bufwriter.rs - source
Description: Source of the Rust file `library/std/src/io/buffered/bufwriter.rs`.
Keywords: rust, rustlang, rust-lang

      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388

    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();
            }
        }
    }