Struct `BufReader`

struct BufReader<R: ?Sized> { ... }

The BufReader<R> struct adds buffering to any reader.

It can be excessively inefficient to work directly with a Read instance. For example, every call to read on TcpStream results in a system call. A BufReader<R> performs large, infrequent reads on the underlying Read and maintains an in-memory buffer of the results.

BufReader<R> can improve the speed of programs that make small and repeated read calls to the same file or network socket. It does not help when reading very large amounts at once, or reading just one or a few times. It also provides no advantage when reading from a source that is already in memory, like a [Vec]<u8>.

When the BufReader<R> is dropped, the contents of its buffer will be discarded. Creating multiple instances of a BufReader<R> on the same stream can cause data loss. Reading from the underlying reader after unwrapping the BufReader<R> with BufReader::into_inner can also cause data loss.

Examples

use std::io::prelude::*;
use std::io::BufReader;
use std::fs::File;

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

    let mut line = String::new();
    let len = reader.read_line(&mut line)?;
    println!("First line is {len} bytes long");
    Ok(())
}

Implementations

`impl<R: ?Sized + Seek> BufReader<R>`

fn seek_relative(self: &mut Self, offset: i64) -> Result<()>: Seeks relative to the current position. If the new position lies within the buffer, the buffer will not be flushed, allowing for more efficient seeks. This method does not return the location of the underlying reader, so the caller must track this information themselves if it is required.

`impl<R: ?Sized> BufReader<R>`

fn get_ref(self: &Self) -> &R

Gets a reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

Examples

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f1 = File::open("log.txt")?;
    let reader = BufReader::new(f1);

    let f2 = reader.get_ref();
    Ok(())
}

fn get_mut(self: &mut Self) -> &mut R

Gets a mutable reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

Examples

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f1 = File::open("log.txt")?;
    let mut reader = BufReader::new(f1);

    let f2 = reader.get_mut();
    Ok(())
}

fn buffer(self: &Self) -> &[u8]

Returns a reference to the internally buffered data.

Unlike fill_buf, this will not attempt to fill the buffer if it is empty.

Examples

use std::io::{BufReader, BufRead};
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f = File::open("log.txt")?;
    let mut reader = BufReader::new(f);
    assert!(reader.buffer().is_empty());

    if reader.fill_buf()?.len() > 0 {
        assert!(!reader.buffer().is_empty());
    }
    Ok(())
}

fn capacity(self: &Self) -> usize

Returns the number of bytes the internal buffer can hold at once.

Examples

use std::io::{BufReader, BufRead};
use std::fs::File;

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

    let capacity = reader.capacity();
    let buffer = reader.fill_buf()?;
    assert!(buffer.len() <= capacity);
    Ok(())
}

fn into_inner(self: Self) -> R where R: Sized

Unwraps this BufReader<R>, returning the underlying reader.

Note that any leftover data in the internal buffer is lost. Therefore, a following read from the underlying reader may lead to data loss.

Examples

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f1 = File::open("log.txt")?;
    let reader = BufReader::new(f1);

    let f2 = reader.into_inner();
    Ok(())
}

`impl<R: Read + ?Sized> BufReader<R>`

fn peek(self: &mut Self, n: usize) -> Result<&[u8]>

Attempt to look ahead n bytes.

n must be less than or equal to capacity.

The returned slice may be less than n bytes long if end of file is reached.

After calling this method, you may call consume with a value less than or equal to n to advance over some or all of the returned bytes.

Examples

#![feature(bufreader_peek)]
use std::io::{Read, BufReader};

let mut bytes = &b"oh, hello there"[..];
let mut rdr = BufReader::with_capacity(6, &mut bytes);
assert_eq!(rdr.peek(2).unwrap(), b"oh");
let mut buf = [0; 4];
rdr.read(&mut buf[..]).unwrap();
assert_eq!(&buf, b"oh, ");
assert_eq!(rdr.peek(5).unwrap(), b"hello");
let mut s = String::new();
rdr.read_to_string(&mut s).unwrap();
assert_eq!(&s, "hello there");
assert_eq!(rdr.peek(1).unwrap().len(), 0);

`impl<R: Read> BufReader<R>`

fn new(inner: R) -> BufReader<R>

Creates a new BufReader<R> with a default buffer capacity. The default is currently 8 KiB, but may change in the future.

Examples

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f = File::open("log.txt")?;
    let reader = BufReader::new(f);
    Ok(())
}

fn with_capacity(capacity: usize, inner: R) -> BufReader<R>

Creates a new BufReader<R> with the specified buffer capacity.

Examples

Creating a buffer with ten bytes of capacity:

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f = File::open("log.txt")?;
    let reader = BufReader::with_capacity(10, f);
    Ok(())
}

`impl<R> Debug for BufReader<R>`

fn fmt(self: &Self, fmt: &mut Formatter<'_>) -> Result

`impl<R> Freeze for BufReader<R>`

`impl<R> RefUnwindSafe for BufReader<R>`

`impl<R> Send for BufReader<R>`

`impl<R> Sync for BufReader<R>`

`impl<R> Unpin for BufReader<R>`

`impl<R> UnsafeUnpin for BufReader<R>`

`impl<R> UnwindSafe for BufReader<R>`

`impl<R: ?Sized + Read> BufRead for BufReader<R>`

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

`impl<R: ?Sized + Read> Read for BufReader<R>`

fn read(self: &mut Self, buf: &mut [u8]) -> Result<usize>
fn read_buf(self: &mut Self, cursor: BorrowedCursor<'_>) -> Result<()>
fn read_exact(self: &mut Self, buf: &mut [u8]) -> Result<()>
fn read_buf_exact(self: &mut Self, cursor: BorrowedCursor<'_>) -> Result<()>
fn read_vectored(self: &mut Self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
fn is_read_vectored(self: &Self) -> bool
fn read_to_end(self: &mut Self, buf: &mut Vec<u8>) -> Result<usize>
fn read_to_string(self: &mut Self, buf: &mut String) -> Result<usize>

`impl<R: ?Sized + Seek> Seek for BufReader<R>`

fn seek(self: &mut Self, pos: SeekFrom) -> Result<u64>

Seek to an offset, in bytes, in the underlying reader.

The position used for seeking with [SeekFrom::Current](_) is the position the underlying reader would be at if the BufReader<R> had no internal buffer.

Seeking always discards the internal buffer, even if the seek position would otherwise fall within it. This guarantees that calling [BufReader::into_inner()] immediately after a seek yields the underlying reader at the same position.

To seek without discarding the internal buffer, use BufReader::seek_relative.

See std::io::Seek for more details.

Note: In the edge case where you're seeking with [SeekFrom::Current](n) where n minus the internal buffer length overflows an i64, two seeks will be performed instead of one. If the second seek returns Err, the underlying reader will be left at the same position it would have if you called seek with [SeekFrom::Current](0).

fn stream_position(self: &mut Self) -> Result<u64>

Returns the current seek position from the start of the stream.

The value returned is equivalent to self.seek(SeekFrom::Current(0)) but does not flush the internal buffer. Due to this optimization the function does not guarantee that calling .into_inner() immediately afterwards will yield the underlying reader at the same position. Use BufReader::seek instead if you require that guarantee.

Panics

This function will panic if the position of the inner reader is smaller than the amount of buffered data. That can happen if the inner reader has an incorrect implementation of Seek::stream_position, or if the position has gone out of sync due to calling Seek::seek directly on the underlying reader.

Example

use std::{
    io::{self, BufRead, BufReader, Seek},
    fs::File,
};

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

    let before = f.stream_position()?;
    f.read_line(&mut String::new())?;
    let after = f.stream_position()?;

    println!("The first line was {} bytes long", after - before);
    Ok(())
}

fn seek_relative(self: &mut Self, offset: i64) -> Result<()>

Seeks relative to the current position.

If the new position lies within the buffer, the buffer will not be flushed, allowing for more efficient seeks. This method does not return the location of the underlying reader, so the caller must track this information themselves if it is required.

`impl<T> Any for BufReader<R>`

fn type_id(self: &Self) -> TypeId

`impl<T> Borrow for BufReader<R>`

fn borrow(self: &Self) -> &T

`impl<T> BorrowMut for BufReader<R>`

fn borrow_mut(self: &mut Self) -> &mut T

`impl<T> From for BufReader<R>`

fn from(t: T) -> T: Returns the argument unchanged.

`impl<T, U> Into for BufReader<R>`

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

`impl<T, U> TryFrom for BufReader<R>`

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

`impl<T, U> TryInto for BufReader<R>`

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>