util.rs.html -- source
Description: Source to the Rust file `library/std/src/io/util.rs`.
Keywords: rust, rustlang, rust-lang

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    #![allow(missing_copy_implementations)]

    use crate::fmt;
    use crate::io::{self, BufRead, ErrorKind, Initializer, IoSlice, IoSliceMut, Read, Write};
    use crate::mem::MaybeUninit;

    /// Copies the entire contents of a reader into a writer.
    ///
    /// This function will continuously read data from `reader` and then
    /// write it into `writer` in a streaming fashion until `reader`
    /// returns EOF.
    ///
    /// On success, the total number of bytes that were copied from
    /// `reader` to `writer` is returned.
    ///
    /// If you’re wanting to copy the contents of one file to another and you’re
    /// working with filesystem paths, see the [`fs::copy`] function.
    ///
    /// [`fs::copy`]: crate::fs::copy
    ///
    /// # Errors
    ///
    /// This function will return an error immediately if any call to [`read`] or
    /// [`write`] returns an error. All instances of [`ErrorKind::Interrupted`] are
    /// handled by this function and the underlying operation is retried.
    ///
    /// [`read`]: Read::read
    /// [`write`]: Write::write
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut reader: &[u8] = b"hello";
    ///     let mut writer: Vec<u8> = vec![];
    ///
    ///     io::copy(&mut reader, &mut writer)?;
    ///
    ///     assert_eq!(&b"hello"[..], &writer[..]);
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> io::Result<u64>
    where
        R: Read,
        W: Write,
    {
        let mut buf = MaybeUninit::<[u8; super::DEFAULT_BUF_SIZE]>::uninit();
        // FIXME(#53491): This is calling `get_mut` and `get_ref` on an uninitialized
        // `MaybeUninit`. Revisit this once we decided whether that is valid or not.
        // This is still technically undefined behavior due to creating a reference
        // to uninitialized data, but within libstd we can rely on more guarantees
        // than if this code were in an external lib.
        unsafe {
            reader.initializer().initialize(buf.get_mut());
        }

        let mut written = 0;
        loop {
            let len = match reader.read(unsafe { buf.get_mut() }) {
                Ok(0) => return Ok(written),
                Ok(len) => len,
                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => return Err(e),
            };
            writer.write_all(unsafe { &buf.get_ref()[..len] })?;
            written += len as u64;
        }
    }

    /// A reader which is always at EOF.
    ///
    /// This struct is generally created by calling [`empty()`]. Please see
    /// the documentation of [`empty()`] for more details.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Empty {
        _priv: (),
    }

    /// Constructs a new handle to an empty reader.
    ///
    /// All reads from the returned reader will return [`Ok`]`(0)`.
    ///
    /// # Examples
    ///
    /// A slightly sad example of not reading anything into a buffer:
    ///
    /// ```
    /// use std::io::{self, Read};
    ///
    /// let mut buffer = String::new();
    /// io::empty().read_to_string(&mut buffer).unwrap();
    /// assert!(buffer.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn empty() -> Empty {
        Empty { _priv: () }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Read for Empty {
        #[inline]
        fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
            Ok(0)
        }

        #[inline]
        unsafe fn initializer(&self) -> Initializer {
            Initializer::nop()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl BufRead for Empty {
        #[inline]
        fn fill_buf(&mut self) -> io::Result<&[u8]> {
            Ok(&[])
        }
        #[inline]
        fn consume(&mut self, _n: usize) {}
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl fmt::Debug for Empty {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Empty { .. }")
        }
    }

    /// A reader which yields one byte over and over and over and over and over and...
    ///
    /// This struct is generally created by calling [`repeat()`]. Please
    /// see the documentation of [`repeat()`] for more details.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Repeat {
        byte: u8,
    }

    /// Creates an instance of a reader that infinitely repeats one byte.
    ///
    /// All reads from this reader will succeed by filling the specified buffer with
    /// the given byte.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{self, Read};
    ///
    /// let mut buffer = [0; 3];
    /// io::repeat(0b101).read_exact(&mut buffer).unwrap();
    /// assert_eq!(buffer, [0b101, 0b101, 0b101]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn repeat(byte: u8) -> Repeat {
        Repeat { byte }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Read for Repeat {
        #[inline]
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            for slot in &mut *buf {
                *slot = self.byte;
            }
            Ok(buf.len())
        }

        #[inline]
        fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
            let mut nwritten = 0;
            for buf in bufs {
                nwritten += self.read(buf)?;
            }
            Ok(nwritten)
        }

        #[inline]
        fn is_read_vectored(&self) -> bool {
            true
        }

        #[inline]
        unsafe fn initializer(&self) -> Initializer {
            Initializer::nop()
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl fmt::Debug for Repeat {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Repeat { .. }")
        }
    }

    /// A writer which will move data into the void.
    ///
    /// This struct is generally created by calling [`sink`]. Please
    /// see the documentation of [`sink()`] for more details.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Sink {
        _priv: (),
    }

    /// Creates an instance of a writer which will successfully consume all data.
    ///
    /// All calls to [`write`] on the returned instance will return `Ok(buf.len())`
    /// and the contents of the buffer will not be inspected.
    ///
    /// [`write`]: Write::write
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::io::{self, Write};
    ///
    /// let buffer = vec![1, 2, 3, 5, 8];
    /// let num_bytes = io::sink().write(&buffer).unwrap();
    /// assert_eq!(num_bytes, 5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn sink() -> Sink {
        Sink { _priv: () }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Write for Sink {
        #[inline]
        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
            Ok(buf.len())
        }

        #[inline]
        fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
            let total_len = bufs.iter().map(|b| b.len()).sum();
            Ok(total_len)
        }

        #[inline]
        fn is_write_vectored(&self) -> bool {
            true
        }

        #[inline]
        fn flush(&mut self) -> io::Result<()> {
            Ok(())
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl fmt::Debug for Sink {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Sink { .. }")
        }
    }

    #[cfg(test)]
    mod tests {
        use crate::io::prelude::*;
        use crate::io::{copy, empty, repeat, sink};

        #[test]
        fn copy_copies() {
            let mut r = repeat(0).take(4);
            let mut w = sink();
            assert_eq!(copy(&mut r, &mut w).unwrap(), 4);

            let mut r = repeat(0).take(1 << 17);
            assert_eq!(copy(&mut r as &mut dyn Read, &mut w as &mut dyn Write).unwrap(), 1 << 17);
        }

        #[test]
        fn sink_sinks() {
            let mut s = sink();
            assert_eq!(s.write(&[]).unwrap(), 0);
            assert_eq!(s.write(&[0]).unwrap(), 1);
            assert_eq!(s.write(&[0; 1024]).unwrap(), 1024);
            assert_eq!(s.by_ref().write(&[0; 1024]).unwrap(), 1024);
        }

        #[test]
        fn empty_reads() {
            let mut e = empty();
            assert_eq!(e.read(&mut []).unwrap(), 0);
            assert_eq!(e.read(&mut [0]).unwrap(), 0);
            assert_eq!(e.read(&mut [0; 1024]).unwrap(), 0);
            assert_eq!(e.by_ref().read(&mut [0; 1024]).unwrap(), 0);
        }

        #[test]
        fn repeat_repeats() {
            let mut r = repeat(4);
            let mut b = [0; 1024];
            assert_eq!(r.read(&mut b).unwrap(), 1024);
            assert!(b.iter().all(|b| *b == 4));
        }

        #[test]
        fn take_some_bytes() {
            assert_eq!(repeat(4).take(100).bytes().count(), 100);
            assert_eq!(repeat(4).take(100).bytes().next().unwrap().unwrap(), 4);
            assert_eq!(repeat(1).take(10).chain(repeat(2).take(10)).bytes().count(), 20);
        }
    }