library/std/src/io/copy.rs

   1 use super::{BorrowedBuf, BufWriter, ErrorKind, Read, Result, Write, DEFAULT_BUF_SIZE};
   2 use crate::mem::MaybeUninit;
   3
   4 /// Copies the entire contents of a reader into a writer.
   5 ///
   6 /// This function will continuously read data from `reader` and then
   7 /// write it into `writer` in a streaming fashion until `reader`
   8 /// returns EOF.
   9 ///
  10 /// On success, the total number of bytes that were copied from
  11 /// `reader` to `writer` is returned.
  12 ///
  13 /// If you’re wanting to copy the contents of one file to another and you’re
  14 /// working with filesystem paths, see the [`fs::copy`] function.
  15 ///
  16 /// [`fs::copy`]: crate::fs::copy
  17 ///
  18 /// # Errors
  19 ///
  20 /// This function will return an error immediately if any call to [`read`] or
  21 /// [`write`] returns an error. All instances of [`ErrorKind::Interrupted`] are
  22 /// handled by this function and the underlying operation is retried.
  23 ///
  24 /// [`read`]: Read::read
  25 /// [`write`]: Write::write
  26 ///
  27 /// # Examples
  28 ///
  29 /// ```
  30 /// use std::io;
  31 ///
  32 /// fn main() -> io::Result<()> {
  33 ///     let mut reader: &[u8] = b"hello";
  34 ///     let mut writer: Vec<u8> = vec![];
  35 ///
  36 ///     io::copy(&mut reader, &mut writer)?;
  37 ///
  38 ///     assert_eq!(&b"hello"[..], &writer[..]);
  39 ///     Ok(())
  40 /// }
  41 /// ```
  42 ///
  43 /// # Platform-specific behavior
  44 ///
  45 /// On Linux (including Android), this function uses `copy_file_range(2)`,
  46 /// `sendfile(2)` or `splice(2)` syscalls to move data directly between file
  47 /// descriptors if possible.
  48 ///
  49 /// Note that platform-specific behavior [may change in the future][changes].
  50 ///
  51 /// [changes]: crate::io#platform-specific-behavior
  52 #[stable(feature = "rust1", since = "1.0.0")]
  53 pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
  54 where
  55     R: Read,
  56     W: Write,
  57 {
  58     cfg_if::cfg_if! {
  59         if #[cfg(any(target_os = "linux", target_os = "android"))] {
  60             crate::sys::kernel_copy::copy_spec(reader, writer)
  61         } else {
  62             generic_copy(reader, writer)
  63         }
  64     }
  65 }
  66
  67 /// The userspace read-write-loop implementation of `io::copy` that is used when
  68 /// OS-specific specializations for copy offloading are not available or not applicable.
  69 pub(crate) fn generic_copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
  70 where
  71     R: Read,
  72     W: Write,
  73 {
  74     BufferedCopySpec::copy_to(reader, writer)
  75 }
  76
  77 /// Specialization of the read-write loop that either uses a stack buffer
  78 /// or reuses the internal buffer of a BufWriter
  79 trait BufferedCopySpec: Write {
  80     fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64>;
  81 }
  82
  83 impl<W: Write + ?Sized> BufferedCopySpec for W {
  84     default fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
  85         stack_buffer_copy(reader, writer)
  86     }
  87 }
  88
  89 impl<I: Write> BufferedCopySpec for BufWriter<I> {
  90     fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
  91         if writer.capacity() < DEFAULT_BUF_SIZE {
  92             return stack_buffer_copy(reader, writer);
  93         }
  94
  95         let mut len = 0;
  96         let mut init = 0;
  97
  98         loop {
  99             let buf = writer.buffer_mut();
 100             let mut read_buf: BorrowedBuf<'_> = buf.spare_capacity_mut().into();
 101
 102             unsafe {
 103                 // SAFETY: init is either 0 or the init_len from the previous iteration.
 104                 read_buf.set_init(init);
 105             }
 106
 107             if read_buf.capacity() >= DEFAULT_BUF_SIZE {
 108                 let mut cursor = read_buf.unfilled();
 109                 match reader.read_buf(cursor.reborrow()) {
 110                     Ok(()) => {
 111                         let bytes_read = cursor.written();
 112
 113                         if bytes_read == 0 {
 114                             return Ok(len);
 115                         }
 116
 117                         init = read_buf.init_len() - bytes_read;
 118                         len += bytes_read as u64;
 119
 120                         // SAFETY: BorrowedBuf guarantees all of its filled bytes are init
 121                         unsafe { buf.set_len(buf.len() + bytes_read) };
 122
 123                         // Read again if the buffer still has enough capacity, as BufWriter itself would do
 124                         // This will occur if the reader returns short reads
 125                     }
 126                     Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
 127                     Err(e) => return Err(e),
 128                 }
 129             } else {
 130                 writer.flush_buf()?;
 131                 init = 0;
 132             }
 133         }
 134     }
 135 }
 136
 137 fn stack_buffer_copy<R: Read + ?Sized, W: Write + ?Sized>(
 138     reader: &mut R,
 139     writer: &mut W,
 140 ) -> Result<u64> {
 141     let buf: &mut [_] = &mut [MaybeUninit::uninit(); DEFAULT_BUF_SIZE];
 142     let mut buf: BorrowedBuf<'_> = buf.into();
 143
 144     let mut len = 0;
 145
 146     loop {
 147         match reader.read_buf(buf.unfilled()) {
 148             Ok(()) => {}
 149             Err(e) if e.kind() == ErrorKind::Interrupted => continue,
 150             Err(e) => return Err(e),
 151         };
 152
 153         if buf.filled().is_empty() {
 154             break;
 155         }
 156
 157         len += buf.filled().len() as u64;
 158         writer.write_all(buf.filled())?;
 159         buf.clear();
 160     }
 161
 162     Ok(len)
 163 }