Module tokio::io

Traits, helpers, and type definitions for asynchronous I/O functionality.

This module is the asynchronous version of std::io. Primarily, it defines two traits, AsyncRead and AsyncWrite, which are asynchronous versions of the Read and Write traits in the standard library.

AsyncRead and AsyncWrite

Like the standard library’s Read and Write traits, AsyncRead and AsyncWrite provide the most general interface for reading and writing input and output. Unlike the standard library’s traits, however, they are asynchronous — meaning that reading from or writing to a tokio::io type will yield to the Tokio scheduler when IO is not ready, rather than blocking. This allows other tasks to run while waiting on IO.

Another difference is that AsyncRead and AsyncWrite only contain core methods needed to provide asynchronous reading and writing functionality. Instead, utility methods are defined in the AsyncReadExt and AsyncWriteExt extension traits. These traits are automatically implemented for all values that implement AsyncRead and AsyncWrite respectively.

End users will rarely interact directly with AsyncRead and AsyncWrite. Instead, they will use the async functions defined in the extension traits. Library authors are expected to implement AsyncRead and AsyncWrite in order to provide types that behave like byte streams.

Even with these differences, Tokio’s AsyncRead and AsyncWrite traits can be used in almost exactly the same manner as the standard library’s Read and Write. Most types in the standard library that implement Read and Write have asynchronous equivalents in tokio that implement AsyncRead and AsyncWrite, such as File and TcpStream.

For example, the standard library documentation introduces Read by demonstrating reading some bytes from a std::fs::File. We can do the same with tokio::fs::File:

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 = [0; 10];

    // read up to 10 bytes
    let n = f.read(&mut buffer).await?;

    println!("The bytes: {:?}", &buffer[..n]);
    Ok(())
}

Buffered Readers and Writers

Byte-based interfaces are unwieldy and can be inefficient, as we’d need to be making near-constant calls to the operating system. To help with this, std::io comes with support for buffered readers and writers, and therefore, tokio::io does as well.

Tokio provides an async version of the std::io::BufRead trait, AsyncBufRead; and async BufReader and BufWriter structs, which wrap readers and writers. These wrappers use a buffer, reducing the number of calls and providing nicer methods for accessing exactly what you want.

For example, BufReader works with the AsyncBufRead trait to add extra methods to any async reader:

use tokio::io::{self, BufReader, AsyncBufReadExt};
use tokio::fs::File;

#[tokio::main]
async fn main() -> io::Result<()> {
    let f = File::open("foo.txt").await?;
    let mut reader = BufReader::new(f);
    let mut buffer = String::new();

    // read a line into buffer
    reader.read_line(&mut buffer).await?;

    println!("{}", buffer);
    Ok(())
}

BufWriter doesn’t add any new ways of writing; it just buffers every call to write. However, you must flush BufWriter to ensure that any buffered data is written.

use tokio::io::{self, BufWriter, AsyncWriteExt};
use tokio::fs::File;

#[tokio::main]
async fn main() -> io::Result<()> {
    let f = File::create("foo.txt").await?;
    {
        let mut writer = BufWriter::new(f);

        // Write a byte to the buffer.
        writer.write(&[42u8]).await?;

        // Flush the buffer before it goes out of scope.
        writer.flush().await?;

    } // Unless flushed or shut down, the contents of the buffer is discarded on drop.

    Ok(())
}

Implementing AsyncRead and AsyncWrite

Because they are traits, we can implement AsyncRead and AsyncWrite for our own types, as well. Note that these traits must only be implemented for non-blocking I/O types that integrate with the futures type system. In other words, these types must never block the thread, and instead the current task is notified when the I/O resource is ready.

Conversion to and from Stream/Sink

It is often convenient to encapsulate the reading and writing of bytes in a Stream or Sink of data.

Tokio provides simple wrappers for converting AsyncRead to Stream and vice-versa in the tokio-util crate, see ReaderStream and StreamReader.

There are also utility traits that abstract the asynchronous buffering necessary to write your own adaptors for encoding and decoding bytes to/from your structured data, allowing to transform something that implements AsyncRead/AsyncWrite into a Stream/Sink, see Decoder and Encoder in the tokio-util::codec module.

Standard input and output

Tokio provides asynchronous APIs to standard input, output, and error. These APIs are very similar to the ones provided by std, but they also implement AsyncRead and AsyncWrite.

Note that the standard input / output APIs must be used from the context of the Tokio runtime, as they require Tokio-specific features to function. Calling these functions outside of a Tokio runtime will panic.

std re-exports

Additionally, Error, ErrorKind, Result, and SeekFrom are re-exported from std::io for ease of use.

Re-exports

• pub use std::io::Error;
• pub use std::io::ErrorKind;
• pub use std::io::Result;
• pub use std::io::SeekFrom;

Structs

• BufReader
  The BufReader struct adds buffering to any reader.
• BufStream
  Wraps a type that is AsyncWrite and AsyncRead, and buffers its input and output.
• BufWriter
  Wraps a writer and buffers its output.
• DuplexStream
  A bidirectional pipe to read and write bytes in memory.
• Empty
  Empty ignores any data written via AsyncWrite, and will always be empty (returning zero bytes) when read via AsyncRead.
• Join
  Joins two values implementing AsyncRead and AsyncWrite into a single handle.
• Lines
  Reads lines from an AsyncBufRead.
• ReadBuf
  A wrapper around a byte buffer that is incrementally filled and initialized.
• ReadHalf
  The readable half of a value returned from split.
• Repeat
  An async reader which yields one byte over and over and over and over and over and…
• SimplexStream
  A unidirectional pipe to read and write bytes in memory.
• Sink
  An async writer which will move data into the void.
• Split
  Splitter for the split method.
• Take
  Stream for the take method.
• WriteHalf
  The writable half of a value returned from split.

Traits

• AsyncBufRead
  Reads bytes asynchronously.
• AsyncBufReadExt
  An extension trait which adds utility methods to AsyncBufRead types.
• AsyncRead
  Reads bytes from a source.
• AsyncReadExt
  Reads bytes from a source.
• AsyncSeek
  Seek bytes asynchronously.
• AsyncSeekExt
  An extension trait that adds utility methods to AsyncSeek types.
• AsyncWrite
  Writes bytes asynchronously.
• AsyncWriteExt
  Writes bytes to a sink.

Functions

• copy
  Asynchronously copies the entire contents of a reader into a writer.
• copy_bidirectional
  Copies data in both directions between a and b.
• copy_bidirectional_with_sizes
  Copies data in both directions between a and b using buffers of the specified size.
• copy_buf
  Asynchronously copies the entire contents of a reader into a writer.
• duplex
  Create a new pair of DuplexStreams that act like a pair of connected sockets.
• empty
  Creates a value that is always at EOF for reads, and ignores all data written.
• join
  Join two values implementing AsyncRead and AsyncWrite into a single handle.
• repeat
  Creates an instance of an async reader that infinitely repeats one byte.
• simplex
  Creates unidirectional buffer that acts like in memory pipe.
• sink
  Creates an instance of an async writer which will successfully consume all data.
• split
  Splits a single value implementing AsyncRead + AsyncWrite into separate AsyncRead and AsyncWrite handles.