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< h1 id = "五种兵器" > < a class = "header" href = "#五种兵器" > 五种兵器< / a > < / h1 >
< p > 古龙有一部小说，名为“七种兵器“，其中每一种都精妙绝伦，令人闻风丧胆，而 < code > unsafe< / code > 也有五种兵器，它们可以让你拥有其它代码无法实现的能力，同时它们也像七种兵器一样令人闻风丧胆，下面一起来看看庐山真面目。< / p >
< h2 id = "解引用裸指针" > < a class = "header" href = "#解引用裸指针" > 解引用裸指针< / a > < / h2 >
< p > 裸指针(raw pointer， < code > *const T< / code > 和 < code > *mut T< / code > ，它们分别代表了不可变和可变。< / p >
< p > 大家在之前学过 < code > *< / code > 操作符，知道它可以用于解引用，但是在裸指针 < code > *const T< / code > 中，这里的 < code > *< / code > 只是类型名称的一部分，并没有解引用的含义。< / p >
< p > 至此，我们已经学过三种类似指针的概念：引用、智能指针和裸指针。与前两者不同，裸指针：< / p >
< ul >
< li > 可以绕过 Rust 的借用规则，可以同时拥有一个数据的可变、不可变指针，甚至还能拥有多个可变的指针< / li >
< li > 并不能保证指向合法的内存< / li >
< li > 可以是 < code > null< / code > < / li >
< li > 没有实现任何自动的回收 (drop)< / li >
< / ul >
< p > 总之，裸指针跟 C 指针是非常像的，使用它需要以牺牲安全性为前提，但我们获得了更好的性能，也可以跟其它语言或硬件打交道。< / p >
< h4 id = "基于引用创建裸指针" > < a class = "header" href = "#基于引用创建裸指针" > 基于引用创建裸指针< / a > < / h4 >
< p > 下面的代码< strong > 基于值的引用< / strong > 同时创建了可变和不可变的裸指针：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let mut num = 5;
let r1 = & num as *const i32;
let r2 = & mut num as *mut i32;
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > as< / code > 可以用于强制类型转换，在< a href = "https://course.rs/basic/converse.html" > 之前章节< / a > 中有讲解。在这里，我们将引用 < code > & num / & mut num< / code > 强转为相应的裸指针 < code > *const i32 / *mut i32< / code > 。< / p >
< p > 细心的同学可能会发现，在这段代码中并没有 < code > unsafe< / code > 的身影，原因在于：< strong > 创建裸指针是安全的行为，而解引用裸指针才是不安全的行为< / strong > :< / 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 >
< h4 id = "基于内存地址创建裸指针" > < a class = "header" href = "#基于内存地址创建裸指针" > 基于内存地址创建裸指针< / a > < / h4 >
< p > 在上面例子中，我们基于现有的引用来创建裸指针，这种行为是很安全的。但是接下来的方式就不安全了：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let address = 0x012345usize;
let r = address as *const i32;
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里基于一个内存地址来创建裸指针，可以想像，这种行为是相当危险的。试图使用任意的内存地址往往是一种未定义的行为(undefined behavior)，因为该内存地址有可能存在值，也有可能没有，就算有值，也大概率不是你需要的值。< / p >
< p > 同时编译器也有可能会优化这段代码，会造成没有任何内存访问发生，甚至程序还可能发生段错误(segmentation fault)。< strong > 总之，你几乎没有好的理由像上面这样实现代码，虽然它是可行的< / strong > 。< / p >
< p > 如果真的要使用内存地址，也是类似下面的用法，先取地址，再使用，而不是凭空捏造一个地址：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::{slice::from_raw_parts, str::from_utf8_unchecked};
// 获取字符串的内存地址和长度
fn get_memory_location() -> (usize, usize) {
let string = "Hello World!";
let pointer = string.as_ptr() as usize;
let length = string.len();
(pointer, length)
}
// 在指定的内存地址读取字符串
fn get_str_at_location(pointer: usize, length: usize) -> & 'static str {
unsafe { from_utf8_unchecked(from_raw_parts(pointer as *const u8, length)) }
}
fn main() {
let (pointer, length) = get_memory_location();
let message = get_str_at_location(pointer, length);
println!(
"The {} bytes at 0x{:X} stored: {}",
length, pointer, message
);
// 如果大家想知道为何处理裸指针需要 `unsafe`，可以试着反注释以下代码
// let message = get_str_at_location(1000, 10);
}< / code > < / pre >
< p > 以上代码同时还演示了访问非法内存地址会发生什么，大家可以试着去反注释这段代码试试。< / p >
< h4 id = "使用--解引用" > < a class = "header" href = "#使用--解引用" > 使用 * 解引用< / a > < / h4 >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let a = 1;
let b: *const i32 = & a as *const i32;
let c: *const i32 = & a;
unsafe {
println!("{}", *c);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 使用 < code > *< / code > 可以对裸指针进行解引用，由于该指针的内存安全性并没有任何保证，因此我们需要使用 < code > unsafe< / code > 来包裹解引用的逻辑(切记，< code > unsafe< / code > 语句块的范围一定要尽可能的小，具体原因在上一章节有讲)。< / p >
< p > 以上代码另一个值得注意的点就是：除了使用 < code > as< / code > 来显式的转换，我们还使用了隐式的转换方式 < code > let c: *const i32 = & a;< / code > 。在实际使用中，我们建议使用 < code > as< / code > 来转换，因为这种显式的方式更有助于提醒用户：你在使用的指针是裸指针，需要小心。< / p >
< h4 id = "基于智能指针创建裸指针" > < a class = "header" href = "#基于智能指针创建裸指针" > 基于智能指针创建裸指针< / a > < / h4 >
< p > 还有一种创建裸指针的方式，那就是基于智能指针来创建:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let a: Box< i32> = Box::new(10);
// 需要先解引用a
let b: *const i32 = & *a;
// 使用 into_raw 来创建
let c: *const i32 = Box::into_raw(a);
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "小结" > < a class = "header" href = "#小结" > 小结< / a > < / h4 >
< p > 像之前代码演示的那样，使用裸指针可以让我们创建两个可变指针都指向同一个数据，如果使用安全的 Rust， ， ， ， ， ， ， ！ < / p >
< p > 既然这么危险，为何还要使用裸指针？除了之前提到的性能等原因，还有一个重要用途就是跟 < code > C< / code > 语言的代码进行交互( FFI )，在讲解 FFI 之前，先来看看如何调用 unsafe 函数或方法。< / p >
< h2 id = "调用-unsafe-函数或方法" > < a class = "header" href = "#调用-unsafe-函数或方法" > 调用 unsafe 函数或方法< / a > < / h2 >
< p > < code > unsafe< / code > 函数从外表上来看跟普通函数并无区别，唯一的区别就是它需要使用 < code > unsafe fn< / code > 来进行定义。这种定义方式是为了告诉调用者：当调用此函数时，你需要注意它的相关需求，因为 Rust 无法担保调用者在使用该函数时能满足它所需的一切需求。< / p >
< p > 强制调用者加上 < code > unsafe< / code > 语句块，就可以让他清晰的认识到，正在调用一个不安全的函数，需要小心看看文档，看看函数有哪些特别的要求需要被满足。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > unsafe fn dangerous() {}
fn main() {
dangerous();
}< / code > < / pre >
< p > 如果试图像上面这样调用，编译器就会报错：< / p >
< pre > < code class = "language-shell" > error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
--> src/main.rs:3:5
|
3 | dangerous();
| ^^^^^^^^^^^ call to unsafe function
< / code > < / pre >
< p > 按照报错提示，加上 < code > unsafe< / code > 语句块后，就能顺利执行了:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > unsafe {
dangerous();
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 道理很简单，但一定要牢记在心：< strong > 使用 unsafe 声明的函数时，一定要看看相关的文档，确定自己没有遗漏什么< / strong > 。< / p >
< p > 还有，< code > unsafe< / code > 无需俄罗斯套娃，在 < code > unsafe< / code > 函数体中使用 < code > unsafe< / code > 语句块是多余的行为。< / p >
< h2 id = "用安全抽象包裹-unsafe-代码" > < a class = "header" href = "#用安全抽象包裹-unsafe-代码" > 用安全抽象包裹 unsafe 代码< / a > < / h2 >
< p > 一个函数包含了 < code > unsafe< / code > 代码不代表我们需要将整个函数都定义为 < code > unsafe fn< / code > 。事实上，在标准库中有大量的安全函数，它们内部都包含了 < code > unsafe< / code > 代码块，下面我们一起来看看一个很好用的标准库函数：< code > split_at_mut< / code > 。< / p >
< p > 大家可以想象一下这个场景：需要将一个数组分成两个切片，且每一个切片都要求是可变的。类似需求在安全 Rust 中是很难实现的，因为要对同一个数组做两个可变借用：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn split_at_mut(slice: & mut [i32], mid: usize) -> (& mut [i32], & mut [i32]) {
let len = slice.len();
assert!(mid < = len);
(& mut slice[..mid], & mut slice[mid..])
}
fn main() {
let mut v = vec![1, 2, 3, 4, 5, 6];
let r = & mut v[..];
let (a, b) = split_at_mut(r, 3);
assert_eq!(a, & mut [1, 2, 3]);
assert_eq!(b, & mut [4, 5, 6]);
}< / code > < / pre >
< p > 上面代码一眼看过去就知道会报错，因为我们试图在自定义的 < code > split_at_mut< / code > 函数中，可变借用 < code > slice< / code > 两次：< / p >
< pre > < code class = "language-shell" > error[E0499]: cannot borrow `*slice` as mutable more than once at a time
--> src/main.rs:6:30
|
1 | fn split_at_mut(slice: & mut [i32], mid: usize) -> (& mut [i32], & mut [i32]) {
| - let's call the lifetime of this reference `'1`
...
6 | (& mut slice[..mid], & mut slice[mid..])
| -------------------------^^^^^--------
| | | |
| | | second mutable borrow occurs here
| | first mutable borrow occurs here
| returning this value requires that `*slice` is borrowed for `'1`
< / code > < / pre >
< p > 对于 Rust 的借用检查器来说，它无法理解我们是分别借用了同一个切片的两个不同部分，但事实上，这种行为是没任何问题的，毕竟两个借用没有任何重叠之处。总之，不太聪明的 Rust 编译器阻碍了我们用这种简单且安全的方式去实现，那只能剑走偏锋，试试 < code > unsafe< / code > 了。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::slice;
fn split_at_mut(slice: & mut [i32], mid: usize) -> (& mut [i32], & mut [i32]) {
let len = slice.len();
let ptr = slice.as_mut_ptr();
assert!(mid < = len);
unsafe {
(
slice::from_raw_parts_mut(ptr, mid),
slice::from_raw_parts_mut(ptr.add(mid), len - mid),
)
}
}
fn main() {
let mut v = vec![1, 2, 3, 4, 5, 6];
let r = & mut v[..];
let (a, b) = split_at_mut(r, 3);
assert_eq!(a, & mut [1, 2, 3]);
assert_eq!(b, & mut [4, 5, 6]);
}< / code > < / pre >
< p > 相比安全实现，这段代码就显得没那么好理解了，甚至于我们还需要像 C 语言那样，通过指针地址的偏移去控制数组的分割。< / p >
< ul >
< li > < code > as_mut_ptr< / code > 会返回指向 < code > slice< / code > 首地址的裸指针 < code > *mut i32< / code > < / li >
< li > < code > slice::from_raw_parts_mut< / code > 函数通过指针和长度来创建一个新的切片，简单来说，该切片的初始地址是 < code > ptr< / code > ，长度为 < code > mid< / code > < / li >
< li > < code > ptr.add(mid)< / code > 可以获取第二个切片的初始地址，由于切片中的元素是 < code > i32< / code > 类型，每个元素都占用了 4 个字节的内存大小，因此我们不能简单的用 < code > ptr + mid< / code > 来作为初始地址，而应该使用 < code > ptr + 4 * mid< / code > ，但是这种使用方式并不安全，因此 < code > .add< / code > 方法是最佳选择< / li >
< / ul >
< p > 由于 < code > slice::from_raw_parts_mut< / code > 使用裸指针作为参数，因此它是一个 < code > unsafe fn< / code > ，我们在使用它时，就必须用 < code > unsafe< / code > 语句块进行包裹，类似的，< code > .add< / code > 方法也是如此(还是那句话，不要将无关的代码包含在 < code > unsafe< / code > 语句块中)。< / p >
< p > 部分同学可能会有疑问，那这段代码我们怎么保证 < code > unsafe< / code > 中使用的裸指针 < code > ptr< / code > 和 < code > ptr.add(mid)< / code > 是合法的呢？秘诀就在于 < code > assert!(mid < = len);< / code > ，通过这个断言，我们保证了裸指针一定指向了 < code > slice< / code > 切片中的某个元素，而不是一个莫名其妙的内存地址。< / p >
< p > 再回到我们的主题：< strong > 虽然 split_at_mut 使用了 < code > unsafe< / code > ，但我们无需将其声明为 < code > unsafe fn< / code > < / strong > ，这种情况下就是使用安全的抽象包裹 < code > unsafe< / code > 代码，这里的 < code > unsafe< / code > 使用是非常安全的，因为我们从合法数据中创建了的合法指针。< / p >
< p > 与之对比，下面的代码就非常危险了:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::slice;
let address = 0x01234usize;
let r = address as *mut i32;
let slice: & [i32] = unsafe { slice::from_raw_parts_mut(r, 10000) };
println!("{:?}",slice);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码从一个任意的内存地址，创建了一个 10000 长度的 < code > i32< / code > 切片，我们无法保证切片中的元素都是合法的 < code > i32< / code > 值，这种访问就是一种未定义行为(UB = undefined behavior)。< / p >
< pre > < code class = "language-shell" > zsh: segmentation fault
< / code > < / pre >
< p > 不出所料，运行后看到了一个段错误。< / p >
< h2 id = "ffi" > < a class = "header" href = "#ffi" > FFI< / a > < / h2 >
< p > < code > FFI< / code > （ ） ， ， < code > JNI（ ） < / code > 。< / p >
< p > < code > FFI< / code > 之所以存在是由于现实中很多代码库都是由不同语言编写的，如果我们需要使用某个库，但是它是由其它语言编写的，那么往往只有两个选择：< / p >
< ul >
< li > 对该库进行重写或者移植< / li >
< li > 使用 < code > FFI< / code > < / li >
< / ul >
< p > 前者相当不错，但是在很多时候，并没有那么多时间去重写，因此 < code > FFI< / code > 就成了最佳选择。回到 Rust 语言上，由于这门语言依然很年轻，一些生态是缺失的，我们在写一些不是那么大众的项目时，可能会同时遇到没有相应的 Rust 库可用的尴尬境况，此时通过 < code > FFI< / code > 去调用 C 语言的库就成了相当棒的选择。< / p >
< p > 还有在将 C/C++ 的代码重构为 Rust 时，先将相关代码引入到 Rust 项目中，然后逐步重构，也是不错的(为什么用不错来形容？因为重构一个有一定规模的 C/C++ 项目远没有想象中美好，因此最好的选择还是对于新项目使用 Rust 实现，老项目。。就让它先运行着吧)。< / p >
< p > 当然，除了 < code > FFI< / code > 还有一个办法可以解决跨语言调用的问题， ， ， ， < / p >
< p > 言归正传，之前我们提到 < code > unsafe< / code > 的另一个重要目的就是对 < code > FFI< / code > 提供支持，它的全称是 < code > Foreign Function Interface< / code > ，顾名思义，通过 < code > FFI< / code > , 我们的 Rust 代码可以跟其它语言的外部代码进行交互。< / p >
< p > 下面的例子演示了如何调用 C 标准库中的 < code > abs< / code > 函数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > extern "C" {
fn abs(input: i32) -> i32;
}
fn main() {
unsafe {
println!("Absolute value of -3 according to C: {}", abs(-3));
}
}< / code > < / pre >
< p > C 语言的代码定义在了 < code > extern< / code > 代码块中， 而 < code > extern< / code > 必须使用 < code > unsafe< / code > 才能进行进行调用，原因在于其它语言的代码并不会强制执行 Rust 的规则，因此 Rust 无法对这些代码进行检查，最终还是要靠开发者自己来保证代码的正确性和程序的安全性。< / p >
< h4 id = "abi" > < a class = "header" href = "#abi" > ABI< / a > < / h4 >
< p > 在 < code > extern "C"< / code > 代码块中，我们列出了想要调用的外部函数的签名。其中 < code > "C"< / code > 定义了外部函数所使用的< strong > 应用二进制接口< / strong > < code > ABI< / code > (Application Binary Interface)： < code > ABI< / code > 定义了如何在汇编层面来调用该函数。在所有 < code > ABI< / code > 中， < / p >
< h4 id = "在其它语言中调用-rust-函数" > < a class = "header" href = "#在其它语言中调用-rust-函数" > 在其它语言中调用 Rust 函数< / a > < / h4 >
< p > 在 Rust 中调用其它语言的函数是让 Rust 利用其他语言的生态，那反过来可以吗？其他语言可以利用 Rust 的生态不？答案是肯定的。< / p >
< p > 我们可以使用 < code > extern< / code > 来创建一个接口，其它语言可以通过该接口来调用相关的 Rust 函数。但是此处的语法与之前有所不同，之前用的是语句块，而这里是在函数定义时加上 < code > extern< / code > 关键字，当然，别忘了指定相应的 < code > ABI< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[no_mangle]
pub extern "C" fn call_from_c() {
println!("Just called a Rust function from C!");
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面的代码可以让 < code > call_from_c< / code > 函数被 < code > C< / code > 语言的代码调用，当然，前提是将其编译成一个共享库，然后链接到 C 语言中。< / p >
< p > 这里还有一个比较奇怪的注解 < code > #[no_mangle]< / code > ，它用于告诉 Rust 编译器：不要乱改函数的名称。 < code > Mangling< / code > 的定义是：当 Rust 因为编译需要去修改函数的名称，例如为了让名称包含更多的信息，这样其它的编译部分就能从该名称获取相应的信息，这种修改会导致函数名变得相当不可读。< / p >
< p > 因此，为了让 Rust 函数能顺利被其它语言调用，我们必须要禁止掉该功能。< / p >
< h2 id = "访问或修改一个可变的静态变量" > < a class = "header" href = "#访问或修改一个可变的静态变量" > 访问或修改一个可变的静态变量< / a > < / h2 >
< p > 这部分我们在之前的< a href = "https://course.rs/advance/global-variable.html#静态变量" > 全局变量章节< / a > 中有过详细介绍，这里就不再赘述，大家可以前往此章节阅读。< / p >
< h2 id = "实现-unsafe-特征" > < a class = "header" href = "#实现-unsafe-特征" > 实现 unsafe 特征< / a > < / h2 >
< p > 说实话，< code > unsafe< / code > 的特征确实不多见，如果大家还记得的话，我们在之前的 < a href = "https://course.rs/advance/concurrency-with-threads/send-sync.html#为裸指针实现sync" > Send 和 Sync< / a > 章节中实现过 < code > unsafe< / code > 特征 < code > Send< / code > 。< / p >
< p > 之所以会有 < code > unsafe< / code > 的特征，是因为该特征至少有一个方法包含有编译器无法验证的内容。< code > unsafe< / code > 特征的声明很简单：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > unsafe trait Foo {
// 方法列表
}
unsafe impl Foo for i32 {
// 实现相应的方法
}
fn main() {}< / code > < / pre >
< p > 通过 < code > unsafe impl< / code > 的使用，我们告诉编译器：相应的正确性由我们自己来保证。< / p >
< p > 再回到刚提到的 < code > Send< / code > 特征，若我们的类型中的所有字段都实现了 < code > Send< / code > 特征，那该类型也会自动实现 < code > Send< / code > 。但是如果我们想要为某个类型手动实现 < code > Send< / code > ，例如为裸指针，那么就必须使用 < code > unsafe< / code > ，相关的代码在之前的链接中也有，大家可以移步查看。< / p >
< p > 总之，< code > Send< / code > 特征标记为 < code > unsafe< / code > 是因为 Rust 无法验证我们的类型是否能在线程间安全的传递，因此就需要通过 < code > unsafe< / code > 来告诉编译器，它无需操心，剩下的交给我们自己来处理。< / p >
< h2 id = "访问-union-中的字段" > < a class = "header" href = "#访问-union-中的字段" > 访问 union 中的字段< / a > < / h2 >
< p > 截止目前，我们还没有介绍过 < code > union< / code > ，原因很简单，它主要用于跟 < code > C< / code > 代码进行交互。< / p >
< p > 访问 < code > union< / code > 的字段是不安全的，因为 Rust 无法保证当前存储在 < code > union< / code > 实例中的数据类型。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[repr(C)]
union MyUnion {
f1: u32,
f2: f32,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上从可以看出，< code > union< / code > 的使用方式跟结构体确实很相似，但是前者的所有字段都共享同一个存储空间，意味着往 < code > union< / code > 的某个字段写入值，会导致其它字段的值会被覆盖。< / p >
< p > 关于 < code > union< / code > 的更多信息，可以在< a href = "https://doc.rust-lang.org/reference/items/unions.html" > 这里查看< / a > 。< / p >
< h2 id = "一些实用工具库" > < a class = "header" href = "#一些实用工具库" > 一些实用工具(库)< / a > < / h2 >
< p > 由于 < code > unsafe< / code > 和 < code > FFI< / code > 在 Rust 的使用场景中是相当常见的(例如相对于 Go 的 < code > unsafe< / code > 来说)，因此社区已经开发出了相当一部分实用的工具，可以改善相应的开发体验。< / p >
< h4 id = "rust-bindgen-和-cbindgen" > < a class = "header" href = "#rust-bindgen-和-cbindgen" > rust-bindgen 和 cbindgen< / a > < / h4 >
< p > 对于 < code > FFI< / code > 调用来说，保证接口的正确性是非常重要的，这两个库可以帮我们自动生成相应的接口，其中 < a href = "https://github.com/rust-lang/rust-bindgen" > < code > rust-bindgen< / code > < / a > 用于在 Rust 中访问 C 代码，而 < a href = "https://github.com/eqrion/cbindgen/" > < code > cbindgen< / code > < / a > 则反之。< / p >
< p > 下面以 < code > rust-bindgen< / code > 为例，来看看如何自动生成调用 C 的代码，首先下面是 C 代码:< / p >
< pre > < code class = "language-c" > typedef struct Doggo {
int many;
char wow;
} Doggo;
void eleven_out_of_ten_majestic_af(Doggo* pupper);
< / code > < / pre >
< p > 下面是自动生成的可以调用上面代码的 Rust 代码：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > /* automatically generated by rust-bindgen 0.99.9 */
#[repr(C)]
pub struct Doggo {
pub many: ::std::os::raw::c_int,
pub wow: ::std::os::raw::c_char,
}
extern "C" {
pub fn eleven_out_of_ten_majestic_af(pupper: *mut Doggo);
}
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "cxx" > < a class = "header" href = "#cxx" > cxx< / a > < / h4 >
< p > 如果需要跟 C++ 代码交互，非常推荐使用 < a href = "https://github.com/dtolnay/cxx" > < code > cxx< / code > < / a > ，它提供了双向的调用，最大的优点就是安全：是的，你无需通过 < code > unsafe< / code > 来使用它！< / p >
< h4 id = "miri" > < a class = "header" href = "#miri" > Miri< / a > < / h4 >
< p > < a href = "https://github.com/rust-lang/miri" > < code > miri< / code > < / a > 可以生成 Rust 的中间层表示 MIR， ， < / p >
< p > 可以通过 < code > rustup component add miri< / code > 来安装它，并通过 < code > cargo miri< / code > 来使用，同时还可以使用 < code > cargo miri test< / code > 来运行测试代码。< / p >
< p > < code > miri< / code > 可以帮助我们检查常见的未定义行为(UB = Undefined Behavior)，以下列出了一部分:< / p >
< ul >
< li > 内存越界检查和内存释放后再使用(use-after-free)< / li >
< li > 使用未初始化的数据< / li >
< li > 数据竞争< / li >
< li > 内存对齐问题< / li >
< / ul >
< p > 但是需要注意的是，它只能帮助识别被执行代码路径的风险，那些未被执行到的代码是没办法被识别的。< / p >
< h4 id = "clippy" > < a class = "header" href = "#clippy" > Clippy< / a > < / h4 >
< p > 官方的 < a href = "https://github.com/rust-lang/rust-clippy" > < code > clippy< / code > < / a > 检查器提供了有限的 < code > unsafe< / code > 支持，虽然不多，但是至少有一定帮助。例如 < code > missing_safety_docs< / code > 检查可以帮助我们检查哪些 < code > unsafe< / code > 函数遗漏了文档。< / p >
< p > 需要注意的是： Rust 编译器并不会默认开启所有检查，大家可以调用 < code > rustc -W help< / code > 来看看最新的信息。< / p >
< h4 id = "prusti" > < a class = "header" href = "#prusti" > Prusti< / a > < / h4 >
< p > < a href = "https://viperproject.github.io/prusti-dev/user-guide/" > < code > prusti< / code > < / a > 需要大家自己来构建一个证明，然后通过它证明代码中的不变量是正确被使用的，当你在安全代码中使用不安全的不变量时，就会非常有用。具体的使用文档见< a href = "https://viperproject.github.io/prusti-dev/user-guide/" > 这里< / a > 。< / p >
< h4 id = "模糊测试fuzz-testing" > < a class = "header" href = "#模糊测试fuzz-testing" > 模糊测试(fuzz testing)< / a > < / h4 >
< p > 在 < a href = "https://rust-fuzz.github.io/book/" > Rust Fuzz Book< / a > 中列出了一些 Rust 可以使用的模糊测试方法。< / p >
< p > 同时，我们还可以使用 < a href = "https://github.com/jakubadamw/rutenspitz" > < code > rutenspitz< / code > < / a > 这个过程宏来测试有状态的代码，例如数据结构。< / p >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 至此，< code > unsafe< / code > 的五种兵器已介绍完毕，大家是否意犹未尽？我想说的是，就算意犹未尽，也没有其它兵器了。< / p >
< p > 就像上一章中所提到的，< code > unsafe< / code > 只应该用于这五种场景，其它场景，你应该坚决的使用安全的代码，否则就会像 < code > actix-web< / code > 的前作者一样，被很多人议论，甚至被喷。。。< / p >
< p > 总之，能不使用 < code > unsafe< / code > 一定不要使用，就算使用也要控制好边界，让范围尽可能的小，就像本章的例子一样，只有真的需要 < code > unsafe< / code > 的代码，才应该包含其中, 而不是将无关代码也纳入进来。< / p >
< h2 id = "进一步学习" > < a class = "header" href = "#进一步学习" > 进一步学习< / a > < / h2 >
< ol >
< li > < a href = "https://blog.logrocket.com/unsafe-rust-how-and-when-not-to-use-it/" > Unsafe Rust: How and when (not) to use it< / a > < / li >
< / ol >
< / main >
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