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< h1 id = "io" > < a class = "header" href = "#io" > I/O< / a > < / h1 >
< p > 本章节中我们将深入学习 Tokio 中的 I/O 操作，了解它的原理以及该如何使用。< / p >
< p > Tokio 中的 I/O 操作和 < code > std< / code > 在使用方式上几无区别，最大的区别就是前者是异步的，例如 Tokio 的读写特征分别是 < code > AsyncRead< / code > 和 < code > AsyncWrite< / code > :< / p >
< ul >
< li > 有部分类型按照自己的所需实现了它们: < code > TcpStream< / code > ， < code > File< / code > ， < code > Stdout< / code > < / li >
< li > 还有数据结构也实现了它们：< code > Vec< u8> < / code > 、< code > & [u8]< / code > ，这样就可以直接使用这些数据结构作为读写器( reader / writer)< / li >
< / ul >
< h2 id = "asyncread-和-asyncwrite" > < a class = "header" href = "#asyncread-和-asyncwrite" > AsyncRead 和 AsyncWrite< / a > < / h2 >
< p > 这两个特征为字节流的异步读写提供了便利，通常我们会使用 < code > AsyncReadExt< / code > 和 < code > AsyncWriteExt< / code > 提供的工具方法，这些方法都使用 < code > async< / code > 声明，且需要通过 < code > .await< / code > 进行调用，< / p >
< h4 id = "async-fn-read" > < a class = "header" href = "#async-fn-read" > async fn read< / a > < / h4 >
< p > < code > AsyncReadExt::read< / code > 是一个异步方法可以将数据读入缓冲区( < code > buffer< / code > )中，然后返回读取的字节数。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::fs::File;
use tokio::io::{self, AsyncReadExt};
#[tokio::main]
async fn main() -> io::Result< ()> {
let mut f = File::open("foo.txt").await?;
let mut buffer = [0; 10];
// 由于 buffer 的长度限制，当次的 `read` 调用最多可以从文件中读取 10 个字节的数据
let n = f.read(& mut buffer[..]).await?;
println!("The bytes: {:?}", & buffer[..n]);
Ok(())
}< / code > < / pre >
< p > 需要注意的是：当 < code > read< / code > 返回 < code > Ok(0)< / code > 时，意味着字节流( stream )已经关闭，在这之后继续调用 < code > read< / code > 会立刻完成，依然获取到返回值 < code > Ok(0)< / code > 。 例如，字节流如果是 < code > TcpStream< / code > 类型，那 < code > Ok(0)< / code > 说明该< strong > 连接的读取端已经被关闭< / strong > (写入端关闭，会报其它的错误)。< / p >
< h4 id = "async-fn-read_to_end" > < a class = "header" href = "#async-fn-read_to_end" > async fn read_to_end< / a > < / h4 >
< p > < code > AsyncReadExt::read_to_end< / code > 方法会从字节流中读取所有的字节，直到遇到 < code > EOF< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::io::{self, AsyncReadExt};
use tokio::fs::File;
#[tokio::main]
async fn main() -> io::Result< ()> {
let mut f = File::open("foo.txt").await?;
let mut buffer = Vec::new();
// 读取整个文件的内容
f.read_to_end(& mut buffer).await?;
Ok(())
}< / code > < / pre >
< h4 id = "async-fn-write" > < a class = "header" href = "#async-fn-write" > async fn write< / a > < / h4 >
< p > < code > AsyncWriteExt::write< / code > 异步方法会尝试将缓冲区的内容写入到写入器( < code > writer< / code > )中，同时返回写入的字节数:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::io::{self, AsyncWriteExt};
use tokio::fs::File;
#[tokio::main]
async fn main() -> io::Result< ()> {
let mut file = File::create("foo.txt").await?;
let n = file.write(b"some bytes").await?;
println!("Wrote the first {} bytes of 'some bytes'.", n);
Ok(())
}< / code > < / pre >
< p > 上面代码很清晰，但是大家可能会疑惑 < code > b"some bytes"< / code > 是什么意思。这种写法可以将一个 < code > & str< / code > 字符串转变成一个字节数组：< code > & [u8;10]< / code > ，然后 < code > write< / code > 方法又会将这个 < code > & [u8;10]< / code > 的数组类型隐式强转为数组切片: < code > & [u8]< / code > 。< / p >
< h4 id = "async-fn-write_all" > < a class = "header" href = "#async-fn-write_all" > async fn write_all< / a > < / h4 >
< p > < code > AsyncWriteExt::write_all< / code > 将缓冲区的内容全部写入到写入器中：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::io::{self, AsyncWriteExt};
use tokio::fs::File;
#[tokio::main]
async fn main() -> io::Result< ()> {
let mut file = File::create("foo.txt").await?;
file.write_all(b"some bytes").await?;
Ok(())
}< / code > < / pre >
< p > 以上只是部分方法，实际上还有一些实用的方法由于篇幅有限无法列出，大家可以通过 < a href = "https://docs.rs/tokio/latest/tokio/io/index.html" > API 文档< / a > 查看完整的列表。< / p >
< h2 id = "实用函数" > < a class = "header" href = "#实用函数" > 实用函数< / a > < / h2 >
< p > 另外，和标准库一样， < code > tokio::io< / code > 模块包含了多个实用的函数或 API， < / p >
< p > 例如，< code > tokio::io::copy< / code > 异步的将读取器( < code > reader< / code > )中的内容拷贝到写入器( < code > writer< / code > )中。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::fs::File;
use tokio::io;
#[tokio::main]
async fn main() -> io::Result< ()> {
let mut reader: & [u8] = b"hello";
let mut file = File::create("foo.txt").await?;
io::copy(& mut reader, & mut file).await?;
Ok(())
}< / code > < / pre >
< p > 还记得我们之前提到的字节数组 < code > & [u8]< / code > 实现了 < code > AsyncRead< / code > 吗？正因为这个原因，所以这里可以直接将 < code > & u8< / code > 用作读取器。< / p >
< h2 id = "回声服务-echo-" > < a class = "header" href = "#回声服务-echo-" > 回声服务( Echo )< / a > < / h2 >
< p > 就如同写代码必写 < code > hello, world< / code > ，实现 web 服务器，往往会选择实现一个回声服务。该服务会将用户的输入内容直接返回给用户，就像回声壁一样。< / p >
< p > 具体来说，就是从用户建立的 TCP 连接的 socket 中读取到数据，然后立刻将同样的数据写回到该 socket 中。因此客户端会收到和自己发送的数据一模一样的回复。< / p >
< p > 下面我们将使用两种稍有不同的方法实现该回声服务。< / p >
< h4 id = "使用-iocopy" > < a class = "header" href = "#使用-iocopy" > 使用 < code > io::copy()< / code > < / a > < / h4 >
< p > 先来创建一个新的 bin 文件，用于运行我们的回声服务：< / p >
< pre > < code class = "language-console" > touch src/bin/echo-server-copy.rs
< / code > < / pre >
< p > 然后可以通过以下命令运行它(跟上一章节的方式相同)： < / p >
< pre > < code class = "language-console" > cargo run --bin echo-server-copy
< / code > < / pre >
< p > 至于客户端，可以简单的使用 < code > telnet< / code > 的方式来连接，或者也可以使用 < code > tokio::net::TcpStream< / code > ，它的< a href = "https://docs.rs/tokio/1/tokio/net/struct.TcpStream.html#examples" > 文档示例< / a > 非常适合大家进行参考。< / p >
< p > 先来实现一下基本的服务器框架：通过 loop 循环接收 TCP 连接，然后为每一条连接创建一个单独的任务去处理。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::io;
use tokio::net::TcpListener;
#[tokio::main]
async fn main() -> io::Result< ()> {
let listener = TcpListener::bind("127.0.0.1:6142").await?;
loop {
let (mut socket, _) = listener.accept().await?;
tokio::spawn(async move {
// 在这里拷贝数据
});
}
}< / code > < / pre >
< p > 下面，来看看重头戏 < code > io::copy< / code > ，它有两个参数：一个读取器，一个写入器，然后将读取器中的数据直接拷贝到写入器中，类似的实现代码如下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > io::copy(& mut socket, & mut socket).await
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码相信大家一眼就能看出问题，由于我们的读取器和写入器都是同一个 socket， ， < / p >
< h5 id = "分离读写器" > < a class = "header" href = "#分离读写器" > 分离读写器< / a > < / h5 >
< p > 显然，使用同一个 socket 是不行的，为了实现目标功能，必须将 < code > socket< / code > 分离成一个读取器和写入器。< / p >
< p > 任何一个读写器( reader + writer )都可以使用 < code > io::split< / code > 方法进行分离，最终返回一个读取器和写入器，这两者可以独自的使用，例如可以放入不同的任务中。< / p >
< p > 例如，我们的回声客户端可以这样实现，以实现同时并发读写：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
use tokio::net::TcpStream;
#[tokio::main]
async fn main() -> io::Result< ()> {
let socket = TcpStream::connect("127.0.0.1:6142").await?;
let (mut rd, mut wr) = io::split(socket);
// 创建异步任务，在后台写入数据
tokio::spawn(async move {
wr.write_all(b"hello\r\n").await?;
wr.write_all(b"world\r\n").await?;
// 有时，我们需要给予 Rust 一些类型暗示，它才能正确的推导出类型
Ok::< _, io::Error> (())
});
let mut buf = vec![0; 128];
loop {
let n = rd.read(& mut buf).await?;
if n == 0 {
break;
}
println!("GOT {:?}", & buf[..n]);
}
Ok(())
}< / code > < / pre >
< p > 实际上，< code > io::split< / code > 可以用于任何同时实现了 < code > AsyncRead< / code > 和 < code > AsyncWrite< / code > 的值，它的内部使用了 < code > Arc< / code > 和 < code > Mutex< / code > 来实现相应的功能。如果大家觉得这种实现有些重，可以使用 Tokio 提供的 < code > TcpStream< / code > ，它提供了两种方式进行分离:< / p >
< ul >
< li > < a href = "https://docs.rs/tokio/1.15.0/tokio/net/struct.TcpStream.html#method.split" > < code > TcpStream::split< / code > < / a > 会获取字节流的引用，然后将其分离成一个读取器和写入器。但由于使用了引用的方式，它们俩必须和 < code > split< / code > 在同一个任务中。 优点就是，这种实现没有性能开销，因为无需 < code > Arc< / code > 和 < code > Mutex< / code > 。< / li >
< li > < a href = "https://docs.rs/tokio/1.15.0/tokio/net/struct.TcpStream.html#method.into_split" > < code > TcpStream::into_split< / code > < / a > 还提供了一种分离实现，分离出来的结果可以在任务间移动，内部是通过 < code > Arc< / code > 实现< / li >
< / ul >
< p > 再来分析下我们的使用场景，由于 < code > io::copy()< / code > 调用时所在的任务和 < code > split< / code > 所在的任务是同一个，因此可以使用性能最高的 < code > TcpStream::split< / code > :< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > tokio::spawn(async move {
let (mut rd, mut wr) = socket.split();
if io::copy(& mut rd, & mut wr).await.is_err() {
eprintln!("failed to copy");
}
});
< span class = "boring" > }< / span > < / code > < / pre >
< p > 使用 < code > io::copy< / code > 实现的完整代码见< a href = "https://github.com/tokio-rs/website/blob/master/tutorial-code/io/src/echo-server-copy.rs" > 此处< / a > 。< / p >
< h4 id = "手动拷贝" > < a class = "header" href = "#手动拷贝" > 手动拷贝< / a > < / h4 >
< p > 程序员往往拥有一颗手动干翻一切的心，因此如果你不想用 < code > io::copy< / code > 来简单实现，还可以自己手动去拷贝数据:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
use tokio::net::TcpListener;
#[tokio::main]
async fn main() -> io::Result< ()> {
let listener = TcpListener::bind("127.0.0.1:6142").await?;
loop {
let (mut socket, _) = listener.accept().await?;
tokio::spawn(async move {
let mut buf = vec![0; 1024];
loop {
match socket.read(& mut buf).await {
// 返回值 `Ok(0)` 说明对端已经关闭
Ok(0) => return,
Ok(n) => {
// Copy the data back to socket
// 将数据拷贝回 socket 中
if socket.write_all(& buf[..n]).await.is_err() {
// 非预期错误，由于我们这里无需再做什么，因此直接停止处理
return;
}
}
Err(_) => {
// 非预期错误，由于我们无需再做什么，因此直接停止处理
return;
}
}
}
});
}
}< / code > < / pre >
< p > 建议这段代码放入一个和之前 < code > io::copy< / code > 不同的文件中 < code > src/bin/echo-server.rs< / code > ， < code > cargo run --bin echo-server< / code > 运行。< / p >
< p > 下面一起来看看这段代码有哪些值得注意的地方。首先，由于使用了 < code > write_all< / code > 和 < code > read< / code > 方法，需要先将对应的特征引入到当前作用域内:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
< span class = "boring" > }< / span > < / code > < / pre >
< h5 id = "在堆上分配缓冲区" > < a class = "header" href = "#在堆上分配缓冲区" > 在堆上分配缓冲区< / a > < / h5 >
< p > 在上面代码中，我们需要将数据从 < code > socket< / code > 中读取到一个缓冲区 < code > buffer< / code > 中：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let mut buf = vec![0; 1024];
< span class = "boring" > }< / span > < / code > < / pre >
< p > 可以看到，此处的缓冲区是一个 < code > Vec< / code > 动态数组，它的数据是存储在堆上，而不是栈上(若改成 < code > let mut buf = [0; 1024];< / code > ，则存储在栈上)。< / p >
< p > 在之前，我们提到过一个数据如果想在 < code > .await< / code > 调用过程中存在，那它必须存储在当前任务内。在我们的代码中，< code > buf< / code > 会在 < code > .await< / code > 调用过程中被使用，因此它必须要存储在任务内。< / p >
< p > 若该缓冲区数组创建在栈上，那每条连接所对应的任务的内部数据结构看上去可能如下所示：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct Task {
task: enum {
AwaitingRead {
socket: TcpStream,
buf: [BufferType],
},
AwaitingWriteAll {
socket: TcpStream,
buf: [BufferType],
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 可以看到，栈数组要被使用，就必须存储在相应的结构体内，其中两个结构体分别持有了不同的栈数组 < code > [BufferType]< / code > ，这种方式会导致任务结构变得很大。特别地，我们选择缓冲区长度往往会使用分页长度(page size)，因此使用栈数组会导致任务的内存大小变得很奇怪甚至糟糕：< code > $page-size + 一些额外的字节< / code > 。< / p >
< p > 当然，编译器会帮助我们做一些优化。例如，会进一步优化 < code > async< / code > 语句块的布局，而不是像上面一样简单的使用 < code > enum< / code > 。在实践中，变量也不会在枚举成员间移动。< / p >
< p > 但是再怎么优化，任务的结构体至少也会跟其中的栈数组一样大，因此通常情况下，使用堆上的缓冲区会高效实用的多。< / p >
< blockquote >
< p > 当任务因为调度在线程间移动时，存储在栈上的数据需要进行保存和恢复，过大的栈上变量会带来不小的数据拷贝开销< / p >
< p > 因此，存储大量数据的变量最好放到堆上< / p >
< / blockquote >
< h5 id = "处理-eof" > < a class = "header" href = "#处理-eof" > 处理 EOF< / a > < / h5 >
< p > 当 TCP 连接的读取端关闭后，再调用 < code > read< / code > 方法会返回 < code > Ok(0)< / code > 。此时，再继续下去已经没有意义，因此我们需要退出循环。忘记在 EOF 时退出读取循环，是网络编程中一个常见的 bug :< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > loop {
match socket.read(& mut buf).await {
Ok(0) => return,
// ... 其余错误处理
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 大家不妨深入思考下，如果没有退出循环会怎么样？之前我们提到过，一旦读取端关闭后，那后面的 < code > read< / code > 调用就会立即返回 < code > Ok(0)< / code > ，而不会阻塞等待，因此这种无阻塞循环会最终导致 CPU 立刻跑到 100% ，并将一直持续下去，直到程序关闭。< / p >
< / main >
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