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Showing content from https://doc.rust-lang.org/nightly/std/io/struct.BufReader.html below:

BufReader in std::io - Rust

pub 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.

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(())
}

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

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(())
}

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

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(())
}

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.

#![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);

Gets a reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

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(())
}

Gets a mutable reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

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(())
}

Returns a reference to the internally buffered data.

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

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(())
}

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

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(())
}

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.

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(())
}

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.

Returns the contents of the internal buffer, filling it with more data, via

methods, if empty.

Marks the given

of additional bytes from the internal buffer as having been read. Subsequent calls to

only return bytes that have not been marked as read.

Checks if there is any data left to be

.

Reads all bytes into

until the delimiter

or EOF is reached.

Skips all bytes until the delimiter

or EOF is reached.

Reads all bytes until a newline (the

byte) is reached, and append them to the provided

buffer.

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

.

Returns an iterator over the lines of this reader.

Pull some bytes from this source into the specified buffer, returning how many bytes were read.

Pull some bytes from this source into the specified buffer.

Reads the exact number of bytes required to fill

.

Reads the exact number of bytes required to fill

.

Like

, except that it reads into a slice of buffers.

Determines if this

er has an efficient

implementation.

Reads all bytes until EOF in this source, placing them into

.

Reads all bytes until EOF in this source, appending them to

.

Creates a “by reference” adaptor for this instance of

.

Creates an adapter which will chain this stream with another.

Creates an adapter which will read at most

bytes from it.

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).

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.

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.

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(())
}

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.

Rewind to the beginning of a stream.

Returns the length of this stream (in bytes).

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