pub struct BufReader<R: ?Sized> { /* 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 {len} bytes long");
Ok(())
}
RunImplementations§
source§impl<R: Read> BufReader<R>
impl<R: Read> BufReader<R>
source§impl<R: ?Sized> BufReader<R>
impl<R: ?Sized> BufReader<R>
1.0.0 · sourcepub fn get_mut(&mut self) -> &mut R
pub fn get_mut(&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(())
}
Run1.37.0 · sourcepub fn buffer(&self) -> &[u8] ⓘ
pub fn buffer(&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(())
}
Run1.46.0 · sourcepub fn capacity(&self) -> usize
pub fn capacity(&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(())
}
Run1.0.0 · sourcepub fn into_inner(self) -> Rwhere
R: Sized,
pub fn into_inner(self) -> Rwhere
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(())
}
Runsource§impl<R: ?Sized + Seek> BufReader<R>
impl<R: ?Sized + Seek> BufReader<R>
1.53.0 · sourcepub fn seek_relative(&mut self, offset: i64) -> Result<()>
pub fn seek_relative(&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.
Trait Implementations§
1.0.0 · source§impl<R: ?Sized + Read> BufRead for BufReader<R>
impl<R: ?Sized + Read> BufRead for BufReader<R>
source§fn fill_buf(&mut self) -> Result<&[u8]>
fn fill_buf(&mut self) -> Result<&[u8]>
source§fn consume(&mut self, amt: usize)
fn consume(&mut self, amt: usize)
amt
bytes have been consumed from the buffer,
so they should no longer be returned in calls to read
. Read moresource§fn has_data_left(&mut self) -> Result<bool>
fn has_data_left(&mut self) -> Result<bool>
buf_read_has_data_left
#86423)Read
has any data left to be read. Read more1.0.0 · source§impl<R: ?Sized + Read> Read for BufReader<R>
impl<R: ?Sized + Read> Read for BufReader<R>
source§fn read(&mut self, buf: &mut [u8]) -> Result<usize>
fn read(&mut self, buf: &mut [u8]) -> Result<usize>
source§fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>
fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>
read_buf
#78485)source§fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>
fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>
buf
. Read moresource§fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>
fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>
read_buf
#78485)cursor
. Read moresource§fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
read
, except that it reads into a slice of buffers. Read moresource§fn is_read_vectored(&self) -> bool
fn is_read_vectored(&self) -> bool
can_vector
#69941)1.0.0 · source§impl<R: ?Sized + Seek> Seek for BufReader<R>
impl<R: ?Sized + Seek> Seek for BufReader<R>
source§fn seek(&mut self, pos: SeekFrom) -> Result<u64>
fn seek(&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)
.
source§fn stream_position(&mut self) -> Result<u64>
fn stream_position(&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(())
}
Runsource§fn seek_relative(&mut self, offset: i64) -> Result<()>
fn seek_relative(&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.