1 //! Traits, helpers, and type definitions for core I/O functionality.
3 //! The `std::io` module contains a number of common things you'll need
4 //! when doing input and output. The most core part of this module is
5 //! the [`Read`] and [`Write`] traits, which provide the
6 //! most general interface for reading and writing input and output.
10 //! Because they are traits, [`Read`] and [`Write`] are implemented by a number
11 //! of other types, and you can implement them for your types too. As such,
12 //! you'll see a few different types of I/O throughout the documentation in
13 //! this module: [`File`]s, [`TcpStream`]s, and sometimes even [`Vec<T>`]s. For
14 //! example, [`Read`] adds a [`read`][`Read::read`] method, which we can use on
19 //! use std::io::prelude::*;
20 //! use std::fs::File;
22 //! fn main() -> io::Result<()> {
23 //! let mut f = File::open("foo.txt")?;
24 //! let mut buffer = [0; 10];
26 //! // read up to 10 bytes
27 //! let n = f.read(&mut buffer)?;
29 //! println!("The bytes: {:?}", &buffer[..n]);
34 //! [`Read`] and [`Write`] are so important, implementors of the two traits have a
35 //! nickname: readers and writers. So you'll sometimes see 'a reader' instead
36 //! of 'a type that implements the [`Read`] trait'. Much easier!
38 //! ## Seek and BufRead
40 //! Beyond that, there are two important traits that are provided: [`Seek`]
41 //! and [`BufRead`]. Both of these build on top of a reader to control
42 //! how the reading happens. [`Seek`] lets you control where the next byte is
47 //! use std::io::prelude::*;
48 //! use std::io::SeekFrom;
49 //! use std::fs::File;
51 //! fn main() -> io::Result<()> {
52 //! let mut f = File::open("foo.txt")?;
53 //! let mut buffer = [0; 10];
55 //! // skip to the last 10 bytes of the file
56 //! f.seek(SeekFrom::End(-10))?;
58 //! // read up to 10 bytes
59 //! let n = f.read(&mut buffer)?;
61 //! println!("The bytes: {:?}", &buffer[..n]);
66 //! [`BufRead`] uses an internal buffer to provide a number of other ways to read, but
67 //! to show it off, we'll need to talk about buffers in general. Keep reading!
69 //! ## BufReader and BufWriter
71 //! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
72 //! making near-constant calls to the operating system. To help with this,
73 //! `std::io` comes with two structs, [`BufReader`] and [`BufWriter`], which wrap
74 //! readers and writers. The wrapper uses a buffer, reducing the number of
75 //! calls and providing nicer methods for accessing exactly what you want.
77 //! For example, [`BufReader`] works with the [`BufRead`] trait to add extra
78 //! methods to any reader:
82 //! use std::io::prelude::*;
83 //! use std::io::BufReader;
84 //! use std::fs::File;
86 //! fn main() -> io::Result<()> {
87 //! let f = File::open("foo.txt")?;
88 //! let mut reader = BufReader::new(f);
89 //! let mut buffer = String::new();
91 //! // read a line into buffer
92 //! reader.read_line(&mut buffer)?;
94 //! println!("{}", buffer);
99 //! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
100 //! to [`write`][`Write::write`]:
104 //! use std::io::prelude::*;
105 //! use std::io::BufWriter;
106 //! use std::fs::File;
108 //! fn main() -> io::Result<()> {
109 //! let f = File::create("foo.txt")?;
111 //! let mut writer = BufWriter::new(f);
113 //! // write a byte to the buffer
114 //! writer.write(&[42])?;
116 //! } // the buffer is flushed once writer goes out of scope
122 //! ## Standard input and output
124 //! A very common source of input is standard input:
129 //! fn main() -> io::Result<()> {
130 //! let mut input = String::new();
132 //! io::stdin().read_line(&mut input)?;
134 //! println!("You typed: {}", input.trim());
139 //! Note that you cannot use the [`?` operator] in functions that do not return
140 //! a [`Result<T, E>`][`Result`]. Instead, you can call [`.unwrap()`]
141 //! or `match` on the return value to catch any possible errors:
146 //! let mut input = String::new();
148 //! io::stdin().read_line(&mut input).unwrap();
151 //! And a very common source of output is standard output:
155 //! use std::io::prelude::*;
157 //! fn main() -> io::Result<()> {
158 //! io::stdout().write(&[42])?;
163 //! Of course, using [`io::stdout`] directly is less common than something like
166 //! ## Iterator types
168 //! A large number of the structures provided by `std::io` are for various
169 //! ways of iterating over I/O. For example, [`Lines`] is used to split over
174 //! use std::io::prelude::*;
175 //! use std::io::BufReader;
176 //! use std::fs::File;
178 //! fn main() -> io::Result<()> {
179 //! let f = File::open("foo.txt")?;
180 //! let reader = BufReader::new(f);
182 //! for line in reader.lines() {
183 //! println!("{}", line?);
191 //! There are a number of [functions][functions-list] that offer access to various
192 //! features. For example, we can use three of these functions to copy everything
193 //! from standard input to standard output:
198 //! fn main() -> io::Result<()> {
199 //! io::copy(&mut io::stdin(), &mut io::stdout())?;
204 //! [functions-list]: #functions-1
208 //! Last, but certainly not least, is [`io::Result`]. This type is used
209 //! as the return type of many `std::io` functions that can cause an error, and
210 //! can be returned from your own functions as well. Many of the examples in this
211 //! module use the [`?` operator]:
216 //! fn read_input() -> io::Result<()> {
217 //! let mut input = String::new();
219 //! io::stdin().read_line(&mut input)?;
221 //! println!("You typed: {}", input.trim());
227 //! The return type of `read_input()`, [`io::Result<()>`][`io::Result`], is a very
228 //! common type for functions which don't have a 'real' return value, but do want to
229 //! return errors if they happen. In this case, the only purpose of this function is
230 //! to read the line and print it, so we use `()`.
232 //! ## Platform-specific behavior
234 //! Many I/O functions throughout the standard library are documented to indicate
235 //! what various library or syscalls they are delegated to. This is done to help
236 //! applications both understand what's happening under the hood as well as investigate
237 //! any possibly unclear semantics. Note, however, that this is informative, not a binding
238 //! contract. The implementation of many of these functions are subject to change over
239 //! time and may call fewer or more syscalls/library functions.
241 //! [`File`]: crate::fs::File
242 //! [`TcpStream`]: crate::net::TcpStream
243 //! [`io::stdout`]: stdout
244 //! [`io::Result`]: self::Result
245 //! [`?` operator]: ../../book/appendix-02-operators.html
246 //! [`Result`]: crate::result::Result
247 //! [`.unwrap()`]: crate::result::Result::unwrap
249 #![stable(feature = "rust1", since = "1.0.0")]
256 use crate::ops::{Deref, DerefMut};
261 use crate::sys_common::memchr;
263 #[stable(feature = "rust1", since = "1.0.0")]
264 pub use self::buffered::IntoInnerError;
265 #[stable(feature = "rust1", since = "1.0.0")]
266 pub use self::buffered::{BufReader, BufWriter, LineWriter};
267 #[stable(feature = "rust1", since = "1.0.0")]
268 pub use self::copy::copy;
269 #[stable(feature = "rust1", since = "1.0.0")]
270 pub use self::cursor::Cursor;
271 #[stable(feature = "rust1", since = "1.0.0")]
272 pub use self::error::{Error, ErrorKind, Result};
273 #[unstable(feature = "internal_output_capture", issue = "none")]
274 #[doc(no_inline, hidden)]
275 pub use self::stdio::set_output_capture;
276 #[stable(feature = "rust1", since = "1.0.0")]
277 pub use self::stdio::{stderr, stdin, stdout, Stderr, Stdin, Stdout};
278 #[stable(feature = "rust1", since = "1.0.0")]
279 pub use self::stdio::{StderrLock, StdinLock, StdoutLock};
280 #[unstable(feature = "print_internals", issue = "none")]
281 pub use self::stdio::{_eprint, _print};
282 #[stable(feature = "rust1", since = "1.0.0")]
283 pub use self::util::{empty, repeat, sink, Empty, Repeat, Sink};
294 const DEFAULT_BUF_SIZE: usize = crate::sys_common::io::DEFAULT_BUF_SIZE;
296 pub(crate) fn cleanup() {
301 buf: &'a mut Vec<u8>,
305 impl Drop for Guard<'_> {
308 self.buf.set_len(self.len);
313 // A few methods below (read_to_string, read_line) will append data into a
314 // `String` buffer, but we need to be pretty careful when doing this. The
315 // implementation will just call `.as_mut_vec()` and then delegate to a
316 // byte-oriented reading method, but we must ensure that when returning we never
317 // leave `buf` in a state such that it contains invalid UTF-8 in its bounds.
319 // To this end, we use an RAII guard (to protect against panics) which updates
320 // the length of the string when it is dropped. This guard initially truncates
321 // the string to the prior length and only after we've validated that the
322 // new contents are valid UTF-8 do we allow it to set a longer length.
324 // The unsafety in this function is twofold:
326 // 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
328 // 2. We're passing a raw buffer to the function `f`, and it is expected that
329 // the function only *appends* bytes to the buffer. We'll get undefined
330 // behavior if existing bytes are overwritten to have non-UTF-8 data.
331 fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
333 F: FnOnce(&mut Vec<u8>) -> Result<usize>,
336 let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec() };
338 if str::from_utf8(&g.buf[g.len..]).is_err() {
340 Err(Error::new_const(ErrorKind::InvalidData, &"stream did not contain valid UTF-8"))
349 // This uses an adaptive system to extend the vector when it fills. We want to
350 // avoid paying to allocate and zero a huge chunk of memory if the reader only
351 // has 4 bytes while still making large reads if the reader does have a ton
352 // of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
353 // time is 4,500 times (!) slower than a default reservation size of 32 if the
354 // reader has a very small amount of data to return.
356 // Because we're extending the buffer with uninitialized data for trusted
357 // readers, we need to make sure to truncate that if any of this panics.
358 fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
359 read_to_end_with_reservation(r, buf, |_| 32)
362 fn read_to_end_with_reservation<R, F>(
365 mut reservation_size: F,
369 F: FnMut(&R) -> usize,
371 let start_len = buf.len();
372 let mut g = Guard { len: buf.len(), buf };
374 if g.len == g.buf.len() {
376 // FIXME(danielhenrymantilla): #42788
378 // - This creates a (mut) reference to a slice of
379 // _uninitialized_ integers, which is **undefined behavior**
381 // - Only the standard library gets to soundly "ignore" this,
382 // based on its privileged knowledge of unstable rustc
384 g.buf.reserve(reservation_size(r));
385 let capacity = g.buf.capacity();
386 g.buf.set_len(capacity);
387 r.initializer().initialize(&mut g.buf[g.len..]);
391 let buf = &mut g.buf[g.len..];
393 Ok(0) => return Ok(g.len - start_len),
395 // We can't allow bogus values from read. If it is too large, the returned vec could have its length
396 // set past its capacity, or if it overflows the vec could be shortened which could create an invalid
397 // string if this is called via read_to_string.
398 assert!(n <= buf.len());
401 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
402 Err(e) => return Err(e),
407 pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
409 F: FnOnce(&mut [u8]) -> Result<usize>,
411 let buf = bufs.iter_mut().find(|b| !b.is_empty()).map_or(&mut [][..], |b| &mut **b);
415 pub(crate) fn default_write_vectored<F>(write: F, bufs: &[IoSlice<'_>]) -> Result<usize>
417 F: FnOnce(&[u8]) -> Result<usize>,
419 let buf = bufs.iter().find(|b| !b.is_empty()).map_or(&[][..], |b| &**b);
423 pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [u8]) -> Result<()> {
424 while !buf.is_empty() {
425 match this.read(buf) {
431 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
432 Err(e) => return Err(e),
436 Err(Error::new_const(ErrorKind::UnexpectedEof, &"failed to fill whole buffer"))
442 /// The `Read` trait allows for reading bytes from a source.
444 /// Implementors of the `Read` trait are called 'readers'.
446 /// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
447 /// will attempt to pull bytes from this source into a provided buffer. A
448 /// number of other methods are implemented in terms of [`read()`], giving
449 /// implementors a number of ways to read bytes while only needing to implement
452 /// Readers are intended to be composable with one another. Many implementors
453 /// throughout [`std::io`] take and provide types which implement the `Read`
456 /// Please note that each call to [`read()`] may involve a system call, and
457 /// therefore, using something that implements [`BufRead`], such as
458 /// [`BufReader`], will be more efficient.
462 /// [`File`]s implement `Read`:
466 /// use std::io::prelude::*;
467 /// use std::fs::File;
469 /// fn main() -> io::Result<()> {
470 /// let mut f = File::open("foo.txt")?;
471 /// let mut buffer = [0; 10];
473 /// // read up to 10 bytes
474 /// f.read(&mut buffer)?;
476 /// let mut buffer = Vec::new();
477 /// // read the whole file
478 /// f.read_to_end(&mut buffer)?;
480 /// // read into a String, so that you don't need to do the conversion.
481 /// let mut buffer = String::new();
482 /// f.read_to_string(&mut buffer)?;
484 /// // and more! See the other methods for more details.
489 /// Read from [`&str`] because [`&[u8]`][prim@slice] implements `Read`:
493 /// use std::io::prelude::*;
495 /// fn main() -> io::Result<()> {
496 /// let mut b = "This string will be read".as_bytes();
497 /// let mut buffer = [0; 10];
499 /// // read up to 10 bytes
500 /// b.read(&mut buffer)?;
502 /// // etc... it works exactly as a File does!
507 /// [`read()`]: Read::read
508 /// [`&str`]: prim@str
509 /// [`std::io`]: self
510 /// [`File`]: crate::fs::File
511 #[stable(feature = "rust1", since = "1.0.0")]
512 #[doc(notable_trait)]
514 /// Pull some bytes from this source into the specified buffer, returning
515 /// how many bytes were read.
517 /// This function does not provide any guarantees about whether it blocks
518 /// waiting for data, but if an object needs to block for a read and cannot,
519 /// it will typically signal this via an [`Err`] return value.
521 /// If the return value of this method is [`Ok(n)`], then implementations must
522 /// guarantee that `0 <= n <= buf.len()`. A nonzero `n` value indicates
523 /// that the buffer `buf` has been filled in with `n` bytes of data from this
524 /// source. If `n` is `0`, then it can indicate one of two scenarios:
526 /// 1. This reader has reached its "end of file" and will likely no longer
527 /// be able to produce bytes. Note that this does not mean that the
528 /// reader will *always* no longer be able to produce bytes. As an example,
529 /// on Linux, this method will call the `recv` syscall for a [`TcpStream`],
530 /// where returning zero indicates the connection was shut down correctly. While
531 /// for [`File`], it is possible to reach the end of file and get zero as result,
532 /// but if more data is appended to the file, future calls to `read` will return
534 /// 2. The buffer specified was 0 bytes in length.
536 /// It is not an error if the returned value `n` is smaller than the buffer size,
537 /// even when the reader is not at the end of the stream yet.
538 /// This may happen for example because fewer bytes are actually available right now
539 /// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
541 /// As this trait is safe to implement, callers cannot rely on `n <= buf.len()` for safety.
542 /// Extra care needs to be taken when `unsafe` functions are used to access the read bytes.
543 /// Callers have to ensure that no unchecked out-of-bounds accesses are possible even if
546 /// No guarantees are provided about the contents of `buf` when this
547 /// function is called, implementations cannot rely on any property of the
548 /// contents of `buf` being true. It is recommended that *implementations*
549 /// only write data to `buf` instead of reading its contents.
551 /// Correspondingly, however, *callers* of this method may not assume any guarantees
552 /// about how the implementation uses `buf`. The trait is safe to implement,
553 /// so it is possible that the code that's supposed to write to the buffer might also read
554 /// from it. It is your responsibility to make sure that `buf` is initialized
555 /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
556 /// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
558 /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
562 /// If this function encounters any form of I/O or other error, an error
563 /// variant will be returned. If an error is returned then it must be
564 /// guaranteed that no bytes were read.
566 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
567 /// operation should be retried if there is nothing else to do.
571 /// [`File`]s implement `Read`:
574 /// [`File`]: crate::fs::File
575 /// [`TcpStream`]: crate::net::TcpStream
579 /// use std::io::prelude::*;
580 /// use std::fs::File;
582 /// fn main() -> io::Result<()> {
583 /// let mut f = File::open("foo.txt")?;
584 /// let mut buffer = [0; 10];
586 /// // read up to 10 bytes
587 /// let n = f.read(&mut buffer[..])?;
589 /// println!("The bytes: {:?}", &buffer[..n]);
593 #[stable(feature = "rust1", since = "1.0.0")]
594 fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
596 /// Like `read`, except that it reads into a slice of buffers.
598 /// Data is copied to fill each buffer in order, with the final buffer
599 /// written to possibly being only partially filled. This method must
600 /// behave equivalently to a single call to `read` with concatenated
603 /// The default implementation calls `read` with either the first nonempty
604 /// buffer provided, or an empty one if none exists.
605 #[stable(feature = "iovec", since = "1.36.0")]
606 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
607 default_read_vectored(|b| self.read(b), bufs)
610 /// Determines if this `Read`er has an efficient `read_vectored`
613 /// If a `Read`er does not override the default `read_vectored`
614 /// implementation, code using it may want to avoid the method all together
615 /// and coalesce writes into a single buffer for higher performance.
617 /// The default implementation returns `false`.
618 #[unstable(feature = "can_vector", issue = "69941")]
619 fn is_read_vectored(&self) -> bool {
623 /// Determines if this `Read`er can work with buffers of uninitialized
626 /// The default implementation returns an initializer which will zero
629 /// If a `Read`er guarantees that it can work properly with uninitialized
630 /// memory, it should call [`Initializer::nop()`]. See the documentation for
631 /// [`Initializer`] for details.
633 /// The behavior of this method must be independent of the state of the
634 /// `Read`er - the method only takes `&self` so that it can be used through
639 /// This method is unsafe because a `Read`er could otherwise return a
640 /// non-zeroing `Initializer` from another `Read` type without an `unsafe`
642 #[unstable(feature = "read_initializer", issue = "42788")]
644 unsafe fn initializer(&self) -> Initializer {
645 Initializer::zeroing()
648 /// Read all bytes until EOF in this source, placing them into `buf`.
650 /// All bytes read from this source will be appended to the specified buffer
651 /// `buf`. This function will continuously call [`read()`] to append more data to
652 /// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
653 /// non-[`ErrorKind::Interrupted`] kind.
655 /// If successful, this function will return the total number of bytes read.
659 /// If this function encounters an error of the kind
660 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
663 /// If any other read error is encountered then this function immediately
664 /// returns. Any bytes which have already been read will be appended to
669 /// [`File`]s implement `Read`:
671 /// [`read()`]: Read::read
673 /// [`File`]: crate::fs::File
677 /// use std::io::prelude::*;
678 /// use std::fs::File;
680 /// fn main() -> io::Result<()> {
681 /// let mut f = File::open("foo.txt")?;
682 /// let mut buffer = Vec::new();
684 /// // read the whole file
685 /// f.read_to_end(&mut buffer)?;
690 /// (See also the [`std::fs::read`] convenience function for reading from a
693 /// [`std::fs::read`]: crate::fs::read
694 #[stable(feature = "rust1", since = "1.0.0")]
695 fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
696 read_to_end(self, buf)
699 /// Read all bytes until EOF in this source, appending them to `buf`.
701 /// If successful, this function returns the number of bytes which were read
702 /// and appended to `buf`.
706 /// If the data in this stream is *not* valid UTF-8 then an error is
707 /// returned and `buf` is unchanged.
709 /// See [`read_to_end`] for other error semantics.
711 /// [`read_to_end`]: Read::read_to_end
715 /// [`File`]s implement `Read`:
717 /// [`File`]: crate::fs::File
721 /// use std::io::prelude::*;
722 /// use std::fs::File;
724 /// fn main() -> io::Result<()> {
725 /// let mut f = File::open("foo.txt")?;
726 /// let mut buffer = String::new();
728 /// f.read_to_string(&mut buffer)?;
733 /// (See also the [`std::fs::read_to_string`] convenience function for
734 /// reading from a file.)
736 /// [`std::fs::read_to_string`]: crate::fs::read_to_string
737 #[stable(feature = "rust1", since = "1.0.0")]
738 fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
739 // Note that we do *not* call `.read_to_end()` here. We are passing
740 // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
741 // method to fill it up. An arbitrary implementation could overwrite the
742 // entire contents of the vector, not just append to it (which is what
743 // we are expecting).
745 // To prevent extraneously checking the UTF-8-ness of the entire buffer
746 // we pass it to our hardcoded `read_to_end` implementation which we
747 // know is guaranteed to only read data into the end of the buffer.
748 append_to_string(buf, |b| read_to_end(self, b))
751 /// Read the exact number of bytes required to fill `buf`.
753 /// This function reads as many bytes as necessary to completely fill the
754 /// specified buffer `buf`.
756 /// No guarantees are provided about the contents of `buf` when this
757 /// function is called, implementations cannot rely on any property of the
758 /// contents of `buf` being true. It is recommended that implementations
759 /// only write data to `buf` instead of reading its contents. The
760 /// documentation on [`read`] has a more detailed explanation on this
765 /// If this function encounters an error of the kind
766 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
769 /// If this function encounters an "end of file" before completely filling
770 /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
771 /// The contents of `buf` are unspecified in this case.
773 /// If any other read error is encountered then this function immediately
774 /// returns. The contents of `buf` are unspecified in this case.
776 /// If this function returns an error, it is unspecified how many bytes it
777 /// has read, but it will never read more than would be necessary to
778 /// completely fill the buffer.
782 /// [`File`]s implement `Read`:
784 /// [`read`]: Read::read
785 /// [`File`]: crate::fs::File
789 /// use std::io::prelude::*;
790 /// use std::fs::File;
792 /// fn main() -> io::Result<()> {
793 /// let mut f = File::open("foo.txt")?;
794 /// let mut buffer = [0; 10];
796 /// // read exactly 10 bytes
797 /// f.read_exact(&mut buffer)?;
801 #[stable(feature = "read_exact", since = "1.6.0")]
802 fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
803 default_read_exact(self, buf)
806 /// Creates a "by reference" adaptor for this instance of `Read`.
808 /// The returned adaptor also implements `Read` and will simply borrow this
813 /// [`File`]s implement `Read`:
815 /// [`File`]: crate::fs::File
819 /// use std::io::Read;
820 /// use std::fs::File;
822 /// fn main() -> io::Result<()> {
823 /// let mut f = File::open("foo.txt")?;
824 /// let mut buffer = Vec::new();
825 /// let mut other_buffer = Vec::new();
828 /// let reference = f.by_ref();
830 /// // read at most 5 bytes
831 /// reference.take(5).read_to_end(&mut buffer)?;
833 /// } // drop our &mut reference so we can use f again
835 /// // original file still usable, read the rest
836 /// f.read_to_end(&mut other_buffer)?;
840 #[stable(feature = "rust1", since = "1.0.0")]
841 fn by_ref(&mut self) -> &mut Self
848 /// Transforms this `Read` instance to an [`Iterator`] over its bytes.
850 /// The returned type implements [`Iterator`] where the `Item` is
851 /// [`Result`]`<`[`u8`]`, `[`io::Error`]`>`.
852 /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
853 /// otherwise. EOF is mapped to returning [`None`] from this iterator.
857 /// [`File`]s implement `Read`:
859 /// [`File`]: crate::fs::File
860 /// [`Result`]: crate::result::Result
861 /// [`io::Error`]: self::Error
865 /// use std::io::prelude::*;
866 /// use std::fs::File;
868 /// fn main() -> io::Result<()> {
869 /// let mut f = File::open("foo.txt")?;
871 /// for byte in f.bytes() {
872 /// println!("{}", byte.unwrap());
877 #[stable(feature = "rust1", since = "1.0.0")]
878 fn bytes(self) -> Bytes<Self>
882 Bytes { inner: self }
885 /// Creates an adaptor which will chain this stream with another.
887 /// The returned `Read` instance will first read all bytes from this object
888 /// until EOF is encountered. Afterwards the output is equivalent to the
889 /// output of `next`.
893 /// [`File`]s implement `Read`:
895 /// [`File`]: crate::fs::File
899 /// use std::io::prelude::*;
900 /// use std::fs::File;
902 /// fn main() -> io::Result<()> {
903 /// let mut f1 = File::open("foo.txt")?;
904 /// let mut f2 = File::open("bar.txt")?;
906 /// let mut handle = f1.chain(f2);
907 /// let mut buffer = String::new();
909 /// // read the value into a String. We could use any Read method here,
910 /// // this is just one example.
911 /// handle.read_to_string(&mut buffer)?;
915 #[stable(feature = "rust1", since = "1.0.0")]
916 fn chain<R: Read>(self, next: R) -> Chain<Self, R>
920 Chain { first: self, second: next, done_first: false }
923 /// Creates an adaptor which will read at most `limit` bytes from it.
925 /// This function returns a new instance of `Read` which will read at most
926 /// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
927 /// read errors will not count towards the number of bytes read and future
928 /// calls to [`read()`] may succeed.
932 /// [`File`]s implement `Read`:
934 /// [`File`]: crate::fs::File
936 /// [`read()`]: Read::read
940 /// use std::io::prelude::*;
941 /// use std::fs::File;
943 /// fn main() -> io::Result<()> {
944 /// let mut f = File::open("foo.txt")?;
945 /// let mut buffer = [0; 5];
947 /// // read at most five bytes
948 /// let mut handle = f.take(5);
950 /// handle.read(&mut buffer)?;
954 #[stable(feature = "rust1", since = "1.0.0")]
955 fn take(self, limit: u64) -> Take<Self>
959 Take { inner: self, limit }
963 /// Read all bytes from a [reader][Read] into a new [`String`].
965 /// This is a convenience function for [`Read::read_to_string`]. Using this
966 /// function avoids having to create a variable first and provides more type
967 /// safety since you can only get the buffer out if there were no errors. (If you
968 /// use [`Read::read_to_string`] you have to remember to check whether the read
969 /// succeeded because otherwise your buffer will be empty or only partially full.)
973 /// The downside of this function's increased ease of use and type safety is
974 /// that it gives you less control over performance. For example, you can't
975 /// pre-allocate memory like you can using [`String::with_capacity`] and
976 /// [`Read::read_to_string`]. Also, you can't re-use the buffer if an error
977 /// occurs while reading.
979 /// In many cases, this function's performance will be adequate and the ease of use
980 /// and type safety tradeoffs will be worth it. However, there are cases where you
981 /// need more control over performance, and in those cases you should definitely use
982 /// [`Read::read_to_string`] directly.
986 /// This function forces you to handle errors because the output (the `String`)
987 /// is wrapped in a [`Result`]. See [`Read::read_to_string`] for the errors
988 /// that can occur. If any error occurs, you will get an [`Err`], so you
989 /// don't have to worry about your buffer being empty or partially full.
994 /// #![feature(io_read_to_string)]
997 /// fn main() -> io::Result<()> {
998 /// let stdin = io::read_to_string(&mut io::stdin())?;
999 /// println!("Stdin was:");
1000 /// println!("{}", stdin);
1004 #[unstable(feature = "io_read_to_string", issue = "80218")]
1005 pub fn read_to_string<R: Read>(reader: &mut R) -> Result<String> {
1006 let mut buf = String::new();
1007 reader.read_to_string(&mut buf)?;
1011 /// A buffer type used with `Read::read_vectored`.
1013 /// It is semantically a wrapper around an `&mut [u8]`, but is guaranteed to be
1014 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
1016 #[stable(feature = "iovec", since = "1.36.0")]
1017 #[repr(transparent)]
1018 pub struct IoSliceMut<'a>(sys::io::IoSliceMut<'a>);
1020 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1021 unsafe impl<'a> Send for IoSliceMut<'a> {}
1023 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1024 unsafe impl<'a> Sync for IoSliceMut<'a> {}
1026 #[stable(feature = "iovec", since = "1.36.0")]
1027 impl<'a> fmt::Debug for IoSliceMut<'a> {
1028 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
1029 fmt::Debug::fmt(self.0.as_slice(), fmt)
1033 impl<'a> IoSliceMut<'a> {
1034 /// Creates a new `IoSliceMut` wrapping a byte slice.
1038 /// Panics on Windows if the slice is larger than 4GB.
1039 #[stable(feature = "iovec", since = "1.36.0")]
1041 pub fn new(buf: &'a mut [u8]) -> IoSliceMut<'a> {
1042 IoSliceMut(sys::io::IoSliceMut::new(buf))
1045 /// Advance the internal cursor of the slice.
1049 /// Elements in the slice may be modified if the cursor is not advanced to
1050 /// the end of the slice. For example if we have a slice of buffers with 2
1051 /// `IoSliceMut`s, both of length 8, and we advance the cursor by 10 bytes
1052 /// the first `IoSliceMut` will be untouched however the second will be
1053 /// modified to remove the first 2 bytes (10 - 8).
1058 /// #![feature(io_slice_advance)]
1060 /// use std::io::IoSliceMut;
1061 /// use std::ops::Deref;
1063 /// let mut buf1 = [1; 8];
1064 /// let mut buf2 = [2; 16];
1065 /// let mut buf3 = [3; 8];
1066 /// let mut bufs = &mut [
1067 /// IoSliceMut::new(&mut buf1),
1068 /// IoSliceMut::new(&mut buf2),
1069 /// IoSliceMut::new(&mut buf3),
1072 /// // Mark 10 bytes as read.
1073 /// bufs = IoSliceMut::advance(bufs, 10);
1074 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1075 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1077 #[unstable(feature = "io_slice_advance", issue = "62726")]
1079 pub fn advance<'b>(bufs: &'b mut [IoSliceMut<'a>], n: usize) -> &'b mut [IoSliceMut<'a>] {
1080 // Number of buffers to remove.
1082 // Total length of all the to be removed buffers.
1083 let mut accumulated_len = 0;
1084 for buf in bufs.iter() {
1085 if accumulated_len + buf.len() > n {
1088 accumulated_len += buf.len();
1093 let bufs = &mut bufs[remove..];
1094 if !bufs.is_empty() {
1095 bufs[0].0.advance(n - accumulated_len)
1101 #[stable(feature = "iovec", since = "1.36.0")]
1102 impl<'a> Deref for IoSliceMut<'a> {
1106 fn deref(&self) -> &[u8] {
1111 #[stable(feature = "iovec", since = "1.36.0")]
1112 impl<'a> DerefMut for IoSliceMut<'a> {
1114 fn deref_mut(&mut self) -> &mut [u8] {
1115 self.0.as_mut_slice()
1119 /// A buffer type used with `Write::write_vectored`.
1121 /// It is semantically a wrapper around an `&[u8]`, but is guaranteed to be
1122 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
1124 #[stable(feature = "iovec", since = "1.36.0")]
1125 #[derive(Copy, Clone)]
1126 #[repr(transparent)]
1127 pub struct IoSlice<'a>(sys::io::IoSlice<'a>);
1129 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1130 unsafe impl<'a> Send for IoSlice<'a> {}
1132 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1133 unsafe impl<'a> Sync for IoSlice<'a> {}
1135 #[stable(feature = "iovec", since = "1.36.0")]
1136 impl<'a> fmt::Debug for IoSlice<'a> {
1137 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
1138 fmt::Debug::fmt(self.0.as_slice(), fmt)
1142 impl<'a> IoSlice<'a> {
1143 /// Creates a new `IoSlice` wrapping a byte slice.
1147 /// Panics on Windows if the slice is larger than 4GB.
1148 #[stable(feature = "iovec", since = "1.36.0")]
1150 pub fn new(buf: &'a [u8]) -> IoSlice<'a> {
1151 IoSlice(sys::io::IoSlice::new(buf))
1154 /// Advance the internal cursor of the slice.
1158 /// Elements in the slice may be modified if the cursor is not advanced to
1159 /// the end of the slice. For example if we have a slice of buffers with 2
1160 /// `IoSlice`s, both of length 8, and we advance the cursor by 10 bytes the
1161 /// first `IoSlice` will be untouched however the second will be modified to
1162 /// remove the first 2 bytes (10 - 8).
1167 /// #![feature(io_slice_advance)]
1169 /// use std::io::IoSlice;
1170 /// use std::ops::Deref;
1172 /// let buf1 = [1; 8];
1173 /// let buf2 = [2; 16];
1174 /// let buf3 = [3; 8];
1175 /// let mut bufs = &mut [
1176 /// IoSlice::new(&buf1),
1177 /// IoSlice::new(&buf2),
1178 /// IoSlice::new(&buf3),
1181 /// // Mark 10 bytes as written.
1182 /// bufs = IoSlice::advance(bufs, 10);
1183 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1184 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1185 #[unstable(feature = "io_slice_advance", issue = "62726")]
1187 pub fn advance<'b>(bufs: &'b mut [IoSlice<'a>], n: usize) -> &'b mut [IoSlice<'a>] {
1188 // Number of buffers to remove.
1190 // Total length of all the to be removed buffers.
1191 let mut accumulated_len = 0;
1192 for buf in bufs.iter() {
1193 if accumulated_len + buf.len() > n {
1196 accumulated_len += buf.len();
1201 let bufs = &mut bufs[remove..];
1202 if !bufs.is_empty() {
1203 bufs[0].0.advance(n - accumulated_len)
1209 #[stable(feature = "iovec", since = "1.36.0")]
1210 impl<'a> Deref for IoSlice<'a> {
1214 fn deref(&self) -> &[u8] {
1219 /// A type used to conditionally initialize buffers passed to `Read` methods.
1220 #[unstable(feature = "read_initializer", issue = "42788")]
1222 pub struct Initializer(bool);
1225 /// Returns a new `Initializer` which will zero out buffers.
1226 #[unstable(feature = "read_initializer", issue = "42788")]
1228 pub fn zeroing() -> Initializer {
1232 /// Returns a new `Initializer` which will not zero out buffers.
1236 /// This may only be called by `Read`ers which guarantee that they will not
1237 /// read from buffers passed to `Read` methods, and that the return value of
1238 /// the method accurately reflects the number of bytes that have been
1239 /// written to the head of the buffer.
1240 #[unstable(feature = "read_initializer", issue = "42788")]
1242 pub unsafe fn nop() -> Initializer {
1246 /// Indicates if a buffer should be initialized.
1247 #[unstable(feature = "read_initializer", issue = "42788")]
1249 pub fn should_initialize(&self) -> bool {
1253 /// Initializes a buffer if necessary.
1254 #[unstable(feature = "read_initializer", issue = "42788")]
1256 pub fn initialize(&self, buf: &mut [u8]) {
1257 if self.should_initialize() {
1258 unsafe { ptr::write_bytes(buf.as_mut_ptr(), 0, buf.len()) }
1263 /// A trait for objects which are byte-oriented sinks.
1265 /// Implementors of the `Write` trait are sometimes called 'writers'.
1267 /// Writers are defined by two required methods, [`write`] and [`flush`]:
1269 /// * The [`write`] method will attempt to write some data into the object,
1270 /// returning how many bytes were successfully written.
1272 /// * The [`flush`] method is useful for adaptors and explicit buffers
1273 /// themselves for ensuring that all buffered data has been pushed out to the
1276 /// Writers are intended to be composable with one another. Many implementors
1277 /// throughout [`std::io`] take and provide types which implement the `Write`
1280 /// [`write`]: Write::write
1281 /// [`flush`]: Write::flush
1282 /// [`std::io`]: self
1287 /// use std::io::prelude::*;
1288 /// use std::fs::File;
1290 /// fn main() -> std::io::Result<()> {
1291 /// let data = b"some bytes";
1293 /// let mut pos = 0;
1294 /// let mut buffer = File::create("foo.txt")?;
1296 /// while pos < data.len() {
1297 /// let bytes_written = buffer.write(&data[pos..])?;
1298 /// pos += bytes_written;
1304 /// The trait also provides convenience methods like [`write_all`], which calls
1305 /// `write` in a loop until its entire input has been written.
1307 /// [`write_all`]: Write::write_all
1308 #[stable(feature = "rust1", since = "1.0.0")]
1309 #[doc(notable_trait)]
1311 /// Write a buffer into this writer, returning how many bytes were written.
1313 /// This function will attempt to write the entire contents of `buf`, but
1314 /// the entire write may not succeed, or the write may also generate an
1315 /// error. A call to `write` represents *at most one* attempt to write to
1316 /// any wrapped object.
1318 /// Calls to `write` are not guaranteed to block waiting for data to be
1319 /// written, and a write which would otherwise block can be indicated through
1320 /// an [`Err`] variant.
1322 /// If the return value is [`Ok(n)`] then it must be guaranteed that
1323 /// `n <= buf.len()`. A return value of `0` typically means that the
1324 /// underlying object is no longer able to accept bytes and will likely not
1325 /// be able to in the future as well, or that the buffer provided is empty.
1329 /// Each call to `write` may generate an I/O error indicating that the
1330 /// operation could not be completed. If an error is returned then no bytes
1331 /// in the buffer were written to this writer.
1333 /// It is **not** considered an error if the entire buffer could not be
1334 /// written to this writer.
1336 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
1337 /// write operation should be retried if there is nothing else to do.
1342 /// use std::io::prelude::*;
1343 /// use std::fs::File;
1345 /// fn main() -> std::io::Result<()> {
1346 /// let mut buffer = File::create("foo.txt")?;
1348 /// // Writes some prefix of the byte string, not necessarily all of it.
1349 /// buffer.write(b"some bytes")?;
1355 #[stable(feature = "rust1", since = "1.0.0")]
1356 fn write(&mut self, buf: &[u8]) -> Result<usize>;
1358 /// Like [`write`], except that it writes from a slice of buffers.
1360 /// Data is copied from each buffer in order, with the final buffer
1361 /// read from possibly being only partially consumed. This method must
1362 /// behave as a call to [`write`] with the buffers concatenated would.
1364 /// The default implementation calls [`write`] with either the first nonempty
1365 /// buffer provided, or an empty one if none exists.
1367 /// [`write`]: Write::write
1368 #[stable(feature = "iovec", since = "1.36.0")]
1369 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
1370 default_write_vectored(|b| self.write(b), bufs)
1373 /// Determines if this `Write`r has an efficient [`write_vectored`]
1376 /// If a `Write`r does not override the default [`write_vectored`]
1377 /// implementation, code using it may want to avoid the method all together
1378 /// and coalesce writes into a single buffer for higher performance.
1380 /// The default implementation returns `false`.
1382 /// [`write_vectored`]: Write::write_vectored
1383 #[unstable(feature = "can_vector", issue = "69941")]
1384 fn is_write_vectored(&self) -> bool {
1388 /// Flush this output stream, ensuring that all intermediately buffered
1389 /// contents reach their destination.
1393 /// It is considered an error if not all bytes could be written due to
1394 /// I/O errors or EOF being reached.
1399 /// use std::io::prelude::*;
1400 /// use std::io::BufWriter;
1401 /// use std::fs::File;
1403 /// fn main() -> std::io::Result<()> {
1404 /// let mut buffer = BufWriter::new(File::create("foo.txt")?);
1406 /// buffer.write_all(b"some bytes")?;
1407 /// buffer.flush()?;
1411 #[stable(feature = "rust1", since = "1.0.0")]
1412 fn flush(&mut self) -> Result<()>;
1414 /// Attempts to write an entire buffer into this writer.
1416 /// This method will continuously call [`write`] until there is no more data
1417 /// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
1418 /// returned. This method will not return until the entire buffer has been
1419 /// successfully written or such an error occurs. The first error that is
1420 /// not of [`ErrorKind::Interrupted`] kind generated from this method will be
1423 /// If the buffer contains no data, this will never call [`write`].
1427 /// This function will return the first error of
1428 /// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
1430 /// [`write`]: Write::write
1435 /// use std::io::prelude::*;
1436 /// use std::fs::File;
1438 /// fn main() -> std::io::Result<()> {
1439 /// let mut buffer = File::create("foo.txt")?;
1441 /// buffer.write_all(b"some bytes")?;
1445 #[stable(feature = "rust1", since = "1.0.0")]
1446 fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
1447 while !buf.is_empty() {
1448 match self.write(buf) {
1450 return Err(Error::new_const(
1451 ErrorKind::WriteZero,
1452 &"failed to write whole buffer",
1455 Ok(n) => buf = &buf[n..],
1456 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
1457 Err(e) => return Err(e),
1463 /// Attempts to write multiple buffers into this writer.
1465 /// This method will continuously call [`write_vectored`] until there is no
1466 /// more data to be written or an error of non-[`ErrorKind::Interrupted`]
1467 /// kind is returned. This method will not return until all buffers have
1468 /// been successfully written or such an error occurs. The first error that
1469 /// is not of [`ErrorKind::Interrupted`] kind generated from this method
1470 /// will be returned.
1472 /// If the buffer contains no data, this will never call [`write_vectored`].
1476 /// Unlike [`write_vectored`], this takes a *mutable* reference to
1477 /// a slice of [`IoSlice`]s, not an immutable one. That's because we need to
1478 /// modify the slice to keep track of the bytes already written.
1480 /// Once this function returns, the contents of `bufs` are unspecified, as
1481 /// this depends on how many calls to [`write_vectored`] were necessary. It is
1482 /// best to understand this function as taking ownership of `bufs` and to
1483 /// not use `bufs` afterwards. The underlying buffers, to which the
1484 /// [`IoSlice`]s point (but not the [`IoSlice`]s themselves), are unchanged and
1487 /// [`write_vectored`]: Write::write_vectored
1492 /// #![feature(write_all_vectored)]
1493 /// # fn main() -> std::io::Result<()> {
1495 /// use std::io::{Write, IoSlice};
1497 /// let mut writer = Vec::new();
1498 /// let bufs = &mut [
1499 /// IoSlice::new(&[1]),
1500 /// IoSlice::new(&[2, 3]),
1501 /// IoSlice::new(&[4, 5, 6]),
1504 /// writer.write_all_vectored(bufs)?;
1505 /// // Note: the contents of `bufs` is now undefined, see the Notes section.
1507 /// assert_eq!(writer, &[1, 2, 3, 4, 5, 6]);
1510 #[unstable(feature = "write_all_vectored", issue = "70436")]
1511 fn write_all_vectored(&mut self, mut bufs: &mut [IoSlice<'_>]) -> Result<()> {
1512 // Guarantee that bufs is empty if it contains no data,
1513 // to avoid calling write_vectored if there is no data to be written.
1514 bufs = IoSlice::advance(bufs, 0);
1515 while !bufs.is_empty() {
1516 match self.write_vectored(bufs) {
1518 return Err(Error::new_const(
1519 ErrorKind::WriteZero,
1520 &"failed to write whole buffer",
1523 Ok(n) => bufs = IoSlice::advance(bufs, n),
1524 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
1525 Err(e) => return Err(e),
1531 /// Writes a formatted string into this writer, returning any error
1534 /// This method is primarily used to interface with the
1535 /// [`format_args!()`] macro, but it is rare that this should
1536 /// explicitly be called. The [`write!()`] macro should be favored to
1537 /// invoke this method instead.
1539 /// This function internally uses the [`write_all`] method on
1540 /// this trait and hence will continuously write data so long as no errors
1541 /// are received. This also means that partial writes are not indicated in
1544 /// [`write_all`]: Write::write_all
1548 /// This function will return any I/O error reported while formatting.
1553 /// use std::io::prelude::*;
1554 /// use std::fs::File;
1556 /// fn main() -> std::io::Result<()> {
1557 /// let mut buffer = File::create("foo.txt")?;
1560 /// write!(buffer, "{:.*}", 2, 1.234567)?;
1561 /// // turns into this:
1562 /// buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?;
1566 #[stable(feature = "rust1", since = "1.0.0")]
1567 fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
1568 // Create a shim which translates a Write to a fmt::Write and saves
1569 // off I/O errors. instead of discarding them
1570 struct Adaptor<'a, T: ?Sized + 'a> {
1575 impl<T: Write + ?Sized> fmt::Write for Adaptor<'_, T> {
1576 fn write_str(&mut self, s: &str) -> fmt::Result {
1577 match self.inner.write_all(s.as_bytes()) {
1580 self.error = Err(e);
1587 let mut output = Adaptor { inner: self, error: Ok(()) };
1588 match fmt::write(&mut output, fmt) {
1591 // check if the error came from the underlying `Write` or not
1592 if output.error.is_err() {
1595 Err(Error::new_const(ErrorKind::Other, &"formatter error"))
1601 /// Creates a "by reference" adaptor for this instance of `Write`.
1603 /// The returned adaptor also implements `Write` and will simply borrow this
1609 /// use std::io::Write;
1610 /// use std::fs::File;
1612 /// fn main() -> std::io::Result<()> {
1613 /// let mut buffer = File::create("foo.txt")?;
1615 /// let reference = buffer.by_ref();
1617 /// // we can use reference just like our original buffer
1618 /// reference.write_all(b"some bytes")?;
1622 #[stable(feature = "rust1", since = "1.0.0")]
1623 fn by_ref(&mut self) -> &mut Self
1631 /// The `Seek` trait provides a cursor which can be moved within a stream of
1634 /// The stream typically has a fixed size, allowing seeking relative to either
1635 /// end or the current offset.
1639 /// [`File`]s implement `Seek`:
1641 /// [`File`]: crate::fs::File
1645 /// use std::io::prelude::*;
1646 /// use std::fs::File;
1647 /// use std::io::SeekFrom;
1649 /// fn main() -> io::Result<()> {
1650 /// let mut f = File::open("foo.txt")?;
1652 /// // move the cursor 42 bytes from the start of the file
1653 /// f.seek(SeekFrom::Start(42))?;
1657 #[stable(feature = "rust1", since = "1.0.0")]
1659 /// Seek to an offset, in bytes, in a stream.
1661 /// A seek beyond the end of a stream is allowed, but behavior is defined
1662 /// by the implementation.
1664 /// If the seek operation completed successfully,
1665 /// this method returns the new position from the start of the stream.
1666 /// That position can be used later with [`SeekFrom::Start`].
1670 /// Seeking can fail, for example because it might involve flushing a buffer.
1672 /// Seeking to a negative offset is considered an error.
1673 #[stable(feature = "rust1", since = "1.0.0")]
1674 fn seek(&mut self, pos: SeekFrom) -> Result<u64>;
1676 /// Rewind to the beginning of a stream.
1678 /// This is a convenience method, equivalent to `seek(SeekFrom::Start(0))`.
1682 /// Rewinding can fail, for example because it might involve flushing a buffer.
1687 /// #![feature(seek_rewind)]
1688 /// use std::io::{Read, Seek, Write};
1689 /// use std::fs::OpenOptions;
1691 /// let mut f = OpenOptions::new()
1695 /// .open("foo.txt").unwrap();
1697 /// let hello = "Hello!\n";
1698 /// write!(f, "{}", hello).unwrap();
1699 /// f.rewind().unwrap();
1701 /// let mut buf = String::new();
1702 /// f.read_to_string(&mut buf).unwrap();
1703 /// assert_eq!(&buf, hello);
1705 #[unstable(feature = "seek_rewind", issue = "85149")]
1706 fn rewind(&mut self) -> Result<()> {
1707 self.seek(SeekFrom::Start(0))?;
1711 /// Returns the length of this stream (in bytes).
1713 /// This method is implemented using up to three seek operations. If this
1714 /// method returns successfully, the seek position is unchanged (i.e. the
1715 /// position before calling this method is the same as afterwards).
1716 /// However, if this method returns an error, the seek position is
1719 /// If you need to obtain the length of *many* streams and you don't care
1720 /// about the seek position afterwards, you can reduce the number of seek
1721 /// operations by simply calling `seek(SeekFrom::End(0))` and using its
1722 /// return value (it is also the stream length).
1724 /// Note that length of a stream can change over time (for example, when
1725 /// data is appended to a file). So calling this method multiple times does
1726 /// not necessarily return the same length each time.
1731 /// #![feature(seek_stream_len)]
1733 /// io::{self, Seek},
1737 /// fn main() -> io::Result<()> {
1738 /// let mut f = File::open("foo.txt")?;
1740 /// let len = f.stream_len()?;
1741 /// println!("The file is currently {} bytes long", len);
1745 #[unstable(feature = "seek_stream_len", issue = "59359")]
1746 fn stream_len(&mut self) -> Result<u64> {
1747 let old_pos = self.stream_position()?;
1748 let len = self.seek(SeekFrom::End(0))?;
1750 // Avoid seeking a third time when we were already at the end of the
1751 // stream. The branch is usually way cheaper than a seek operation.
1753 self.seek(SeekFrom::Start(old_pos))?;
1759 /// Returns the current seek position from the start of the stream.
1761 /// This is equivalent to `self.seek(SeekFrom::Current(0))`.
1767 /// io::{self, BufRead, BufReader, Seek},
1771 /// fn main() -> io::Result<()> {
1772 /// let mut f = BufReader::new(File::open("foo.txt")?);
1774 /// let before = f.stream_position()?;
1775 /// f.read_line(&mut String::new())?;
1776 /// let after = f.stream_position()?;
1778 /// println!("The first line was {} bytes long", after - before);
1782 #[stable(feature = "seek_convenience", since = "1.51.0")]
1783 fn stream_position(&mut self) -> Result<u64> {
1784 self.seek(SeekFrom::Current(0))
1788 /// Enumeration of possible methods to seek within an I/O object.
1790 /// It is used by the [`Seek`] trait.
1791 #[derive(Copy, PartialEq, Eq, Clone, Debug)]
1792 #[stable(feature = "rust1", since = "1.0.0")]
1794 /// Sets the offset to the provided number of bytes.
1795 #[stable(feature = "rust1", since = "1.0.0")]
1796 Start(#[stable(feature = "rust1", since = "1.0.0")] u64),
1798 /// Sets the offset to the size of this object plus the specified number of
1801 /// It is possible to seek beyond the end of an object, but it's an error to
1802 /// seek before byte 0.
1803 #[stable(feature = "rust1", since = "1.0.0")]
1804 End(#[stable(feature = "rust1", since = "1.0.0")] i64),
1806 /// Sets the offset to the current position plus the specified number of
1809 /// It is possible to seek beyond the end of an object, but it's an error to
1810 /// seek before byte 0.
1811 #[stable(feature = "rust1", since = "1.0.0")]
1812 Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
1815 fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
1818 let (done, used) = {
1819 let available = match r.fill_buf() {
1821 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
1822 Err(e) => return Err(e),
1824 match memchr::memchr(delim, available) {
1826 buf.extend_from_slice(&available[..=i]);
1830 buf.extend_from_slice(available);
1831 (false, available.len())
1837 if done || used == 0 {
1843 /// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
1844 /// to perform extra ways of reading.
1846 /// For example, reading line-by-line is inefficient without using a buffer, so
1847 /// if you want to read by line, you'll need `BufRead`, which includes a
1848 /// [`read_line`] method as well as a [`lines`] iterator.
1852 /// A locked standard input implements `BufRead`:
1856 /// use std::io::prelude::*;
1858 /// let stdin = io::stdin();
1859 /// for line in stdin.lock().lines() {
1860 /// println!("{}", line.unwrap());
1864 /// If you have something that implements [`Read`], you can use the [`BufReader`
1865 /// type][`BufReader`] to turn it into a `BufRead`.
1867 /// For example, [`File`] implements [`Read`], but not `BufRead`.
1868 /// [`BufReader`] to the rescue!
1870 /// [`File`]: crate::fs::File
1871 /// [`read_line`]: BufRead::read_line
1872 /// [`lines`]: BufRead::lines
1875 /// use std::io::{self, BufReader};
1876 /// use std::io::prelude::*;
1877 /// use std::fs::File;
1879 /// fn main() -> io::Result<()> {
1880 /// let f = File::open("foo.txt")?;
1881 /// let f = BufReader::new(f);
1883 /// for line in f.lines() {
1884 /// println!("{}", line.unwrap());
1890 #[stable(feature = "rust1", since = "1.0.0")]
1891 pub trait BufRead: Read {
1892 /// Returns the contents of the internal buffer, filling it with more data
1893 /// from the inner reader if it is empty.
1895 /// This function is a lower-level call. It needs to be paired with the
1896 /// [`consume`] method to function properly. When calling this
1897 /// method, none of the contents will be "read" in the sense that later
1898 /// calling `read` may return the same contents. As such, [`consume`] must
1899 /// be called with the number of bytes that are consumed from this buffer to
1900 /// ensure that the bytes are never returned twice.
1902 /// [`consume`]: BufRead::consume
1904 /// An empty buffer returned indicates that the stream has reached EOF.
1908 /// This function will return an I/O error if the underlying reader was
1909 /// read, but returned an error.
1913 /// A locked standard input implements `BufRead`:
1917 /// use std::io::prelude::*;
1919 /// let stdin = io::stdin();
1920 /// let mut stdin = stdin.lock();
1922 /// let buffer = stdin.fill_buf().unwrap();
1924 /// // work with buffer
1925 /// println!("{:?}", buffer);
1927 /// // ensure the bytes we worked with aren't returned again later
1928 /// let length = buffer.len();
1929 /// stdin.consume(length);
1931 #[stable(feature = "rust1", since = "1.0.0")]
1932 fn fill_buf(&mut self) -> Result<&[u8]>;
1934 /// Tells this buffer that `amt` bytes have been consumed from the buffer,
1935 /// so they should no longer be returned in calls to `read`.
1937 /// This function is a lower-level call. It needs to be paired with the
1938 /// [`fill_buf`] method to function properly. This function does
1939 /// not perform any I/O, it simply informs this object that some amount of
1940 /// its buffer, returned from [`fill_buf`], has been consumed and should
1941 /// no longer be returned. As such, this function may do odd things if
1942 /// [`fill_buf`] isn't called before calling it.
1944 /// The `amt` must be `<=` the number of bytes in the buffer returned by
1949 /// Since `consume()` is meant to be used with [`fill_buf`],
1950 /// that method's example includes an example of `consume()`.
1952 /// [`fill_buf`]: BufRead::fill_buf
1953 #[stable(feature = "rust1", since = "1.0.0")]
1954 fn consume(&mut self, amt: usize);
1956 /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
1958 /// This function will read bytes from the underlying stream until the
1959 /// delimiter or EOF is found. Once found, all bytes up to, and including,
1960 /// the delimiter (if found) will be appended to `buf`.
1962 /// If successful, this function will return the total number of bytes read.
1964 /// This function is blocking and should be used carefully: it is possible for
1965 /// an attacker to continuously send bytes without ever sending the delimiter
1970 /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
1971 /// will otherwise return any errors returned by [`fill_buf`].
1973 /// If an I/O error is encountered then all bytes read so far will be
1974 /// present in `buf` and its length will have been adjusted appropriately.
1976 /// [`fill_buf`]: BufRead::fill_buf
1980 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1981 /// this example, we use [`Cursor`] to read all the bytes in a byte slice
1982 /// in hyphen delimited segments:
1985 /// use std::io::{self, BufRead};
1987 /// let mut cursor = io::Cursor::new(b"lorem-ipsum");
1988 /// let mut buf = vec![];
1990 /// // cursor is at 'l'
1991 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1992 /// .expect("reading from cursor won't fail");
1993 /// assert_eq!(num_bytes, 6);
1994 /// assert_eq!(buf, b"lorem-");
1997 /// // cursor is at 'i'
1998 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1999 /// .expect("reading from cursor won't fail");
2000 /// assert_eq!(num_bytes, 5);
2001 /// assert_eq!(buf, b"ipsum");
2004 /// // cursor is at EOF
2005 /// let num_bytes = cursor.read_until(b'-', &mut buf)
2006 /// .expect("reading from cursor won't fail");
2007 /// assert_eq!(num_bytes, 0);
2008 /// assert_eq!(buf, b"");
2010 #[stable(feature = "rust1", since = "1.0.0")]
2011 fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
2012 read_until(self, byte, buf)
2015 /// Read all bytes until a newline (the `0xA` byte) is reached, and append
2016 /// them to the provided buffer.
2018 /// This function will read bytes from the underlying stream until the
2019 /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes
2020 /// up to, and including, the delimiter (if found) will be appended to
2023 /// If successful, this function will return the total number of bytes read.
2025 /// If this function returns [`Ok(0)`], the stream has reached EOF.
2027 /// This function is blocking and should be used carefully: it is possible for
2028 /// an attacker to continuously send bytes without ever sending a newline
2035 /// This function has the same error semantics as [`read_until`] and will
2036 /// also return an error if the read bytes are not valid UTF-8. If an I/O
2037 /// error is encountered then `buf` may contain some bytes already read in
2038 /// the event that all data read so far was valid UTF-8.
2040 /// [`read_until`]: BufRead::read_until
2044 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2045 /// this example, we use [`Cursor`] to read all the lines in a byte slice:
2048 /// use std::io::{self, BufRead};
2050 /// let mut cursor = io::Cursor::new(b"foo\nbar");
2051 /// let mut buf = String::new();
2053 /// // cursor is at 'f'
2054 /// let num_bytes = cursor.read_line(&mut buf)
2055 /// .expect("reading from cursor won't fail");
2056 /// assert_eq!(num_bytes, 4);
2057 /// assert_eq!(buf, "foo\n");
2060 /// // cursor is at 'b'
2061 /// let num_bytes = cursor.read_line(&mut buf)
2062 /// .expect("reading from cursor won't fail");
2063 /// assert_eq!(num_bytes, 3);
2064 /// assert_eq!(buf, "bar");
2067 /// // cursor is at EOF
2068 /// let num_bytes = cursor.read_line(&mut buf)
2069 /// .expect("reading from cursor won't fail");
2070 /// assert_eq!(num_bytes, 0);
2071 /// assert_eq!(buf, "");
2073 #[stable(feature = "rust1", since = "1.0.0")]
2074 fn read_line(&mut self, buf: &mut String) -> Result<usize> {
2075 // Note that we are not calling the `.read_until` method here, but
2076 // rather our hardcoded implementation. For more details as to why, see
2077 // the comments in `read_to_end`.
2078 append_to_string(buf, |b| read_until(self, b'\n', b))
2081 /// Returns an iterator over the contents of this reader split on the byte
2084 /// The iterator returned from this function will return instances of
2085 /// [`io::Result`]`<`[`Vec<u8>`]`>`. Each vector returned will *not* have
2086 /// the delimiter byte at the end.
2088 /// This function will yield errors whenever [`read_until`] would have
2089 /// also yielded an error.
2091 /// [`io::Result`]: self::Result
2092 /// [`read_until`]: BufRead::read_until
2096 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2097 /// this example, we use [`Cursor`] to iterate over all hyphen delimited
2098 /// segments in a byte slice
2101 /// use std::io::{self, BufRead};
2103 /// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
2105 /// let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
2106 /// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
2107 /// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
2108 /// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
2109 /// assert_eq!(split_iter.next(), None);
2111 #[stable(feature = "rust1", since = "1.0.0")]
2112 fn split(self, byte: u8) -> Split<Self>
2116 Split { buf: self, delim: byte }
2119 /// Returns an iterator over the lines of this reader.
2121 /// The iterator returned from this function will yield instances of
2122 /// [`io::Result`]`<`[`String`]`>`. Each string returned will *not* have a newline
2123 /// byte (the `0xA` byte) or `CRLF` (`0xD`, `0xA` bytes) at the end.
2125 /// [`io::Result`]: self::Result
2129 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2130 /// this example, we use [`Cursor`] to iterate over all the lines in a byte
2134 /// use std::io::{self, BufRead};
2136 /// let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");
2138 /// let mut lines_iter = cursor.lines().map(|l| l.unwrap());
2139 /// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
2140 /// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
2141 /// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
2142 /// assert_eq!(lines_iter.next(), None);
2147 /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
2148 #[stable(feature = "rust1", since = "1.0.0")]
2149 fn lines(self) -> Lines<Self>
2157 /// Adaptor to chain together two readers.
2159 /// This struct is generally created by calling [`chain`] on a reader.
2160 /// Please see the documentation of [`chain`] for more details.
2162 /// [`chain`]: Read::chain
2163 #[stable(feature = "rust1", since = "1.0.0")]
2165 pub struct Chain<T, U> {
2171 impl<T, U> Chain<T, U> {
2172 /// Consumes the `Chain`, returning the wrapped readers.
2178 /// use std::io::prelude::*;
2179 /// use std::fs::File;
2181 /// fn main() -> io::Result<()> {
2182 /// let mut foo_file = File::open("foo.txt")?;
2183 /// let mut bar_file = File::open("bar.txt")?;
2185 /// let chain = foo_file.chain(bar_file);
2186 /// let (foo_file, bar_file) = chain.into_inner();
2190 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2191 pub fn into_inner(self) -> (T, U) {
2192 (self.first, self.second)
2195 /// Gets references to the underlying readers in this `Chain`.
2201 /// use std::io::prelude::*;
2202 /// use std::fs::File;
2204 /// fn main() -> io::Result<()> {
2205 /// let mut foo_file = File::open("foo.txt")?;
2206 /// let mut bar_file = File::open("bar.txt")?;
2208 /// let chain = foo_file.chain(bar_file);
2209 /// let (foo_file, bar_file) = chain.get_ref();
2213 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2214 pub fn get_ref(&self) -> (&T, &U) {
2215 (&self.first, &self.second)
2218 /// Gets mutable references to the underlying readers in this `Chain`.
2220 /// Care should be taken to avoid modifying the internal I/O state of the
2221 /// underlying readers as doing so may corrupt the internal state of this
2228 /// use std::io::prelude::*;
2229 /// use std::fs::File;
2231 /// fn main() -> io::Result<()> {
2232 /// let mut foo_file = File::open("foo.txt")?;
2233 /// let mut bar_file = File::open("bar.txt")?;
2235 /// let mut chain = foo_file.chain(bar_file);
2236 /// let (foo_file, bar_file) = chain.get_mut();
2240 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2241 pub fn get_mut(&mut self) -> (&mut T, &mut U) {
2242 (&mut self.first, &mut self.second)
2246 #[stable(feature = "rust1", since = "1.0.0")]
2247 impl<T: Read, U: Read> Read for Chain<T, U> {
2248 fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
2249 if !self.done_first {
2250 match self.first.read(buf)? {
2251 0 if !buf.is_empty() => self.done_first = true,
2255 self.second.read(buf)
2258 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
2259 if !self.done_first {
2260 match self.first.read_vectored(bufs)? {
2261 0 if bufs.iter().any(|b| !b.is_empty()) => self.done_first = true,
2265 self.second.read_vectored(bufs)
2268 unsafe fn initializer(&self) -> Initializer {
2269 let initializer = self.first.initializer();
2270 if initializer.should_initialize() { initializer } else { self.second.initializer() }
2274 #[stable(feature = "chain_bufread", since = "1.9.0")]
2275 impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
2276 fn fill_buf(&mut self) -> Result<&[u8]> {
2277 if !self.done_first {
2278 match self.first.fill_buf()? {
2279 buf if buf.is_empty() => {
2280 self.done_first = true;
2282 buf => return Ok(buf),
2285 self.second.fill_buf()
2288 fn consume(&mut self, amt: usize) {
2289 if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
2293 impl<T, U> SizeHint for Chain<T, U> {
2294 fn lower_bound(&self) -> usize {
2295 SizeHint::lower_bound(&self.first) + SizeHint::lower_bound(&self.second)
2298 fn upper_bound(&self) -> Option<usize> {
2299 match (SizeHint::upper_bound(&self.first), SizeHint::upper_bound(&self.second)) {
2300 (Some(first), Some(second)) => Some(first + second),
2306 /// Reader adaptor which limits the bytes read from an underlying reader.
2308 /// This struct is generally created by calling [`take`] on a reader.
2309 /// Please see the documentation of [`take`] for more details.
2311 /// [`take`]: Read::take
2312 #[stable(feature = "rust1", since = "1.0.0")]
2314 pub struct Take<T> {
2320 /// Returns the number of bytes that can be read before this instance will
2325 /// This instance may reach `EOF` after reading fewer bytes than indicated by
2326 /// this method if the underlying [`Read`] instance reaches EOF.
2332 /// use std::io::prelude::*;
2333 /// use std::fs::File;
2335 /// fn main() -> io::Result<()> {
2336 /// let f = File::open("foo.txt")?;
2338 /// // read at most five bytes
2339 /// let handle = f.take(5);
2341 /// println!("limit: {}", handle.limit());
2345 #[stable(feature = "rust1", since = "1.0.0")]
2346 pub fn limit(&self) -> u64 {
2350 /// Sets the number of bytes that can be read before this instance will
2351 /// return EOF. This is the same as constructing a new `Take` instance, so
2352 /// the amount of bytes read and the previous limit value don't matter when
2353 /// calling this method.
2359 /// use std::io::prelude::*;
2360 /// use std::fs::File;
2362 /// fn main() -> io::Result<()> {
2363 /// let f = File::open("foo.txt")?;
2365 /// // read at most five bytes
2366 /// let mut handle = f.take(5);
2367 /// handle.set_limit(10);
2369 /// assert_eq!(handle.limit(), 10);
2373 #[stable(feature = "take_set_limit", since = "1.27.0")]
2374 pub fn set_limit(&mut self, limit: u64) {
2378 /// Consumes the `Take`, returning the wrapped reader.
2384 /// use std::io::prelude::*;
2385 /// use std::fs::File;
2387 /// fn main() -> io::Result<()> {
2388 /// let mut file = File::open("foo.txt")?;
2390 /// let mut buffer = [0; 5];
2391 /// let mut handle = file.take(5);
2392 /// handle.read(&mut buffer)?;
2394 /// let file = handle.into_inner();
2398 #[stable(feature = "io_take_into_inner", since = "1.15.0")]
2399 pub fn into_inner(self) -> T {
2403 /// Gets a reference to the underlying reader.
2409 /// use std::io::prelude::*;
2410 /// use std::fs::File;
2412 /// fn main() -> io::Result<()> {
2413 /// let mut file = File::open("foo.txt")?;
2415 /// let mut buffer = [0; 5];
2416 /// let mut handle = file.take(5);
2417 /// handle.read(&mut buffer)?;
2419 /// let file = handle.get_ref();
2423 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2424 pub fn get_ref(&self) -> &T {
2428 /// Gets a mutable reference to the underlying reader.
2430 /// Care should be taken to avoid modifying the internal I/O state of the
2431 /// underlying reader as doing so may corrupt the internal limit of this
2438 /// use std::io::prelude::*;
2439 /// use std::fs::File;
2441 /// fn main() -> io::Result<()> {
2442 /// let mut file = File::open("foo.txt")?;
2444 /// let mut buffer = [0; 5];
2445 /// let mut handle = file.take(5);
2446 /// handle.read(&mut buffer)?;
2448 /// let file = handle.get_mut();
2452 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2453 pub fn get_mut(&mut self) -> &mut T {
2458 #[stable(feature = "rust1", since = "1.0.0")]
2459 impl<T: Read> Read for Take<T> {
2460 fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
2461 // Don't call into inner reader at all at EOF because it may still block
2462 if self.limit == 0 {
2466 let max = cmp::min(buf.len() as u64, self.limit) as usize;
2467 let n = self.inner.read(&mut buf[..max])?;
2468 self.limit -= n as u64;
2472 unsafe fn initializer(&self) -> Initializer {
2473 self.inner.initializer()
2476 fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
2477 // Pass in a reservation_size closure that respects the current value
2478 // of limit for each read. If we hit the read limit, this prevents the
2479 // final zero-byte read from allocating again.
2480 read_to_end_with_reservation(self, buf, |self_| cmp::min(self_.limit, 32) as usize)
2484 #[stable(feature = "rust1", since = "1.0.0")]
2485 impl<T: BufRead> BufRead for Take<T> {
2486 fn fill_buf(&mut self) -> Result<&[u8]> {
2487 // Don't call into inner reader at all at EOF because it may still block
2488 if self.limit == 0 {
2492 let buf = self.inner.fill_buf()?;
2493 let cap = cmp::min(buf.len() as u64, self.limit) as usize;
2497 fn consume(&mut self, amt: usize) {
2498 // Don't let callers reset the limit by passing an overlarge value
2499 let amt = cmp::min(amt as u64, self.limit) as usize;
2500 self.limit -= amt as u64;
2501 self.inner.consume(amt);
2505 /// An iterator over `u8` values of a reader.
2507 /// This struct is generally created by calling [`bytes`] on a reader.
2508 /// Please see the documentation of [`bytes`] for more details.
2510 /// [`bytes`]: Read::bytes
2511 #[stable(feature = "rust1", since = "1.0.0")]
2513 pub struct Bytes<R> {
2517 #[stable(feature = "rust1", since = "1.0.0")]
2518 impl<R: Read> Iterator for Bytes<R> {
2519 type Item = Result<u8>;
2521 fn next(&mut self) -> Option<Result<u8>> {
2524 return match self.inner.read(slice::from_mut(&mut byte)) {
2526 Ok(..) => Some(Ok(byte)),
2527 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
2528 Err(e) => Some(Err(e)),
2533 fn size_hint(&self) -> (usize, Option<usize>) {
2534 SizeHint::size_hint(&self.inner)
2539 fn lower_bound(&self) -> usize;
2541 fn upper_bound(&self) -> Option<usize>;
2543 fn size_hint(&self) -> (usize, Option<usize>) {
2544 (self.lower_bound(), self.upper_bound())
2548 impl<T> SizeHint for T {
2549 default fn lower_bound(&self) -> usize {
2553 default fn upper_bound(&self) -> Option<usize> {
2558 /// An iterator over the contents of an instance of `BufRead` split on a
2559 /// particular byte.
2561 /// This struct is generally created by calling [`split`] on a `BufRead`.
2562 /// Please see the documentation of [`split`] for more details.
2564 /// [`split`]: BufRead::split
2565 #[stable(feature = "rust1", since = "1.0.0")]
2567 pub struct Split<B> {
2572 #[stable(feature = "rust1", since = "1.0.0")]
2573 impl<B: BufRead> Iterator for Split<B> {
2574 type Item = Result<Vec<u8>>;
2576 fn next(&mut self) -> Option<Result<Vec<u8>>> {
2577 let mut buf = Vec::new();
2578 match self.buf.read_until(self.delim, &mut buf) {
2581 if buf[buf.len() - 1] == self.delim {
2586 Err(e) => Some(Err(e)),
2591 /// An iterator over the lines of an instance of `BufRead`.
2593 /// This struct is generally created by calling [`lines`] on a `BufRead`.
2594 /// Please see the documentation of [`lines`] for more details.
2596 /// [`lines`]: BufRead::lines
2597 #[stable(feature = "rust1", since = "1.0.0")]
2599 pub struct Lines<B> {
2603 #[stable(feature = "rust1", since = "1.0.0")]
2604 impl<B: BufRead> Iterator for Lines<B> {
2605 type Item = Result<String>;
2607 fn next(&mut self) -> Option<Result<String>> {
2608 let mut buf = String::new();
2609 match self.buf.read_line(&mut buf) {
2612 if buf.ends_with('\n') {
2614 if buf.ends_with('\r') {
2620 Err(e) => Some(Err(e)),