Trait `BufRead`

trait BufRead: Read

A BufRead is a type of Reader which has an internal buffer, allowing it to perform extra ways of reading.

For example, reading line-by-line is inefficient without using a buffer, so if you want to read by line, you'll need BufRead, which includes a read_line method as well as a lines iterator.

Examples

A locked standard input implements BufRead:

use std::io;
use std::io::prelude::*;

let stdin = io::stdin();
for line in stdin.lock().lines() {
    println!("{}", line?);
}
# std::io::Result::Ok(())

If you have something that implements Read, you can use the [BufReader type]BufReader to turn it into a BufRead.

For example, File implements Read, but not BufRead. BufReader to the rescue!

use std::io::{self, BufReader};
use std::io::prelude::*;
use std::fs::File;

fn main() -> io::Result<()> {
    let f = File::open("foo.txt")?;
    let f = BufReader::new(f);

    for line in f.lines() {
        let line = line?;
        println!("{line}");
    }

    Ok(())
}

Required Methods

fn fill_buf(self: &mut Self) -> Result<&[u8]>

Returns the contents of the internal buffer, filling it with more data, via Read methods, if empty.

This is a lower-level method and is meant to be used together with consume, which can be used to mark bytes that should not be returned by subsequent calls to read.

Returns an empty buffer when the stream has reached EOF.

Errors

This function will return an I/O error if a Read method was called, but returned an error.

Examples

A locked standard input implements BufRead:

use std::io;
use std::io::prelude::*;

let stdin = io::stdin();
let mut stdin = stdin.lock();

let buffer = stdin.fill_buf()?;

// work with buffer
println!("{buffer:?}");

// mark the bytes we worked with as read
let length = buffer.len();
stdin.consume(length);
# std::io::Result::Ok(())

fn consume(self: &mut Self, amount: usize)

Marks the given amount of additional bytes from the internal buffer as having been read. Subsequent calls to read only return bytes that have not been marked as read.

This is a lower-level method and is meant to be used together with fill_buf, which can be used to fill the internal buffer via Read methods.

It is a logic error if amount exceeds the number of unread bytes in the internal buffer, which is returned by fill_buf.

Examples

Since consume() is meant to be used with fill_buf, that method's example includes an example of consume().

Provided Methods

fn has_data_left(self: &mut Self) -> Result<bool>

Checks if there is any data left to be read.

This function may fill the buffer to check for data, so this function returns Result<bool>, not bool.

The default implementation calls fill_buf and checks that the returned slice is empty (which means that there is no data left, since EOF is reached).

Errors

This function will return an I/O error if a Read method was called, but returned an error.

Examples

#![feature(buf_read_has_data_left)]
use std::io;
use std::io::prelude::*;

let stdin = io::stdin();
let mut stdin = stdin.lock();

while stdin.has_data_left()? {
    let mut line = String::new();
    stdin.read_line(&mut line)?;
    // work with line
    println!("{line:?}");
}
# std::io::Result::Ok(())

fn read_until(self: &mut Self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>

Reads all bytes into buf until the delimiter byte or EOF is reached.

This function will read bytes from the underlying stream until the delimiter or EOF is found. Once found, all bytes up to, and including, the delimiter (if found) will be appended to buf.

If successful, this function will return the total number of bytes read.

This function is blocking and should be used carefully: it is possible for an attacker to continuously send bytes without ever sending the delimiter or EOF.

Errors

This function will ignore all instances of ErrorKind::Interrupted and will otherwise return any errors returned by fill_buf.

If an I/O error is encountered then all bytes read so far will be present in buf and its length will have been adjusted appropriately.

Examples

[std::io::Cursor]Cursor is a type that implements BufRead. In this example, we use Cursor to read all the bytes in a byte slice in hyphen delimited segments:

use std::io::{self, BufRead};

let mut cursor = io::Cursor::new(b"lorem-ipsum");
let mut buf = vec![];

// cursor is at 'l'
let num_bytes = cursor.read_until(b'-', &mut buf)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 6);
assert_eq!(buf, b"lorem-");
buf.clear();

// cursor is at 'i'
let num_bytes = cursor.read_until(b'-', &mut buf)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 5);
assert_eq!(buf, b"ipsum");
buf.clear();

// cursor is at EOF
let num_bytes = cursor.read_until(b'-', &mut buf)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 0);
assert_eq!(buf, b"");

fn skip_until(self: &mut Self, byte: u8) -> Result<usize>

Skips all bytes until the delimiter byte or EOF is reached.

This function will read (and discard) bytes from the underlying stream until the delimiter or EOF is found.

If successful, this function will return the total number of bytes read, including the delimiter byte if found.

This is useful for efficiently skipping data such as NUL-terminated strings in binary file formats without buffering.

This function is blocking and should be used carefully: it is possible for an attacker to continuously send bytes without ever sending the delimiter or EOF.

Errors

This function will ignore all instances of ErrorKind::Interrupted and will otherwise return any errors returned by fill_buf.

If an I/O error is encountered then all bytes read so far will be present in buf and its length will have been adjusted appropriately.

Examples

[std::io::Cursor]Cursor is a type that implements BufRead. In this example, we use Cursor to read some NUL-terminated information about Ferris from a binary string, skipping the fun fact:

use std::io::{self, BufRead};

let mut cursor = io::Cursor::new(b"Ferris\0Likes long walks on the beach\0Crustacean\0!");

// read name
let mut name = Vec::new();
let num_bytes = cursor.read_until(b'\0', &mut name)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 7);
assert_eq!(name, b"Ferris\0");

// skip fun fact
let num_bytes = cursor.skip_until(b'\0')
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 30);

// read animal type
let mut animal = Vec::new();
let num_bytes = cursor.read_until(b'\0', &mut animal)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 11);
assert_eq!(animal, b"Crustacean\0");

// reach EOF
let num_bytes = cursor.skip_until(b'\0')
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 1);

fn read_line(self: &mut Self, buf: &mut String) -> Result<usize>

Reads all bytes until a newline (the 0xA byte) is reached, and append them to the provided String buffer.

Previous content of the buffer will be preserved. To avoid appending to the buffer, you need to clear it first.

This function will read bytes from the underlying stream until the newline delimiter (the 0xA byte) or EOF is found. Once found, all bytes up to, and including, the delimiter (if found) will be appended to buf.

If successful, this function will return the total number of bytes read.

If this function returns Ok(0), the stream has reached EOF.

This function is blocking and should be used carefully: it is possible for an attacker to continuously send bytes without ever sending a newline or EOF. You can use take to limit the maximum number of bytes read.

Errors

This function has the same error semantics as read_until and will also return an error if the read bytes are not valid UTF-8. If an I/O error is encountered then buf may contain some bytes already read in the event that all data read so far was valid UTF-8.

Examples

[std::io::Cursor]Cursor is a type that implements BufRead. In this example, we use Cursor to read all the lines in a byte slice:

use std::io::{self, BufRead};

let mut cursor = io::Cursor::new(b"foo\nbar");
let mut buf = String::new();

// cursor is at 'f'
let num_bytes = cursor.read_line(&mut buf)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 4);
assert_eq!(buf, "foo\n");
buf.clear();

// cursor is at 'b'
let num_bytes = cursor.read_line(&mut buf)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 3);
assert_eq!(buf, "bar");
buf.clear();

// cursor is at EOF
let num_bytes = cursor.read_line(&mut buf)
    .expect("reading from cursor won't fail");
assert_eq!(num_bytes, 0);
assert_eq!(buf, "");

fn split(self: Self, byte: u8) -> Split<Self> where Self: Sized

Returns an iterator over the contents of this reader split on the byte byte.

The iterator returned from this function will return instances of io::Result<[Vec]<u8>>. Each vector returned will not have the delimiter byte at the end.

This function will yield errors whenever read_until would have also yielded an error.

Examples

[std::io::Cursor]Cursor is a type that implements BufRead. In this example, we use Cursor to iterate over all hyphen delimited segments in a byte slice

use std::io::{self, BufRead};

let cursor = io::Cursor::new(b"lorem-ipsum-dolor");

let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
assert_eq!(split_iter.next(), None);

fn lines(self: Self) -> Lines<Self> where Self: Sized

Returns an iterator over the lines of this reader.

The iterator returned from this function will yield instances of io::Result<[String]>. Each string returned will not have a newline byte (the 0xA byte) or CRLF (0xD, 0xA bytes) at the end.

Examples

[std::io::Cursor]Cursor is a type that implements BufRead. In this example, we use Cursor to iterate over all the lines in a byte slice.

use std::io::{self, BufRead};

let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");

let mut lines_iter = cursor.lines().map(|l| l.unwrap());
assert_eq!(lines_iter.next(), Some(String::from("lorem")));
assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
assert_eq!(lines_iter.next(), Some(String::from("dolor")));
assert_eq!(lines_iter.next(), None);

Errors

Each line of the iterator has the same error semantics as BufRead::read_line.

Implementors

impl BufRead for StdinLock<'_>
impl<T> BufRead for Cursor<T>
impl<A: Allocator> BufRead for crate::collections::VecDeque<u8, A>
impl BufRead for Empty
impl<B: BufRead + ?Sized> BufRead for &mut B
impl BufRead for &[u8]
impl<T: BufRead> BufRead for Take<T>
impl<R: ?Sized + Read> BufRead for BufReader<R>
impl<B: BufRead + ?Sized> BufRead for Box<B>
impl<T: BufRead, U: BufRead> BufRead for Chain<T, U>