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use crate::fmt; use crate::io::{ self, Error, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE, }; /// 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`][`TcpStream::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`]: /// /// ```no_run /// 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(); /// } /// ``` /// /// Because 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>`: /// /// ```no_run /// 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(); /// ``` /// /// By 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. /// // HACK(#78696): can't use `crate` for associated items /// [`TcpStream::write`]: super::super::super::net::TcpStream::write /// [`TcpStream`]: crate::net::TcpStream /// [`flush`]: BufWriter::flush #[stable(feature = "rust1", since = "1.0.0")] pub struct BufWriter<W: Write> { inner: Option<W>, buf: Vec<u8>, // #30888: If the inner writer panics in a call to write, we don't want to // write the buffered data a second time in BufWriter's destructor. This // flag tells the Drop impl if it should skip the flush. panicked: bool, } impl<W: Write> BufWriter<W> { /// Creates a new `BufWriter<W>` with a default buffer capacity. The default is currently 8 KB, /// but may change in the future. /// /// # Examples /// /// ```no_run /// use std::io::BufWriter; /// use std::net::TcpStream; /// /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn new(inner: W) -> BufWriter<W> { BufWriter::with_capacity(DEFAULT_BUF_SIZE, inner) } /// Creates a new `BufWriter<W>` with the specified buffer capacity. /// /// # Examples /// /// Creating a buffer with a buffer of a hundred bytes. /// /// ```no_run /// 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); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> { BufWriter { inner: Some(inner), buf: Vec::with_capacity(capacity), panicked: false } } /// 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. pub(super) fn flush_buf(&mut self) -> io::Result<()> { /// Helper struct to ensure the buffer is updated after all the writes /// are complete. It tracks the number of written bytes and drains them /// all from the front of the buffer when dropped. struct BufGuard<'a> { buffer: &'a mut Vec<u8>, written: usize, } impl<'a> BufGuard<'a> { fn new(buffer: &'a mut Vec<u8>) -> Self { Self { buffer, written: 0 } } /// The unwritten part of the buffer fn remaining(&self) -> &[u8] { &self.buffer[self.written..] } /// Flag some bytes as removed from the front of the buffer fn consume(&mut self, amt: usize) { self.written += amt; } /// true if all of the bytes have been written fn done(&self) -> bool { self.written >= self.buffer.len() } } impl Drop for BufGuard<'_> { fn drop(&mut self) { if self.written > 0 { self.buffer.drain(..self.written); } } } let mut guard = BufGuard::new(&mut self.buf); let inner = self.inner.as_mut().unwrap(); while !guard.done() { self.panicked = true; let r = inner.write(guard.remaining()); self.panicked = false; match r { Ok(0) => { return Err(Error::new( ErrorKind::WriteZero, "failed to write the buffered data", )); } Ok(n) => guard.consume(n), Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {} Err(e) => return Err(e), } } Ok(()) } /// 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. pub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize { let available = self.buf.capacity() - self.buf.len(); let amt_to_buffer = available.min(buf.len()); self.buf.extend_from_slice(&buf[..amt_to_buffer]); amt_to_buffer } /// Gets a reference to the underlying writer. /// /// # Examples /// /// ```no_run /// 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_ref(); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn get_ref(&self) -> &W { self.inner.as_ref().unwrap() } /// Gets a mutable reference to the underlying writer. /// /// It is inadvisable to directly write to the underlying writer. /// /// # Examples /// /// ```no_run /// 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(); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn get_mut(&mut self) -> &mut W { self.inner.as_mut().unwrap() } /// Returns a reference to the internally buffered data. /// /// # Examples /// /// ```no_run /// use std::io::BufWriter; /// use std::net::TcpStream; /// /// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // See how many bytes are currently buffered /// let bytes_buffered = buf_writer.buffer().len(); /// ``` #[stable(feature = "bufreader_buffer", since = "1.37.0")] pub fn buffer(&self) -> &[u8] { &self.buf } /// Returns the number of bytes the internal buffer can hold without flushing. /// /// # Examples /// /// ```no_run /// 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(); /// ``` #[stable(feature = "buffered_io_capacity", since = "1.46.0")] pub fn capacity(&self) -> usize { self.buf.capacity() } /// 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 /// /// ```no_run /// 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(); /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn into_inner(mut self) -> Result<W, IntoInnerError<BufWriter<W>>> { match self.flush_buf() { Err(e) => Err(IntoInnerError::new(self, e)), Ok(()) => Ok(self.inner.take().unwrap()), } } } #[stable(feature = "rust1", since = "1.0.0")] impl<W: Write> Write for BufWriter<W> { fn write(&mut self, buf: &[u8]) -> io::Result<usize> { if self.buf.len() + buf.len() > self.buf.capacity() { self.flush_buf()?; } // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919 if buf.len() >= self.buf.capacity() { self.panicked = true; let r = self.get_mut().write(buf); self.panicked = false; r } else { self.buf.extend_from_slice(buf); Ok(buf.len()) } } fn write_all(&mut self, buf: &[u8]) -> io::Result<()> { // Normally, `write_all` just calls `write` in a loop. We can do better // by calling `self.get_mut().write_all()` directly, which avoids // round trips through the buffer in the event of a series of partial // writes in some circumstances. if self.buf.len() + buf.len() > self.buf.capacity() { self.flush_buf()?; } // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919 if buf.len() >= self.buf.capacity() { self.panicked = true; let r = self.get_mut().write_all(buf); self.panicked = false; r } else { self.buf.extend_from_slice(buf); Ok(()) } } fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> { let total_len = bufs.iter().map(|b| b.len()).sum::<usize>(); if self.buf.len() + total_len > self.buf.capacity() { self.flush_buf()?; } // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919 if total_len >= self.buf.capacity() { self.panicked = true; let r = self.get_mut().write_vectored(bufs); self.panicked = false; r } else { bufs.iter().for_each(|b| self.buf.extend_from_slice(b)); Ok(total_len) } } fn is_write_vectored(&self) -> bool { self.get_ref().is_write_vectored() } fn flush(&mut self) -> io::Result<()> { self.flush_buf().and_then(|()| self.get_mut().flush()) } } #[stable(feature = "rust1", since = "1.0.0")] impl<W: Write> fmt::Debug for BufWriter<W> where W: fmt::Debug, { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { fmt.debug_struct("BufWriter") .field("writer", &self.inner.as_ref().unwrap()) .field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity())) .finish() } } #[stable(feature = "rust1", since = "1.0.0")] impl<W: Write + Seek> Seek for BufWriter<W> { /// Seek to the offset, in bytes, in the underlying writer. /// /// Seeking always writes out the internal buffer before seeking. fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> { self.flush_buf()?; self.get_mut().seek(pos) } } #[stable(feature = "rust1", since = "1.0.0")] impl<W: Write> Drop for BufWriter<W> { fn drop(&mut self) { if self.inner.is_some() && !self.panicked { // dtors should not panic, so we ignore a failed flush let _r = self.flush_buf(); } } }