bufreader.rs - source
Description: Source of the Rust file `library/std/src/io/buffered/bufreader.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
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410
    411
    412
    413
    414
    415
    416
    417
    418
    419
    420
    421
    422
    423
    424

    use crate::cmp;
    use crate::fmt;
    use crate::io::{self, BufRead, Initializer, IoSliceMut, Read, Seek, SeekFrom, DEFAULT_BUF_SIZE};

    /// 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`][`TcpStream::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.
    ///
    // HACK(#78696): can't use `crate` for associated items
    /// [`TcpStream::read`]: super::super::super::net::TcpStream::read
    /// [`TcpStream`]: crate::net::TcpStream
    ///
    /// # Examples
    ///
    /// ```no_run
    /// 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(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BufReader<R> {
        inner: R,
        buf: Box<[u8]>,
        pos: usize,
        cap: usize,
    }

    impl<R: Read> BufReader<R> {
        /// Creates a new `BufReader<R>` with a default buffer capacity. The default is currently 8 KB,
        /// but may change in the future.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f = File::open("log.txt")?;
        ///     let reader = BufReader::new(f);
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(inner: R) -> BufReader<R> {
            BufReader::with_capacity(DEFAULT_BUF_SIZE, inner)
        }

        /// Creates a new `BufReader<R>` with the specified buffer capacity.
        ///
        /// # Examples
        ///
        /// Creating a buffer with ten bytes of capacity:
        ///
        /// ```no_run
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f = File::open("log.txt")?;
        ///     let reader = BufReader::with_capacity(10, f);
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
            unsafe {
                let mut buffer = Vec::with_capacity(capacity);
                buffer.set_len(capacity);
                inner.initializer().initialize(&mut buffer);
                BufReader { inner, buf: buffer.into_boxed_slice(), pos: 0, cap: 0 }
            }
        }
    }

    impl<R> BufReader<R> {
        /// Gets a reference to the underlying reader.
        ///
        /// It is inadvisable to directly read from the underlying reader.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// 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.get_ref();
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_ref(&self) -> &R {
            &self.inner
        }

        /// Gets a mutable reference to the underlying reader.
        ///
        /// It is inadvisable to directly read from the underlying reader.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut(&mut self) -> &mut R {
            &mut self.inner
        }

        /// Returns a reference to the internally buffered data.
        ///
        /// Unlike [`fill_buf`], this will not attempt to fill the buffer if it is empty.
        ///
        /// [`fill_buf`]: BufRead::fill_buf
        ///
        /// # Examples
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "bufreader_buffer", since = "1.37.0")]
        pub fn buffer(&self) -> &[u8] {
            &self.buf[self.pos..self.cap]
        }

        /// Returns the number of bytes the internal buffer can hold at once.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "buffered_io_capacity", since = "1.46.0")]
        pub fn capacity(&self) -> usize {
            self.buf.len()
        }

        /// 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
        ///
        /// ```no_run
        /// 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(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(self) -> R {
            self.inner
        }

        /// Invalidates all data in the internal buffer.
        #[inline]
        fn discard_buffer(&mut self) {
            self.pos = 0;
            self.cap = 0;
        }
    }

    impl<R: Seek> BufReader<R> {
        /// 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.
        #[unstable(feature = "bufreader_seek_relative", issue = "31100")]
        pub fn seek_relative(&mut self, offset: i64) -> io::Result<()> {
            let pos = self.pos as u64;
            if offset < 0 {
                if let Some(new_pos) = pos.checked_sub((-offset) as u64) {
                    self.pos = new_pos as usize;
                    return Ok(());
                }
            } else {
                if let Some(new_pos) = pos.checked_add(offset as u64) {
                    if new_pos <= self.cap as u64 {
                        self.pos = new_pos as usize;
                        return Ok(());
                    }
                }
            }
            self.seek(SeekFrom::Current(offset)).map(drop)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Read> Read for BufReader<R> {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            // If we don't have any buffered data and we're doing a massive read
            // (larger than our internal buffer), bypass our internal buffer
            // entirely.
            if self.pos == self.cap && buf.len() >= self.buf.len() {
                self.discard_buffer();
                return self.inner.read(buf);
            }
            let nread = {
                let mut rem = self.fill_buf()?;
                rem.read(buf)?
            };
            self.consume(nread);
            Ok(nread)
        }

        fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
            let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
            if self.pos == self.cap && total_len >= self.buf.len() {
                self.discard_buffer();
                return self.inner.read_vectored(bufs);
            }
            let nread = {
                let mut rem = self.fill_buf()?;
                rem.read_vectored(bufs)?
            };
            self.consume(nread);
            Ok(nread)
        }

        fn is_read_vectored(&self) -> bool {
            self.inner.is_read_vectored()
        }

        // we can't skip unconditionally because of the large buffer case in read.
        unsafe fn initializer(&self) -> Initializer {
            self.inner.initializer()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Read> BufRead for BufReader<R> {
        fn fill_buf(&mut self) -> io::Result<&[u8]> {
            // If we've reached the end of our internal buffer then we need to fetch
            // some more data from the underlying reader.
            // Branch using `>=` instead of the more correct `==`
            // to tell the compiler that the pos..cap slice is always valid.
            if self.pos >= self.cap {
                debug_assert!(self.pos == self.cap);
                self.cap = self.inner.read(&mut self.buf)?;
                self.pos = 0;
            }
            Ok(&self.buf[self.pos..self.cap])
        }

        fn consume(&mut self, amt: usize) {
            self.pos = cmp::min(self.pos + amt, self.cap);
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R> fmt::Debug for BufReader<R>
    where
        R: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("BufReader")
                .field("reader", &self.inner)
                .field("buffer", &format_args!("{}/{}", self.cap - self.pos, self.buf.len()))
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Seek> Seek for BufReader<R> {
        /// 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)`.
        ///
        /// [`std::io::Seek`]: Seek
        fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
            let result: u64;
            if let SeekFrom::Current(n) = pos {
                let remainder = (self.cap - self.pos) as i64;
                // it should be safe to assume that remainder fits within an i64 as the alternative
                // means we managed to allocate 8 exbibytes and that's absurd.
                // But it's not out of the realm of possibility for some weird underlying reader to
                // support seeking by i64::MIN so we need to handle underflow when subtracting
                // remainder.
                if let Some(offset) = n.checked_sub(remainder) {
                    result = self.inner.seek(SeekFrom::Current(offset))?;
                } else {
                    // seek backwards by our remainder, and then by the offset
                    self.inner.seek(SeekFrom::Current(-remainder))?;
                    self.discard_buffer();
                    result = self.inner.seek(SeekFrom::Current(n))?;
                }
            } else {
                // Seeking with Start/End doesn't care about our buffer length.
                result = self.inner.seek(pos)?;
            }
            self.discard_buffer();
            Ok(result)
        }

        /// 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
        ///
        /// ```no_run
        /// #![feature(seek_convenience)]
        /// 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(())
        /// }
        /// ```
        fn stream_position(&mut self) -> io::Result<u64> {
            let remainder = (self.cap - self.pos) as u64;
            self.inner.stream_position().map(|pos| {
                pos.checked_sub(remainder).expect(
                    "overflow when subtracting remaining buffer size from inner stream position",
                )
            })
        }
    }