pub struct BufWriter<W: ?Sized + Write> {
buf: Vec<u8>,
panicked: bool,
inner: W,
}
Expand description
Wraps a writer and buffers its output.
It can be excessively inefficient to work directly with something that
implements Write
. For example, every call to
write
on TcpStream
results in a system call. A
BufWriter<W>
keeps an in-memory buffer of data and writes it to an underlying
writer in large, infrequent batches.
BufWriter<W>
can improve the speed of programs that make small and
repeated write calls to the same file or network socket. It does not
help when writing very large amounts at once, or writing just one or a few
times. It also provides no advantage when writing to a destination that is
in memory, like a Vec<u8>
.
It is critical to call flush
before BufWriter<W>
is dropped. Though
dropping will attempt to flush the contents of the buffer, any errors
that happen in the process of dropping will be ignored. Calling flush
ensures that the buffer is empty and thus dropping will not even attempt
file operations.
Examples
Let’s write the numbers one through ten to a TcpStream
:
use std::io::prelude::*;
use std::net::TcpStream;
let mut stream = TcpStream::connect("127.0.0.1:34254").unwrap();
for i in 0..10 {
stream.write(&[i+1]).unwrap();
}
RunBecause we’re not buffering, we write each one in turn, incurring the
overhead of a system call per byte written. We can fix this with a
BufWriter<W>
:
use std::io::prelude::*;
use std::io::BufWriter;
use std::net::TcpStream;
let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
for i in 0..10 {
stream.write(&[i+1]).unwrap();
}
stream.flush().unwrap();
RunBy wrapping the stream with a BufWriter<W>
, these ten writes are all grouped
together by the buffer and will all be written out in one system call when
the stream
is flushed.
Fields§
§buf: Vec<u8>
§panicked: bool
§inner: W
Implementations§
source§impl<W: Write> BufWriter<W>
impl<W: Write> BufWriter<W>
sourcepub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> ⓘ
pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> ⓘ
Creates a new BufWriter<W>
with at least the specified buffer capacity.
Examples
Creating a buffer with a buffer of at least a hundred bytes.
use std::io::BufWriter;
use std::net::TcpStream;
let stream = TcpStream::connect("127.0.0.1:34254").unwrap();
let mut buffer = BufWriter::with_capacity(100, stream);
Runsourcepub fn into_inner(self) -> Result<W, IntoInnerError<BufWriter<W>>>
pub fn into_inner(self) -> Result<W, IntoInnerError<BufWriter<W>>>
Unwraps this BufWriter<W>
, returning the underlying writer.
The buffer is written out before returning the writer.
Errors
An Err
will be returned if an error occurs while flushing the buffer.
Examples
use std::io::BufWriter;
use std::net::TcpStream;
let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
// unwrap the TcpStream and flush the buffer
let stream = buffer.into_inner().unwrap();
Run1.56.0 · sourcepub fn into_parts(self) -> (W, Result<Vec<u8>, WriterPanicked>)
pub fn into_parts(self) -> (W, Result<Vec<u8>, WriterPanicked>)
Disassembles this BufWriter<W>
, returning the underlying writer, and any buffered but
unwritten data.
If the underlying writer panicked, it is not known what portion of the data was written.
In this case, we return WriterPanicked
for the buffered data (from which the buffer
contents can still be recovered).
into_parts
makes no attempt to flush data and cannot fail.
Examples
use std::io::{BufWriter, Write};
let mut buffer = [0u8; 10];
let mut stream = BufWriter::new(buffer.as_mut());
write!(stream, "too much data").unwrap();
stream.flush().expect_err("it doesn't fit");
let (recovered_writer, buffered_data) = stream.into_parts();
assert_eq!(recovered_writer.len(), 0);
assert_eq!(&buffered_data.unwrap(), b"ata");
Runsource§impl<W: ?Sized + Write> BufWriter<W>
impl<W: ?Sized + Write> BufWriter<W>
sourcepub(in io) fn flush_buf(
&mut self
) -> Result<()>
pub(in io) fn flush_buf( &mut self ) -> Result<()>
Send data in our local buffer into the inner writer, looping as necessary until either it’s all been sent or an error occurs.
Because all the data in the buffer has been reported to our owner as
“successfully written” (by returning nonzero success values from
write
), any 0-length writes from inner
must be reported as i/o
errors from this method.
sourcepub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize
pub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize
Buffer some data without flushing it, regardless of the size of the data. Writes as much as possible without exceeding capacity. Returns the number of bytes written.
sourcepub fn get_mut(&mut self) -> &mut W
pub fn get_mut(&mut self) -> &mut W
Gets a mutable reference to the underlying writer.
It is inadvisable to directly write to the underlying writer.
Examples
use std::io::BufWriter;
use std::net::TcpStream;
let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
// we can use reference just like buffer
let reference = buffer.get_mut();
Runsourcepub(in io) fn buffer_mut(
&mut self
) -> &mut Vec<u8> ⓘ
pub(in io) fn buffer_mut( &mut self ) -> &mut Vec<u8> ⓘ
Returns a mutable reference to the internal buffer.
This can be used to write data directly into the buffer without triggering writers to the underlying writer.
That the buffer is a Vec
is an implementation detail.
Callers should not modify the capacity as there currently is no public API to do so
and thus any capacity changes would be unexpected by the user.
1.46.0 · sourcepub fn capacity(&self) -> usize
pub fn capacity(&self) -> usize
Returns the number of bytes the internal buffer can hold without flushing.
Examples
use std::io::BufWriter;
use std::net::TcpStream;
let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
// Check the capacity of the inner buffer
let capacity = buf_writer.capacity();
// Calculate how many bytes can be written without flushing
let without_flush = capacity - buf_writer.buffer().len();
Runfn write_cold(&mut self, buf: &[u8]) -> Result<usize>
fn write_all_cold(&mut self, buf: &[u8]) -> Result<()>
unsafe fn write_to_buffer_unchecked(&mut self, buf: &[u8])
fn spare_capacity(&self) -> usize
Trait Implementations§
source§impl<T: ?Sized + CopyWrite> CopyWrite for BufWriter<T>
impl<T: ?Sized + CopyWrite> CopyWrite for BufWriter<T>
source§fn properties(&self) -> CopyParams
fn properties(&self) -> CopyParams
source§impl<W: ?Sized + Write + Seek> Seek for BufWriter<W>
impl<W: ?Sized + Write + Seek> Seek for BufWriter<W>
source§fn seek(&mut self, pos: SeekFrom) -> Result<u64>
fn seek(&mut self, pos: SeekFrom) -> Result<u64>
Seek to the offset, in bytes, in the underlying writer.
Seeking always writes out the internal buffer before seeking.
source§impl<W: ?Sized + Write> Write for BufWriter<W>
impl<W: ?Sized + Write> Write for BufWriter<W>
source§fn write(&mut self, buf: &[u8]) -> Result<usize>
fn write(&mut self, buf: &[u8]) -> Result<usize>
source§fn write_all(&mut self, buf: &[u8]) -> Result<()>
fn write_all(&mut self, buf: &[u8]) -> Result<()>
source§fn is_write_vectored(&self) -> bool
fn is_write_vectored(&self) -> bool
can_vector
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