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< h1 id = "unsafe-简介" > < a class = "header" href = "#unsafe-简介" > unsafe 简介< / a > < / h1 >
< 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 >
< 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 >
< h2 id = "为何会有-unsafe" > < a class = "header" href = "#为何会有-unsafe" > 为何会有 unsafe< / a > < / h2 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< h2 id = "unsafe-的超能力" > < a class = "header" href = "#unsafe-的超能力" > unsafe 的超能力< / a > < / h2 >
< 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 >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let mut num = 5;
let r1 = & num as *const i32;
unsafe {
println!("r1 is: {}", *r1);
}
}< / code > < / pre >
< 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 >
< 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 >
< ul >
< li > 解引用裸指针，就如上例所示< / li >
< li > 调用一个 < code > unsafe< / code > 或外部的函数< / li >
< 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 >
< li > 实现一个 < code > unsafe< / code > 特征< / li >
< li > 访问 < code > union< / code > 中的字段< / li >
< / ul >
< p > 在本章中，我们将着重讲解裸指针和 FFI 的使用。< / p >
< 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 >
< 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 >
< 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 >
< 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 >
< h2 id = "谈虎色变" > < a class = "header" href = "#谈虎色变" > 谈虎色变？< / a > < / h2 >
< p > 在网上充斥着这样的言论：< code > 千万不要使用 unsafe， < / 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， ， ， < / p >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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 >
< 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， < code > unsafe< / code > 代码。< / p >
< blockquote >
< p > 忍不住抱怨一句，内存安全方面的 bug ，是真心难查！< / p >
< / blockquote >
< / main >
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