pub struct BufReader<R> { /* private fields */ }Expand description
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 {} bytes long", len);
Ok(())
}Implementations
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(())
}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(())
}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(())
}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(())
}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.
Trait Implementations
Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty. Read more
Tells this buffer that amt bytes have been consumed from the buffer,
so they should no longer be returned in calls to read. Read more
Read all bytes into buf until the delimiter byte or EOF is reached. Read more
Read all bytes until a newline (the 0xA byte) is reached, and append
them to the provided buffer. Read more
Returns an iterator over the contents of this reader split on the byte
byte. Read more
Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more
Read the exact number of bytes required to fill buf. Read more
Like read, except that it reads into a slice of buffers. Read more
Read all bytes until EOF in this source, placing them into buf. Read more
Read all bytes until EOF in this source, appending them to buf. Read more
Creates a “by reference” adaptor for this instance of Read. Read more
Creates an adapter which will chain this stream with another. Read more
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.
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(())
}