1//! Traits, helpers, and type definitions for asynchronous I/O functionality.
2//!
3//! This module is the asynchronous version of `std::io`. Primarily, it
4//! defines two traits, [`AsyncRead`] and [`AsyncWrite`], which are asynchronous
5//! versions of the [`Read`] and [`Write`] traits in the standard library.
6//!
7//! # `AsyncRead` and `AsyncWrite`
8//!
9//! Like the standard library's [`Read`] and [`Write`] traits, [`AsyncRead`] and
10//! [`AsyncWrite`] provide the most general interface for reading and writing
11//! input and output. Unlike the standard library's traits, however, they are
12//! _asynchronous_ — meaning that reading from or writing to a `tokio::io`
13//! type will _yield_ to the Tokio scheduler when IO is not ready, rather than
14//! blocking. This allows other tasks to run while waiting on IO.
15//!
16//! Another difference is that `AsyncRead` and `AsyncWrite` only contain
17//! core methods needed to provide asynchronous reading and writing
18//! functionality. Instead, utility methods are defined in the [`AsyncReadExt`]
19//! and [`AsyncWriteExt`] extension traits. These traits are automatically
20//! implemented for all values that implement `AsyncRead` and `AsyncWrite`
21//! respectively.
22//!
23//! End users will rarely interact directly with `AsyncRead` and
24//! `AsyncWrite`. Instead, they will use the async functions defined in the
25//! extension traits. Library authors are expected to implement `AsyncRead`
26//! and `AsyncWrite` in order to provide types that behave like byte streams.
27//!
28//! Even with these differences, Tokio's `AsyncRead` and `AsyncWrite` traits
29//! can be used in almost exactly the same manner as the standard library's
30//! `Read` and `Write`. Most types in the standard library that implement `Read`
31//! and `Write` have asynchronous equivalents in `tokio` that implement
32//! `AsyncRead` and `AsyncWrite`, such as [`File`] and [`TcpStream`].
33//!
34//! For example, the standard library documentation introduces `Read` by
35//! [demonstrating][std_example] reading some bytes from a [`std::fs::File`]. We
36//! can do the same with [`tokio::fs::File`][`File`]:
37//!
38//! ```no_run
39//! use tokio::io::{self, AsyncReadExt};
40//! use tokio::fs::File;
41//!
42//! #[tokio::main]
43//! async fn main() -> io::Result<()> {
44//! let mut f = File::open("foo.txt").await?;
45//! let mut buffer = [0; 10];
46//!
47//! // read up to 10 bytes
48//! let n = f.read(&mut buffer).await?;
49//!
50//! println!("The bytes: {:?}", &buffer[..n]);
51//! Ok(())
52//! }
53//! ```
54//!
55//! [`File`]: crate::fs::File
56//! [`TcpStream`]: crate::net::TcpStream
57//! [`std::fs::File`]: std::fs::File
58//! [std_example]: std::io#read-and-write
59//!
60//! ## Buffered Readers and Writers
61//!
62//! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
63//! making near-constant calls to the operating system. To help with this,
64//! `std::io` comes with [support for _buffered_ readers and writers][stdbuf],
65//! and therefore, `tokio::io` does as well.
66//!
67//! Tokio provides an async version of the [`std::io::BufRead`] trait,
68//! [`AsyncBufRead`]; and async [`BufReader`] and [`BufWriter`] structs, which
69//! wrap readers and writers. These wrappers use a buffer, reducing the number
70//! of calls and providing nicer methods for accessing exactly what you want.
71//!
72//! For example, [`BufReader`] works with the [`AsyncBufRead`] trait to add
73//! extra methods to any async reader:
74//!
75//! ```no_run
76//! use tokio::io::{self, BufReader, AsyncBufReadExt};
77//! use tokio::fs::File;
78//!
79//! #[tokio::main]
80//! async fn main() -> io::Result<()> {
81//! let f = File::open("foo.txt").await?;
82//! let mut reader = BufReader::new(f);
83//! let mut buffer = String::new();
84//!
85//! // read a line into buffer
86//! reader.read_line(&mut buffer).await?;
87//!
88//! println!("{}", buffer);
89//! Ok(())
90//! }
91//! ```
92//!
93//! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
94//! to [`write`](crate::io::AsyncWriteExt::write). However, you **must** flush
95//! [`BufWriter`] to ensure that any buffered data is written.
96//!
97//! ```no_run
98//! use tokio::io::{self, BufWriter, AsyncWriteExt};
99//! use tokio::fs::File;
100//!
101//! #[tokio::main]
102//! async fn main() -> io::Result<()> {
103//! let f = File::create("foo.txt").await?;
104//! {
105//! let mut writer = BufWriter::new(f);
106//!
107//! // Write a byte to the buffer.
108//! writer.write(&[42u8]).await?;
109//!
110//! // Flush the buffer before it goes out of scope.
111//! writer.flush().await?;
112//!
113//! } // Unless flushed or shut down, the contents of the buffer is discarded on drop.
114//!
115//! Ok(())
116//! }
117//! ```
118//!
119//! [stdbuf]: std::io#bufreader-and-bufwriter
120//! [`std::io::BufRead`]: std::io::BufRead
121//! [`AsyncBufRead`]: crate::io::AsyncBufRead
122//! [`BufReader`]: crate::io::BufReader
123//! [`BufWriter`]: crate::io::BufWriter
124//!
125//! ## Implementing `AsyncRead` and `AsyncWrite`
126//!
127//! Because they are traits, we can implement [`AsyncRead`] and [`AsyncWrite`] for
128//! our own types, as well. Note that these traits must only be implemented for
129//! non-blocking I/O types that integrate with the futures type system. In
130//! other words, these types must never block the thread, and instead the
131//! current task is notified when the I/O resource is ready.
132//!
133//! ## Conversion to and from Stream/Sink
134//!
135//! It is often convenient to encapsulate the reading and writing of bytes in a
136//! [`Stream`] or [`Sink`] of data.
137//!
138//! Tokio provides simple wrappers for converting [`AsyncRead`] to [`Stream`]
139//! and vice-versa in the [tokio-util] crate, see [`ReaderStream`] and
140//! [`StreamReader`].
141//!
142//! There are also utility traits that abstract the asynchronous buffering
143//! necessary to write your own adaptors for encoding and decoding bytes to/from
144//! your structured data, allowing to transform something that implements
145//! [`AsyncRead`]/[`AsyncWrite`] into a [`Stream`]/[`Sink`], see [`Decoder`] and
146//! [`Encoder`] in the [tokio-util::codec] module.
147//!
148//! [tokio-util]: https://docs.rs/tokio-util
149//! [tokio-util::codec]: https://docs.rs/tokio-util/latest/tokio_util/codec/index.html
150//!
151//! # Standard input and output
152//!
153//! Tokio provides asynchronous APIs to standard [input], [output], and [error].
154//! These APIs are very similar to the ones provided by `std`, but they also
155//! implement [`AsyncRead`] and [`AsyncWrite`].
156//!
157//! Note that the standard input / output APIs **must** be used from the
158//! context of the Tokio runtime, as they require Tokio-specific features to
159//! function. Calling these functions outside of a Tokio runtime will panic.
160//!
161//! [input]: fn@stdin
162//! [output]: fn@stdout
163//! [error]: fn@stderr
164//!
165//! # `std` re-exports
166//!
167//! Additionally, [`Error`], [`ErrorKind`], [`Result`], and [`SeekFrom`] are
168//! re-exported from `std::io` for ease of use.
169//!
170//! [`AsyncRead`]: trait@AsyncRead
171//! [`AsyncWrite`]: trait@AsyncWrite
172//! [`AsyncReadExt`]: trait@AsyncReadExt
173//! [`AsyncWriteExt`]: trait@AsyncWriteExt
174//! ["codec"]: https://docs.rs/tokio-util/latest/tokio_util/codec/index.html
175//! [`Encoder`]: https://docs.rs/tokio-util/latest/tokio_util/codec/trait.Encoder.html
176//! [`Decoder`]: https://docs.rs/tokio-util/latest/tokio_util/codec/trait.Decoder.html
177//! [`ReaderStream`]: https://docs.rs/tokio-util/latest/tokio_util/io/struct.ReaderStream.html
178//! [`StreamReader`]: https://docs.rs/tokio-util/latest/tokio_util/io/struct.StreamReader.html
179//! [`Error`]: struct@Error
180//! [`ErrorKind`]: enum@ErrorKind
181//! [`Result`]: type@Result
182//! [`Read`]: std::io::Read
183//! [`SeekFrom`]: enum@SeekFrom
184//! [`Sink`]: https://docs.rs/futures/0.3/futures/sink/trait.Sink.html
185//! [`Stream`]: https://docs.rs/futures/0.3/futures/stream/trait.Stream.html
186//! [`Write`]: std::io::Write
187
188#![cfg_attr(
189 not(all(feature = "rt", feature = "net")),
190 allow(dead_code, unused_imports)
191)]
192
193cfg_io_blocking! {
194 pub(crate) mod blocking;
195}
196
197mod async_buf_read;
198pub use self::async_buf_read::AsyncBufRead;
199
200mod async_read;
201pub use self::async_read::AsyncRead;
202
203mod async_seek;
204pub use self::async_seek::AsyncSeek;
205
206mod async_write;
207pub use self::async_write::AsyncWrite;
208
209mod read_buf;
210pub use self::read_buf::ReadBuf;
211
212// Re-export some types from `std::io` so that users don't have to deal
213// with conflicts when `use`ing `tokio::io` and `std::io`.
214#[doc(no_inline)]
215pub use std::io::{Error, ErrorKind, Result, SeekFrom};
216
217cfg_io_driver_impl! {
218 pub(crate) mod interest;
219 pub(crate) mod ready;
220
221 cfg_net! {
222 pub use interest::Interest;
223 pub use ready::Ready;
224 }
225
226 #[cfg_attr(target_os = "wasi", allow(unused_imports))]
227 mod poll_evented;
228
229 #[cfg(not(loom))]
230 #[cfg_attr(target_os = "wasi", allow(unused_imports))]
231 pub(crate) use poll_evented::PollEvented;
232}
233
234// The bsd module can't be build on Windows, so we completely ignore it, even
235// when building documentation.
236#[cfg(unix)]
237cfg_aio! {
238 /// BSD-specific I/O types.
239 pub mod bsd {
240 mod poll_aio;
241
242 pub use poll_aio::{Aio, AioEvent, AioSource};
243 }
244}
245
246cfg_net_unix! {
247 mod async_fd;
248
249 pub mod unix {
250 //! Asynchronous IO structures specific to Unix-like operating systems.
251 pub use super::async_fd::{AsyncFd, AsyncFdTryNewError, AsyncFdReadyGuard, AsyncFdReadyMutGuard, TryIoError};
252 }
253}
254
255cfg_io_std! {
256 mod stdio_common;
257
258 mod stderr;
259 pub use stderr::{stderr, Stderr};
260
261 mod stdin;
262 pub use stdin::{stdin, Stdin};
263
264 mod stdout;
265 pub use stdout::{stdout, Stdout};
266}
267
268cfg_io_util! {
269 mod split;
270 pub use split::{split, ReadHalf, WriteHalf};
271 mod join;
272 pub use join::{join, Join};
273
274 pub(crate) mod seek;
275 pub(crate) mod util;
276 pub use util::{
277 copy, copy_bidirectional, copy_bidirectional_with_sizes, copy_buf, duplex, empty, repeat, sink, simplex, AsyncBufReadExt, AsyncReadExt, AsyncSeekExt, AsyncWriteExt,
278 BufReader, BufStream, BufWriter, DuplexStream, Empty, Lines, Repeat, Sink, Split, Take, SimplexStream,
279 };
280}
281
282cfg_not_io_util! {
283 cfg_process! {
284 pub(crate) mod util;
285 }
286}
287
288cfg_io_blocking! {
289 /// Types in this module can be mocked out in tests.
290 mod sys {
291 // TODO: don't rename
292 pub(crate) use crate::blocking::spawn_blocking as run;
293 pub(crate) use crate::blocking::JoinHandle as Blocking;
294 }
295}
296