std/io/buffered/bufreader.rs
1mod buffer;
2
3use buffer::Buffer;
4
5use crate::fmt;
6use crate::io::{
7 self, BorrowedCursor, BufRead, DEFAULT_BUF_SIZE, IoSliceMut, Read, Seek, SeekFrom, SizeHint,
8 SpecReadByte, uninlined_slow_read_byte,
9};
10
11/// The `BufReader<R>` struct adds buffering to any reader.
12///
13/// It can be excessively inefficient to work directly with a [`Read`] instance.
14/// For example, every call to [`read`][`TcpStream::read`] on [`TcpStream`]
15/// results in a system call. A `BufReader<R>` performs large, infrequent reads on
16/// the underlying [`Read`] and maintains an in-memory buffer of the results.
17///
18/// `BufReader<R>` can improve the speed of programs that make *small* and
19/// *repeated* read calls to the same file or network socket. It does not
20/// help when reading very large amounts at once, or reading just one or a few
21/// times. It also provides no advantage when reading from a source that is
22/// already in memory, like a <code>[Vec]\<u8></code>.
23///
24/// When the `BufReader<R>` is dropped, the contents of its buffer will be
25/// discarded. Creating multiple instances of a `BufReader<R>` on the same
26/// stream can cause data loss. Reading from the underlying reader after
27/// unwrapping the `BufReader<R>` with [`BufReader::into_inner`] can also cause
28/// data loss.
29///
30/// [`TcpStream::read`]: crate::net::TcpStream::read
31/// [`TcpStream`]: crate::net::TcpStream
32///
33/// # Examples
34///
35/// ```no_run
36/// use std::io::prelude::*;
37/// use std::io::BufReader;
38/// use std::fs::File;
39///
40/// fn main() -> std::io::Result<()> {
41/// let f = File::open("log.txt")?;
42/// let mut reader = BufReader::new(f);
43///
44/// let mut line = String::new();
45/// let len = reader.read_line(&mut line)?;
46/// println!("First line is {len} bytes long");
47/// Ok(())
48/// }
49/// ```
50#[stable(feature = "rust1", since = "1.0.0")]
51pub struct BufReader<R: ?Sized> {
52 buf: Buffer,
53 inner: R,
54}
55
56impl<R: Read> BufReader<R> {
57 /// Creates a new `BufReader<R>` with a default buffer capacity. The default is currently 8 KiB,
58 /// but may change in the future.
59 ///
60 /// # Examples
61 ///
62 /// ```no_run
63 /// use std::io::BufReader;
64 /// use std::fs::File;
65 ///
66 /// fn main() -> std::io::Result<()> {
67 /// let f = File::open("log.txt")?;
68 /// let reader = BufReader::new(f);
69 /// Ok(())
70 /// }
71 /// ```
72 #[stable(feature = "rust1", since = "1.0.0")]
73 pub fn new(inner: R) -> BufReader<R> {
74 BufReader::with_capacity(DEFAULT_BUF_SIZE, inner)
75 }
76
77 pub(crate) fn try_new_buffer() -> io::Result<Buffer> {
78 Buffer::try_with_capacity(DEFAULT_BUF_SIZE)
79 }
80
81 pub(crate) fn with_buffer(inner: R, buf: Buffer) -> Self {
82 Self { inner, buf }
83 }
84
85 /// Creates a new `BufReader<R>` with the specified buffer capacity.
86 ///
87 /// # Examples
88 ///
89 /// Creating a buffer with ten bytes of capacity:
90 ///
91 /// ```no_run
92 /// use std::io::BufReader;
93 /// use std::fs::File;
94 ///
95 /// fn main() -> std::io::Result<()> {
96 /// let f = File::open("log.txt")?;
97 /// let reader = BufReader::with_capacity(10, f);
98 /// Ok(())
99 /// }
100 /// ```
101 #[stable(feature = "rust1", since = "1.0.0")]
102 pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
103 BufReader { inner, buf: Buffer::with_capacity(capacity) }
104 }
105}
106
107impl<R: Read + ?Sized> BufReader<R> {
108 /// Attempt to look ahead `n` bytes.
109 ///
110 /// `n` must be less than or equal to `capacity`.
111 ///
112 /// the returned slice may be less than `n` bytes long if
113 /// end of file is reached.
114 ///
115 /// ## Examples
116 ///
117 /// ```rust
118 /// #![feature(bufreader_peek)]
119 /// use std::io::{Read, BufReader};
120 ///
121 /// let mut bytes = &b"oh, hello"[..];
122 /// let mut rdr = BufReader::with_capacity(6, &mut bytes);
123 /// assert_eq!(rdr.peek(2).unwrap(), b"oh");
124 /// let mut buf = [0; 4];
125 /// rdr.read(&mut buf[..]).unwrap();
126 /// assert_eq!(&buf, b"oh, ");
127 /// assert_eq!(rdr.peek(2).unwrap(), b"he");
128 /// let mut s = String::new();
129 /// rdr.read_to_string(&mut s).unwrap();
130 /// assert_eq!(&s, "hello");
131 /// assert_eq!(rdr.peek(1).unwrap().len(), 0);
132 /// ```
133 #[unstable(feature = "bufreader_peek", issue = "128405")]
134 pub fn peek(&mut self, n: usize) -> io::Result<&[u8]> {
135 assert!(n <= self.capacity());
136 while n > self.buf.buffer().len() {
137 if self.buf.pos() > 0 {
138 self.buf.backshift();
139 }
140 let new = self.buf.read_more(&mut self.inner)?;
141 if new == 0 {
142 // end of file, no more bytes to read
143 return Ok(&self.buf.buffer()[..]);
144 }
145 debug_assert_eq!(self.buf.pos(), 0);
146 }
147 Ok(&self.buf.buffer()[..n])
148 }
149}
150
151impl<R: ?Sized> BufReader<R> {
152 /// Gets a reference to the underlying reader.
153 ///
154 /// It is inadvisable to directly read from the underlying reader.
155 ///
156 /// # Examples
157 ///
158 /// ```no_run
159 /// use std::io::BufReader;
160 /// use std::fs::File;
161 ///
162 /// fn main() -> std::io::Result<()> {
163 /// let f1 = File::open("log.txt")?;
164 /// let reader = BufReader::new(f1);
165 ///
166 /// let f2 = reader.get_ref();
167 /// Ok(())
168 /// }
169 /// ```
170 #[stable(feature = "rust1", since = "1.0.0")]
171 pub fn get_ref(&self) -> &R {
172 &self.inner
173 }
174
175 /// Gets a mutable reference to the underlying reader.
176 ///
177 /// It is inadvisable to directly read from the underlying reader.
178 ///
179 /// # Examples
180 ///
181 /// ```no_run
182 /// use std::io::BufReader;
183 /// use std::fs::File;
184 ///
185 /// fn main() -> std::io::Result<()> {
186 /// let f1 = File::open("log.txt")?;
187 /// let mut reader = BufReader::new(f1);
188 ///
189 /// let f2 = reader.get_mut();
190 /// Ok(())
191 /// }
192 /// ```
193 #[stable(feature = "rust1", since = "1.0.0")]
194 pub fn get_mut(&mut self) -> &mut R {
195 &mut self.inner
196 }
197
198 /// Returns a reference to the internally buffered data.
199 ///
200 /// Unlike [`fill_buf`], this will not attempt to fill the buffer if it is empty.
201 ///
202 /// [`fill_buf`]: BufRead::fill_buf
203 ///
204 /// # Examples
205 ///
206 /// ```no_run
207 /// use std::io::{BufReader, BufRead};
208 /// use std::fs::File;
209 ///
210 /// fn main() -> std::io::Result<()> {
211 /// let f = File::open("log.txt")?;
212 /// let mut reader = BufReader::new(f);
213 /// assert!(reader.buffer().is_empty());
214 ///
215 /// if reader.fill_buf()?.len() > 0 {
216 /// assert!(!reader.buffer().is_empty());
217 /// }
218 /// Ok(())
219 /// }
220 /// ```
221 #[stable(feature = "bufreader_buffer", since = "1.37.0")]
222 pub fn buffer(&self) -> &[u8] {
223 self.buf.buffer()
224 }
225
226 /// Returns the number of bytes the internal buffer can hold at once.
227 ///
228 /// # Examples
229 ///
230 /// ```no_run
231 /// use std::io::{BufReader, BufRead};
232 /// use std::fs::File;
233 ///
234 /// fn main() -> std::io::Result<()> {
235 /// let f = File::open("log.txt")?;
236 /// let mut reader = BufReader::new(f);
237 ///
238 /// let capacity = reader.capacity();
239 /// let buffer = reader.fill_buf()?;
240 /// assert!(buffer.len() <= capacity);
241 /// Ok(())
242 /// }
243 /// ```
244 #[stable(feature = "buffered_io_capacity", since = "1.46.0")]
245 pub fn capacity(&self) -> usize {
246 self.buf.capacity()
247 }
248
249 /// Unwraps this `BufReader<R>`, returning the underlying reader.
250 ///
251 /// Note that any leftover data in the internal buffer is lost. Therefore,
252 /// a following read from the underlying reader may lead to data loss.
253 ///
254 /// # Examples
255 ///
256 /// ```no_run
257 /// use std::io::BufReader;
258 /// use std::fs::File;
259 ///
260 /// fn main() -> std::io::Result<()> {
261 /// let f1 = File::open("log.txt")?;
262 /// let reader = BufReader::new(f1);
263 ///
264 /// let f2 = reader.into_inner();
265 /// Ok(())
266 /// }
267 /// ```
268 #[stable(feature = "rust1", since = "1.0.0")]
269 pub fn into_inner(self) -> R
270 where
271 R: Sized,
272 {
273 self.inner
274 }
275
276 /// Invalidates all data in the internal buffer.
277 #[inline]
278 pub(in crate::io) fn discard_buffer(&mut self) {
279 self.buf.discard_buffer()
280 }
281}
282
283// This is only used by a test which asserts that the initialization-tracking is correct.
284#[cfg(test)]
285impl<R: ?Sized> BufReader<R> {
286 #[allow(missing_docs)]
287 pub fn initialized(&self) -> usize {
288 self.buf.initialized()
289 }
290}
291
292impl<R: ?Sized + Seek> BufReader<R> {
293 /// Seeks relative to the current position. If the new position lies within the buffer,
294 /// the buffer will not be flushed, allowing for more efficient seeks.
295 /// This method does not return the location of the underlying reader, so the caller
296 /// must track this information themselves if it is required.
297 #[stable(feature = "bufreader_seek_relative", since = "1.53.0")]
298 pub fn seek_relative(&mut self, offset: i64) -> io::Result<()> {
299 let pos = self.buf.pos() as u64;
300 if offset < 0 {
301 if let Some(_) = pos.checked_sub((-offset) as u64) {
302 self.buf.unconsume((-offset) as usize);
303 return Ok(());
304 }
305 } else if let Some(new_pos) = pos.checked_add(offset as u64) {
306 if new_pos <= self.buf.filled() as u64 {
307 self.buf.consume(offset as usize);
308 return Ok(());
309 }
310 }
311
312 self.seek(SeekFrom::Current(offset)).map(drop)
313 }
314}
315
316impl<R> SpecReadByte for BufReader<R>
317where
318 Self: Read,
319{
320 #[inline]
321 fn spec_read_byte(&mut self) -> Option<io::Result<u8>> {
322 let mut byte = 0;
323 if self.buf.consume_with(1, |claimed| byte = claimed[0]) {
324 return Some(Ok(byte));
325 }
326
327 // Fallback case, only reached once per buffer refill.
328 uninlined_slow_read_byte(self)
329 }
330}
331
332#[stable(feature = "rust1", since = "1.0.0")]
333impl<R: ?Sized + Read> Read for BufReader<R> {
334 fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
335 // If we don't have any buffered data and we're doing a massive read
336 // (larger than our internal buffer), bypass our internal buffer
337 // entirely.
338 if self.buf.pos() == self.buf.filled() && buf.len() >= self.capacity() {
339 self.discard_buffer();
340 return self.inner.read(buf);
341 }
342 let mut rem = self.fill_buf()?;
343 let nread = rem.read(buf)?;
344 self.consume(nread);
345 Ok(nread)
346 }
347
348 fn read_buf(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
349 // If we don't have any buffered data and we're doing a massive read
350 // (larger than our internal buffer), bypass our internal buffer
351 // entirely.
352 if self.buf.pos() == self.buf.filled() && cursor.capacity() >= self.capacity() {
353 self.discard_buffer();
354 return self.inner.read_buf(cursor);
355 }
356
357 let prev = cursor.written();
358
359 let mut rem = self.fill_buf()?;
360 rem.read_buf(cursor.reborrow())?; // actually never fails
361
362 self.consume(cursor.written() - prev); //slice impl of read_buf known to never unfill buf
363
364 Ok(())
365 }
366
367 // Small read_exacts from a BufReader are extremely common when used with a deserializer.
368 // The default implementation calls read in a loop, which results in surprisingly poor code
369 // generation for the common path where the buffer has enough bytes to fill the passed-in
370 // buffer.
371 fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
372 if self.buf.consume_with(buf.len(), |claimed| buf.copy_from_slice(claimed)) {
373 return Ok(());
374 }
375
376 crate::io::default_read_exact(self, buf)
377 }
378
379 fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
380 if self.buf.consume_with(cursor.capacity(), |claimed| cursor.append(claimed)) {
381 return Ok(());
382 }
383
384 crate::io::default_read_buf_exact(self, cursor)
385 }
386
387 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
388 let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
389 if self.buf.pos() == self.buf.filled() && total_len >= self.capacity() {
390 self.discard_buffer();
391 return self.inner.read_vectored(bufs);
392 }
393 let mut rem = self.fill_buf()?;
394 let nread = rem.read_vectored(bufs)?;
395
396 self.consume(nread);
397 Ok(nread)
398 }
399
400 fn is_read_vectored(&self) -> bool {
401 self.inner.is_read_vectored()
402 }
403
404 // The inner reader might have an optimized `read_to_end`. Drain our buffer and then
405 // delegate to the inner implementation.
406 fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
407 let inner_buf = self.buffer();
408 buf.try_reserve(inner_buf.len())?;
409 buf.extend_from_slice(inner_buf);
410 let nread = inner_buf.len();
411 self.discard_buffer();
412 Ok(nread + self.inner.read_to_end(buf)?)
413 }
414
415 // The inner reader might have an optimized `read_to_end`. Drain our buffer and then
416 // delegate to the inner implementation.
417 fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
418 // In the general `else` case below we must read bytes into a side buffer, check
419 // that they are valid UTF-8, and then append them to `buf`. This requires a
420 // potentially large memcpy.
421 //
422 // If `buf` is empty--the most common case--we can leverage `append_to_string`
423 // to read directly into `buf`'s internal byte buffer, saving an allocation and
424 // a memcpy.
425 if buf.is_empty() {
426 // `append_to_string`'s safety relies on the buffer only being appended to since
427 // it only checks the UTF-8 validity of new data. If there were existing content in
428 // `buf` then an untrustworthy reader (i.e. `self.inner`) could not only append
429 // bytes but also modify existing bytes and render them invalid. On the other hand,
430 // if `buf` is empty then by definition any writes must be appends and
431 // `append_to_string` will validate all of the new bytes.
432 unsafe { crate::io::append_to_string(buf, |b| self.read_to_end(b)) }
433 } else {
434 // We cannot append our byte buffer directly onto the `buf` String as there could
435 // be an incomplete UTF-8 sequence that has only been partially read. We must read
436 // everything into a side buffer first and then call `from_utf8` on the complete
437 // buffer.
438 let mut bytes = Vec::new();
439 self.read_to_end(&mut bytes)?;
440 let string = crate::str::from_utf8(&bytes).map_err(|_| io::Error::INVALID_UTF8)?;
441 *buf += string;
442 Ok(string.len())
443 }
444 }
445}
446
447#[stable(feature = "rust1", since = "1.0.0")]
448impl<R: ?Sized + Read> BufRead for BufReader<R> {
449 fn fill_buf(&mut self) -> io::Result<&[u8]> {
450 self.buf.fill_buf(&mut self.inner)
451 }
452
453 fn consume(&mut self, amt: usize) {
454 self.buf.consume(amt)
455 }
456}
457
458#[stable(feature = "rust1", since = "1.0.0")]
459impl<R> fmt::Debug for BufReader<R>
460where
461 R: ?Sized + fmt::Debug,
462{
463 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
464 fmt.debug_struct("BufReader")
465 .field("reader", &&self.inner)
466 .field(
467 "buffer",
468 &format_args!("{}/{}", self.buf.filled() - self.buf.pos(), self.capacity()),
469 )
470 .finish()
471 }
472}
473
474#[stable(feature = "rust1", since = "1.0.0")]
475impl<R: ?Sized + Seek> Seek for BufReader<R> {
476 /// Seek to an offset, in bytes, in the underlying reader.
477 ///
478 /// The position used for seeking with <code>[SeekFrom::Current]\(_)</code> is the
479 /// position the underlying reader would be at if the `BufReader<R>` had no
480 /// internal buffer.
481 ///
482 /// Seeking always discards the internal buffer, even if the seek position
483 /// would otherwise fall within it. This guarantees that calling
484 /// [`BufReader::into_inner()`] immediately after a seek yields the underlying reader
485 /// at the same position.
486 ///
487 /// To seek without discarding the internal buffer, use [`BufReader::seek_relative`].
488 ///
489 /// See [`std::io::Seek`] for more details.
490 ///
491 /// Note: In the edge case where you're seeking with <code>[SeekFrom::Current]\(n)</code>
492 /// where `n` minus the internal buffer length overflows an `i64`, two
493 /// seeks will be performed instead of one. If the second seek returns
494 /// [`Err`], the underlying reader will be left at the same position it would
495 /// have if you called `seek` with <code>[SeekFrom::Current]\(0)</code>.
496 ///
497 /// [`std::io::Seek`]: Seek
498 fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
499 let result: u64;
500 if let SeekFrom::Current(n) = pos {
501 let remainder = (self.buf.filled() - self.buf.pos()) as i64;
502 // it should be safe to assume that remainder fits within an i64 as the alternative
503 // means we managed to allocate 8 exbibytes and that's absurd.
504 // But it's not out of the realm of possibility for some weird underlying reader to
505 // support seeking by i64::MIN so we need to handle underflow when subtracting
506 // remainder.
507 if let Some(offset) = n.checked_sub(remainder) {
508 result = self.inner.seek(SeekFrom::Current(offset))?;
509 } else {
510 // seek backwards by our remainder, and then by the offset
511 self.inner.seek(SeekFrom::Current(-remainder))?;
512 self.discard_buffer();
513 result = self.inner.seek(SeekFrom::Current(n))?;
514 }
515 } else {
516 // Seeking with Start/End doesn't care about our buffer length.
517 result = self.inner.seek(pos)?;
518 }
519 self.discard_buffer();
520 Ok(result)
521 }
522
523 /// Returns the current seek position from the start of the stream.
524 ///
525 /// The value returned is equivalent to `self.seek(SeekFrom::Current(0))`
526 /// but does not flush the internal buffer. Due to this optimization the
527 /// function does not guarantee that calling `.into_inner()` immediately
528 /// afterwards will yield the underlying reader at the same position. Use
529 /// [`BufReader::seek`] instead if you require that guarantee.
530 ///
531 /// # Panics
532 ///
533 /// This function will panic if the position of the inner reader is smaller
534 /// than the amount of buffered data. That can happen if the inner reader
535 /// has an incorrect implementation of [`Seek::stream_position`], or if the
536 /// position has gone out of sync due to calling [`Seek::seek`] directly on
537 /// the underlying reader.
538 ///
539 /// # Example
540 ///
541 /// ```no_run
542 /// use std::{
543 /// io::{self, BufRead, BufReader, Seek},
544 /// fs::File,
545 /// };
546 ///
547 /// fn main() -> io::Result<()> {
548 /// let mut f = BufReader::new(File::open("foo.txt")?);
549 ///
550 /// let before = f.stream_position()?;
551 /// f.read_line(&mut String::new())?;
552 /// let after = f.stream_position()?;
553 ///
554 /// println!("The first line was {} bytes long", after - before);
555 /// Ok(())
556 /// }
557 /// ```
558 fn stream_position(&mut self) -> io::Result<u64> {
559 let remainder = (self.buf.filled() - self.buf.pos()) as u64;
560 self.inner.stream_position().map(|pos| {
561 pos.checked_sub(remainder).expect(
562 "overflow when subtracting remaining buffer size from inner stream position",
563 )
564 })
565 }
566
567 /// Seeks relative to the current position.
568 ///
569 /// If the new position lies within the buffer, the buffer will not be
570 /// flushed, allowing for more efficient seeks. This method does not return
571 /// the location of the underlying reader, so the caller must track this
572 /// information themselves if it is required.
573 fn seek_relative(&mut self, offset: i64) -> io::Result<()> {
574 self.seek_relative(offset)
575 }
576}
577
578impl<T: ?Sized> SizeHint for BufReader<T> {
579 #[inline]
580 fn lower_bound(&self) -> usize {
581 SizeHint::lower_bound(self.get_ref()) + self.buffer().len()
582 }
583
584 #[inline]
585 fn upper_bound(&self) -> Option<usize> {
586 SizeHint::upper_bound(self.get_ref()).and_then(|up| self.buffer().len().checked_add(up))
587 }
588}