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<main>
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<h1 id="unsafe-简介"><a class="header" href="#unsafe-简介">unsafe 简介</a></h1>
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<p>圣人论迹不论心，论心世上无圣人，对于编程语言而言，亦是如此。</p>
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<p>虽然在本章之前，我们学到的代码都是在编译期就得到了 Rust 的安全保障，但是在其内心深处也隐藏了一些阴暗面，在这些阴暗面里，内存安全就存在一些变数了：当不娴熟的开发者接触到这些阴暗面，就可能写出不安全的代码，因此我们称这种代码为 <code>unsafe</code> 代码块。</p>
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<h2 id="为何会有-unsafe"><a class="header" href="#为何会有-unsafe">为何会有 unsafe</a></h2>
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<p>几乎每个语言都有 <code>unsafe</code> 关键字，但 Rust 语言使用 <code>unsafe</code> 的原因可能与其它编程语言还有所不同。</p>
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<h4 id="过强的编译器"><a class="header" href="#过强的编译器">过强的编译器</a></h4>
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<p>说来尴尬，<code>unsafe</code> 的存在主要是因为 Rust 的静态检查太强了，但是强就算了，它还很保守，这就会导致当编译器在分析代码时，一些正确代码会因为编译器无法分析出它的所有正确性，结果将这段代码拒绝，导致编译错误。</p>
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<p>这种保守的选择确实也没有错，毕竟安全就是要防微杜渐，但是对于使用者来说，就不是那么愉快的事了，特别是当配合 Rust 的所有权系统一起使用时，有个别问题是真的棘手和难以解决。</p>
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<p>举个例子，在之前的自引用章节中，我们就提到了相关的编译检查是很难绕过的，如果想要绕过，最常用的方法之一就是使用 <a href="https://course.rs/advance/circle-self-ref/self-referential.html"><code>unsafe</code> 和 <code>Pin</code></a>。</p>
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<p>好在，当遇到这些情况时，我们可以使用 <code>unsafe</code> 来解决。此时，你需要替代编译器的部分职责对 <code>unsafe</code> 代码的正确性负责，例如在正常代码中不可能遇到的空指针解引用问题在 <code>unsafe</code> 中就可能会遇到，我们需要自己来处理好这些类似的问题。</p>
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<h4 id="特定任务的需要"><a class="header" href="#特定任务的需要">特定任务的需要</a></h4>
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<p>至于 <code>unsafe</code> 存在的另一个原因就是：它必须要存在。原因是计算机底层的一些硬件就是不安全的，如果 Rust 只允许你做安全的操作，那一些任务就无法完成，换句话说，我们还怎么跟 C++ 干架？</p>
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<p>Rust 的一个主要定位就是系统编程，众所周知，系统编程就是底层编程，往往需要直接跟操作系统打交道，甚至于去实现一个操作系统。而为了实现底层系统编程，<code>unsafe</code> 就是必不可少的。</p>
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<p>在了解了为何会有 <code>unsafe</code> 后，我们再来看看，除了这些必要性，<code>unsafe</code> 还能给我们带来哪些超能力。</p>
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<h2 id="unsafe-的超能力"><a class="header" href="#unsafe-的超能力">unsafe 的超能力</a></h2>
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<p>使用 <code>unsafe</code> 非常简单，只需要将对应的代码块标记下即可:</p>
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<pre class="playground"><code class="language-rust edition2021">fn main() {
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let mut num = 5;
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let r1 = &num as *const i32;
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unsafe {
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println!("r1 is: {}", *r1);
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}
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}</code></pre>
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<p>上面代码中, <code>r1</code> 是一个裸指针(raw pointer)，由于它具有破坏 Rust 内存安全的潜力，因此只能在 <code>unsafe</code> 代码块中使用，如果你去掉 <code>unsafe {}</code>，编译器会立刻报错。</p>
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<p>言归正传， <code>unsafe</code> 能赋予我们 5 种超能力，这些能力在安全的 Rust 代码中是无法获取的：</p>
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<ul>
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<li>解引用裸指针，就如上例所示</li>
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<li>调用一个 <code>unsafe</code> 或外部的函数</li>
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<li>访问或修改一个可变的<a href="https://course.rs/advance/global-variable.html#静态变量">静态变量</a></li>
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<li>实现一个 <code>unsafe</code> 特征</li>
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<li>访问 <code>union</code> 中的字段</li>
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</ul>
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<p>在本章中，我们将着重讲解裸指针和 FFI 的使用。</p>
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<h2 id="unsafe-的安全保证"><a class="header" href="#unsafe-的安全保证">unsafe 的安全保证</a></h2>
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<p>曾经在 <code>reddit</code> 上有一个讨论还挺热闹的，是关于 <code>unsafe</code> 的命名是否合适，总之公有公理，婆有婆理，但有一点是不可否认的：虽然名称自带不安全，但是 Rust 依然提供了强大的安全支撑。</p>
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<p>首先，<code>unsafe</code> 并不能绕过 Rust 的借用检查，也不能关闭任何 Rust 的安全检查规则，例如当你在 <code>unsafe</code> 中使用<strong>引用</strong>时，该有的检查一样都不会少。</p>
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<p>因此 <code>unsafe</code> 能给大家提供的也仅仅是之前的 5 种超能力，在使用这 5 种能力时，编译器才不会进行内存安全方面的检查，最典型的就是使用<strong>裸指针</strong>(引用和裸指针有很大的区别)。</p>
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<h2 id="谈虎色变"><a class="header" href="#谈虎色变">谈虎色变？</a></h2>
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<p>在网上充斥着这样的言论：<code>千万不要使用 unsafe，因为它不安全</code>，甚至有些库会以没有 <code>unsafe</code> 代码作为噱头来吸引用户。事实上，大可不必，如果按照这个标准，Rust 的标准库也将不复存在！</p>
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<p>Rust 中的 <code>unsafe</code> 其实没有那么可怕，虽然听上去很不安全，但是实际上 Rust 依然提供了很多机制来帮我们提升了安全性，因此不必像对待 Go 语言的 <code>unsafe</code> 那样去畏惧于使用 Rust 中的 <code>unsafe</code> 。</p>
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<p>大致使用原则总结如下：没必要用时，就不要用，当有必要用时，就大胆用，但是尽量控制好边界，让 <code>unsafe</code> 的范围尽可能小。</p>
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<h2 id="控制-unsafe-的使用边界"><a class="header" href="#控制-unsafe-的使用边界">控制 unsafe 的使用边界</a></h2>
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<p><code>unsafe</code> 不安全，但是该用的时候就要用，在一些时候，它能帮助我们大幅降低代码实现的成本。</p>
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<p>而作为使用者，你的水平决定了 <code>unsafe</code> 到底有多不安全，因此你需要在 <code>unsafe</code> 中小心谨慎地去访问内存。</p>
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<p>即使做到小心谨慎，依然会有出错的可能性，但是 <code>unsafe</code> 语句块决定了：就算内存访问出错了，你也能立刻意识到，错误是在 <code>unsafe</code> 代码块中，而不花大量时间像无头苍蝇一样去寻找问题所在。</p>
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<p>正因为此，写代码时要尽量控制好 <code>unsafe</code> 的边界大小，越小的 <code>unsafe</code> 越会让我们在未来感谢自己当初的选择。</p>
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<p>除了控制边界大小，另一个很常用的方式就是在 <code>unsafe</code> 代码块外包裹一层 <code>safe</code> 的 API，例如一个函数声明为 safe 的，然后在其内部有一块儿是 <code>unsafe</code> 代码。</p>
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<blockquote>
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<p>忍不住抱怨一句，内存安全方面的 bug ，是真心难查！</p>
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</blockquote>
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