong> 模式与模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-all-the-places-for-patterns.html"><strong aria-hidden="true">19.1.</strong> 所有可能会用到模式的位置</a></li><li class="chapter-item expanded "><a href="ch19-02-refutability.html"><strong aria-hidden="true">19.2.</strong> Refutability（可反驳性）: 模式是否会匹配失效</a></li><li class="chapter-item expanded "><a href="ch19-03-pattern-syntax.html"><strong aria-hidden="true">19.3.</strong> 模式语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch20-00-advanced-features.html"><strong aria-hidden="true">20.</strong> 高级特征</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch20-01-unsafe-rust.html"><strong aria-hidden="true">20.1.</strong> 不安全的 Rust</a></li><li class="chapter-item expanded "><a href="ch20-03-advanced-traits.html"><strong aria-hidden="true">20.2.</strong> 高级 trait</a></li><li class="chapter-item expanded "><a href="ch20-04-advanced-types.html"><strong aria-hidden="true">20.3.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="ch20-05-advanced-functions-and-closures.html"><strong aria-hidden="true">20.4.</strong> 高级函数与闭包</a></li><li class="chapter-item expanded "><a href="ch20-06-macros.html"><strong aria-hidden="true">20.5.</strong> 宏</a></li></ol></li><li class="chapter-item expanded "><a href="ch21-00-final-project-a-web-server.html"><strong aria-hidden="true">21.</strong> 最后的项目：构建多线程 web server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch21-01-single-threaded.html"><strong aria-hidden="true">21.1.</strong> 建立单线程 web server</a></li><li class="chapter-item expanded "><a href="ch21-02-multithreaded.html"><strong aria-hidden="true">21.2.</strong> 将单线程 server 变为多线程 server</a></li><li class="chapter-item expanded "><a href="ch21-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">21.3.</strong> 优雅停机与清理</a></li></ol></li><li class="chapter-item expanded "><a href="appendix-00.html"><strong aria-hidden="true">22.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">22.1.</strong> A - 关键字</a></li><li class="chapter-item expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">22.2.</strong> B - 运算符与符号</a></li><li class="chapter-item expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">22.3.</strong> C - 可派生的 trait</a></li><li class="chapter-item expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">22.4.</strong> D - 实用开发工具</a></li><li class="chapter-item expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">22.5.</strong> E - 版本</a></li><li class="chapter-item expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">22.6.</strong> F - 本书译本</a></li><li class="chapter-item expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">22.7.</strong> G - Rust 是如何开发的与 “Nightly Rust”</a></li></ol></li></ol>
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<main>
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<h2 id="共享状态的并发"><a class="header" href="#共享状态的并发">共享状态的并发</a></h2>
|
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<blockquote>
|
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch16-03-shared-state.md">ch16-03-shared-state.md</a>
|
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<br>
|
||
commit 856d89c53a6d69470bb5669c773fdfe6aab6fcc9</p>
|
||
</blockquote>
|
||
<p>消息传递是一个很好的处理并发的方式，但并不是唯一一个。另一种方式是让多个线程访问同一块内存中的数据（共享状态）。再考虑一下 Go 语言文档中的这句口号：“不要通过共享内存来通讯”（“do not communicate by sharing memory.”</p>
|
||
<p>通过共享内存进行通信，会是什么样的代码？此外，为什么喜欢消息传递的人会警告：谨慎使用内存共享？</p>
|
||
<p>在某种程度上，任何编程语言中的信道都类似于单所有权，因为一旦将一个值传送到信道中，将无法再使用这个值。共享内存类似于多所有权：多个线程可以同时访问相同的内存位置。在 15 章中，我们介绍了智能指针可以实现多所有权，然而这会增加额外的复杂性，因为需要管理多个所有者。Rust 的类型系统和所有权规则在正确管理这些问题上提供了极大的帮助：举个例子，让我们来看看 <strong>互斥器</strong>，一个较常见的共享内存并发原语。</p>
|
||
<h3 id="使用互斥器实现同一时刻只允许一个线程访问数据"><a class="header" href="#使用互斥器实现同一时刻只允许一个线程访问数据">使用互斥器，实现同一时刻只允许一个线程访问数据</a></h3>
|
||
<p><strong>互斥器</strong>（<em>mutex</em>）是 互相排斥（<em>mutual exclusion</em>）的缩写。在同一时刻，其只允许一个线程对数据拥有访问权。为了访问互斥器中的数据，线程首先需要通过获取互斥器的 <strong>锁</strong>（<em>lock</em>）来表明其希望访问数据。锁是一个数据结构，作为互斥器的一部分，它记录谁有数据的专属访问权。因此我们讲，互斥器通过锁系统 <strong>保护</strong>（<em>guarding</em>）其数据。</p>
|
||
<p>互斥器以难以使用著称（译注：原文指互斥器在其他编程语言中难以使用），因为你必须记住：</p>
|
||
<ol>
|
||
<li>在使用数据之前，必须获取锁。</li>
|
||
<li>使用完被互斥器所保护的数据之后，必须解锁数据，这样其他线程才能够获取锁。</li>
|
||
</ol>
|
||
<p>作为一个现实中互斥器的例子，想象一下在某个会议的一次小组座谈会中，只有一个麦克风。如果一位成员要发言，他必须请求或表示希望使用麦克风。得到了麦克风后，他可以畅所欲言，讲完后再将麦克风交给下一位希望讲话的成员。如果一位成员结束发言后忘记将麦克风交还，其他人将无法发言。如果对共享麦克风的管理出现了问题，座谈会将无法正常进行！</p>
|
||
<p>正确的管理互斥器异常复杂，这也是许多人之所以热衷于信道的原因。然而，在 Rust 中，得益于类型系统和所有权，我们不会在锁和解锁上出错。</p>
|
||
<h3 id="mutext的-api"><a class="header" href="#mutext的-api"><code>Mutex<T></code>的 API</a></h3>
|
||
<p>我们先从在单线程环境中使用互斥器开始，作为展示其用法的一个例子，如示例 16-12 所示：</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021">use std::sync::Mutex;
|
||
|
||
fn main() {
|
||
let m = Mutex::new(5);
|
||
|
||
{
|
||
let mut num = m.lock().unwrap();
|
||
*num = 6;
|
||
}
|
||
|
||
println!("m = {m:?}");
|
||
}</code></pre></pre>
|
||
<p><span class="caption">示例 16-12: 出于简单的考虑，在一个单线程上下文中探索 <code>Mutex<T></code> 的 API</span></p>
|
||
<p>像很多类型一样，我们使用关联函数 <code>new</code> 来创建一个 <code>Mutex<T></code>。使用 <code>lock</code> 方法来获取锁，从而可以访问互斥器中的数据。这个调用会阻塞当前线程，直到我们拥有锁为止。</p>
|
||
<p>如果另一个线程拥有锁，并且那个线程 panic 了，则 <code>lock</code> 调用会失败。在这种情况下，没人能够再获取锁，所以我们调用 <code>unwrap</code>，使当前线程 panic。</p>
|
||
<p>一旦获取了锁，就可以将返回值（命名为 <code>num</code>）视为一个其内部数据（<code>i32</code>）的可变引用了。类型系统确保了我们在使用 <code>m</code> 中的值之前获取锁。<code>m</code> 的类型是 <code>Mutex<i32></code> 而不是 <code>i32</code>，所以 <strong>必须</strong> 获取锁才能使用这个 <code>i32</code> 值。我们是不会忘记这么做的，因为如果没有获取锁，类型系统就不允许访问内部的 <code>i32</code> 值。</p>
|
||
<p>正如你所猜想的，<code>Mutex<T></code> 是一个智能指针。更准确的说，<code>lock</code> 调用 <strong>返回</strong> 一个叫做 <code>MutexGuard</code> 的智能指针。这个智能指针实现了 <code>Deref</code> 来指向其内部数据；它也实现了 <code>Drop</code>，当 <code>MutexGuard</code> 离开作用域时，自动释放锁（发生在示例 16-12 内部作用域的结尾）。有了这个特性，就不会有忘记释放锁的潜在风险（忘记释放锁会使互斥器无法再被其它线程使用），因为锁的释放是自动发生的。</p>
|
||
<p>释放锁之后，我们可以打印出互斥器内部的 <code>i32</code> 值，并发现我们刚刚已经将其值改为 6。</p>
|
||
<h4 id="在线程间共享-mutext"><a class="header" href="#在线程间共享-mutext">在线程间共享 <code>Mutex<T></code></a></h4>
|
||
<p>现在让我们尝试使用 <code>Mutex<T></code> 在多个线程间共享同一个值。我们将启动 10 个线程，并在各个线程中对同一个计数器值加 1，这样计数器将从 0 变为 10。示例 16-13 中的例子会出现编译错误，而我们将通过这些错误来学习如何使用 <code>Mutex<T></code>，以及 Rust 又是如何帮助我们正确使用的。</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">use std::sync::Mutex;
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let counter = Mutex::new(0);
|
||
let mut handles = vec![];
|
||
|
||
for _ in 0..10 {
|
||
let handle = thread::spawn(move || {
|
||
let mut num = counter.lock().unwrap();
|
||
|
||
*num += 1;
|
||
});
|
||
handles.push(handle);
|
||
}
|
||
|
||
for handle in handles {
|
||
handle.join().unwrap();
|
||
}
|
||
|
||
println!("Result: {}", *counter.lock().unwrap());
|
||
}</code></pre>
|
||
<p><span class="caption">示例 16-13: 程序启动了 10 个线程，每个线程都通过 <code>Mutex<T></code> 来增加计数器的值</span></p>
|
||
<p>这里创建了一个 <code>counter</code> 变量来存放内含 <code>i32</code> 的 <code>Mutex<T></code>，类似示例 16-12 那样。接下来我们遍历整数区间，创建了 10 个线程。我们使用了 <code>thread::spawn</code>，并为所有线程传入了相同的闭包：它们每一个都将调用 <code>lock</code> 方法来获取 <code>Mutex<T></code> 上的锁，接着将互斥器中的值加一。当一个线程结束执行，<code>num</code> 会离开闭包作用域并释放锁，这样另一个线程就可以获取它了。</p>
|
||
<p>在主线程中，我们像示例 16-2 那样收集了所有的 <code>JoinHandle</code>，并调用它们的 <code>join</code> 方法来等待所有线程结束。然后，主线程会获取锁，并打印出程序的结果。</p>
|
||
<p>之前提示过，这个例子不能编译，让我们看看为什么！</p>
|
||
<pre><code class="language-console">$ cargo run
|
||
Compiling shared-state v0.1.0 (file:///projects/shared-state)
|
||
error[E0382]: borrow of moved value: `counter`
|
||
--> src/main.rs:21:29
|
||
|
|
||
5 | let counter = Mutex::new(0);
|
||
| ------- move occurs because `counter` has type `Mutex<i32>`, which does not implement the `Copy` trait
|
||
...
|
||
8 | for _ in 0..10 {
|
||
| -------------- inside of this loop
|
||
9 | let handle = thread::spawn(move || {
|
||
| ------- value moved into closure here, in previous iteration of loop
|
||
...
|
||
21 | println!("Result: {}", *counter.lock().unwrap());
|
||
| ^^^^^^^ value borrowed here after move
|
||
|
|
||
help: consider moving the expression out of the loop so it is only moved once
|
||
|
|
||
8 ~ let mut value = counter.lock();
|
||
9 ~ for _ in 0..10 {
|
||
10 | let handle = thread::spawn(move || {
|
||
11 ~ let mut num = value.unwrap();
|
||
|
|
||
|
||
For more information about this error, try `rustc --explain E0382`.
|
||
error: could not compile `shared-state` (bin "shared-state") due to 1 previous error
|
||
</code></pre>
|
||
<p>错误信息表明 <code>counter</code> 值在上一次循环中被移动了。所以 Rust 告诉我们，不能将 <code>counter</code> 锁的所有权移动到多个线程中。让我们通过一个第 15 章讨论过的多所有权手段，来修复这个编译错误。</p>
|
||
<h4 id="多线程和多所有权"><a class="header" href="#多线程和多所有权">多线程和多所有权</a></h4>
|
||
<p>在第 15 章中，我们用智能指针 <code>Rc<T></code> 来创建引用计数，使得一个值有了多个所有者。让我们做同样的事，看看会发生什么。将示例 16-14 中的 <code>Mutex<T></code> 封装进 <code>Rc<T></code> 中，并在将所有权移入线程之前克隆（clone） <code>Rc<T></code>。</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">use std::rc::Rc;
|
||
use std::sync::Mutex;
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let counter = Rc::new(Mutex::new(0));
|
||
let mut handles = vec![];
|
||
|
||
for _ in 0..10 {
|
||
let counter = Rc::clone(&counter);
|
||
let handle = thread::spawn(move || {
|
||
let mut num = counter.lock().unwrap();
|
||
|
||
*num += 1;
|
||
});
|
||
handles.push(handle);
|
||
}
|
||
|
||
for handle in handles {
|
||
handle.join().unwrap();
|
||
}
|
||
|
||
println!("Result: {}", *counter.lock().unwrap());
|
||
}</code></pre>
|
||
<p><span class="caption">示例 16-14: 尝试使用 <code>Rc<T></code> 来允许多个线程拥有 <code>Mutex<T></code></span></p>
|
||
<p>再一次编译并...出现了不同的错误！编译器真是教会了我们很多！</p>
|
||
<pre><code class="language-console">$ cargo run
|
||
Compiling shared-state v0.1.0 (file:///projects/shared-state)
|
||
error[E0277]: `Rc<Mutex<i32>>` cannot be sent between threads safely
|
||
--> src/main.rs:11:36
|
||
|
|
||
11 | let handle = thread::spawn(move || {
|
||
| ------------- ^------
|
||
| | |
|
||
| ______________________|_____________within this `{closure@src/main.rs:11:36: 11:43}`
|
||
| | |
|
||
| | required by a bound introduced by this call
|
||
12 | | let mut num = counter.lock().unwrap();
|
||
13 | |
|
||
14 | | *num += 1;
|
||
15 | | });
|
||
| |_________^ `Rc<Mutex<i32>>` cannot be sent between threads safely
|
||
|
|
||
= help: within `{closure@src/main.rs:11:36: 11:43}`, the trait `Send` is not implemented for `Rc<Mutex<i32>>`, which is required by `{closure@src/main.rs:11:36: 11:43}: Send`
|
||
note: required because it's used within this closure
|
||
--> src/main.rs:11:36
|
||
|
|
||
11 | let handle = thread::spawn(move || {
|
||
| ^^^^^^^
|
||
note: required by a bound in `spawn`
|
||
--> /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/thread/mod.rs:688:1
|
||
|
||
For more information about this error, try `rustc --explain E0277`.
|
||
error: could not compile `shared-state` (bin "shared-state") due to 1 previous error
|
||
</code></pre>
|
||
<p>哇哦，错误信息太长不看！划重点：第一行错误表明 <code>Rc<Mutex<i32>></code> 不能在线程间安全传递（<code>`Rc<Mutex<i32>>` cannot be sent between threads safely</code>）；编译器也指出了原因：<code>Rc<Mutex<i32>></code> 没有实现 <code>Send</code> trait（<code>the trait `Send` is not implemented for `Rc<Mutex<i32>>`</code>）。下一节我们会讲到 <code>Send</code>：这是一个确保所使用的类型可以用于并发环境的 trait。</p>
|
||
<p>不幸的是，<code>Rc<T></code> 并不能安全的在线程间共享。当 <code>Rc<T></code> 管理引用计数时，它必须在每一个 <code>clone</code> 调用时增加计数，并在每一个克隆体被丢弃时减少计数。<code>Rc<T></code> 并没有使用任何并发原语，无法确保改变计数的操作不会被其他线程打断。这可能使计数出错，并导致诡异的 bug，比如可能会造成内存泄漏，或在使用结束之前就丢弃一个值。我们所需要的是一个与 <code>Rc<T></code> 完全一致，又以线程安全的方式改变引用计数的类型。</p>
|
||
<h4 id="原子引用计数-arct"><a class="header" href="#原子引用计数-arct">原子引用计数 <code>Arc<T></code></a></h4>
|
||
<p>所幸 <code>Arc<T></code> 正是这么一个类似 <code>Rc<T></code> 并可以安全的用于并发环境的类型。字母 “a” 代表 <strong>原子性</strong>（<em>atomic</em>），所以这是一个 <strong>原子引用计数</strong>（<em>atomically reference counted</em>）类型。<strong>原子类型</strong> (Atomics) 是另一类这里还未涉及到的并发原语：请查看标准库中 <a href="https://doc.rust-lang.org/std/sync/atomic/index.html"><code>std::sync::atomic</code></a> 的文档来获取更多细节。目前我们只需要知道：原子类型就像基本类型一样，可以安全的在线程间共享。</p>
|
||
<p>你可能会好奇，为什么不是所有的基本类型都是原子性的？为什么标准库中的类型没有全部默认使用 <code>Arc<T></code> 实现？原因在于，线程安全会造成性能损失，我们希望只在必要时才为此买单。如果只是在单线程中对值进行操作，原子性提供的保证并无必要，而不加入原子性可以使代码运行得更快。</p>
|
||
<p>回到之前的例子：<code>Arc<T></code> 和 <code>Rc<T></code> 有着相同的 API，所以我们只需修改程序中的 <code>use</code> 行、<code>new</code> 调用和 <code>clone</code> 调用。示例 16-15 中的代码最终可以编译和运行：</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021">use std::sync::{Arc, Mutex};
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let counter = Arc::new(Mutex::new(0));
|
||
let mut handles = vec![];
|
||
|
||
for _ in 0..10 {
|
||
let counter = Arc::clone(&counter);
|
||
let handle = thread::spawn(move || {
|
||
let mut num = counter.lock().unwrap();
|
||
|
||
*num += 1;
|
||
});
|
||
handles.push(handle);
|
||
}
|
||
|
||
for handle in handles {
|
||
handle.join().unwrap();
|
||
}
|
||
|
||
println!("Result: {}", *counter.lock().unwrap());
|
||
}</code></pre></pre>
|
||
<p><span class="caption">示例 16-15: 使用 <code>Arc<T></code> 包装一个 <code>Mutex<T></code> 能够实现在多线程之间共享所有权</span></p>
|
||
<p>这会打印出：</p>
|
||
<pre><code class="language-text">Result: 10
|
||
</code></pre>
|
||
<p>成功了！我们从 0 数到了 10，这好像没啥大不了的，不过一路上我们确实学习了很多关于 <code>Mutex<T></code> 和线程安全的内容！这个例子中构建的结构可以用于比增加计数更为复杂的操作。使用这个策略，我们可将计算任务分成独立的部分，并分散到多个线程中，接着使用 <code>Mutex<T></code> 使用各自的运算结果来更新最终的结果。</p>
|
||
<p>注意，对于简单的数值运算，<a href="https://doc.rust-lang.org/std/sync/atomic/index.html">标准库中 <code>std::sync::atomic</code> 模块</a> 提供了比 <code>Mutex<T></code> 更简单的类型。针对基本类型，这些类型提供了安全、并发、原子的操作。在上面的例子中，为了专注于讲明白 <code>Mutex<T></code> 的用法，我们才选择在基本类型上使用 <code>Mutex<T></code>。（译注：对于上面例子中出现的 <code>i32</code> 加法操作，更好的做法是使用 <code>AtomicI32</code> 类型来完成。具体参考文档。）</p>
|
||
<h3 id="refcelltrct-与-mutextarct-的相似性"><a class="header" href="#refcelltrct-与-mutextarct-的相似性"><code>RefCell<T></code>/<code>Rc<T></code> 与 <code>Mutex<T></code>/<code>Arc<T></code> 的相似性</a></h3>
|
||
<p>你可能注意到了，尽管 <code>counter</code> 是不可变的，我们仍然可以获取其内部值的可变引用；这意味着 <code>Mutex<T></code> 提供了内部可变性，就像 <code>Cell</code> 系列类型那样。使用 <code>RefCell<T></code> 可以改变 <code>Rc<T></code> 中内容（在 15 章中讲到过），同样地，使用 <code>Mutex<T></code> 我们也可以改变 <code>Arc<T></code> 中的内容。</p>
|
||
<p>另一个值得注意的细节是，Rust 不能完全避免使用 <code>Mutex<T></code> 所带来的逻辑错误。回忆一下，第 15 章中讲过，使用 <code>Rc<T></code> 就有造成引用循环的风险：两个 <code>Rc<T></code> 值相互引用，造成内存泄漏。同理，<code>Mutex<T></code> 也有造成 <strong>死锁</strong>（<em>deadlock</em>）的风险：当某个操作需要锁住两个资源，而两个线程分别持有两个资源的其中一个锁时，它们会永远相互等待。如果你对这个话题感兴趣，尝试编写一个带有死锁的 Rust 程序，接着研究别的语言中使用互斥器的死锁规避策略，并尝试在 Rust 中实现它们。标准库中 <code>Mutex<T></code> 和 <code>MutexGuard</code> 的 API 文档会提供有用的信息。</p>
|
||
<p>接下来，为了丰富本章的内容，让我们讨论一下 <code>Send</code>和 <code>Sync</code> trait，以及如何对自定义类型使用它们。</p>
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