1 use super::{BufWriter, ErrorKind, Read, Result, Write, DEFAULT_BUF_SIZE};
2 use crate::mem::MaybeUninit;
4 /// Copies the entire contents of a reader into a writer.
6 /// This function will continuously read data from `reader` and then
7 /// write it into `writer` in a streaming fashion until `reader`
10 /// On success, the total number of bytes that were copied from
11 /// `reader` to `writer` is returned.
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.
16 /// [`fs::copy`]: crate::fs::copy
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.
24 /// [`read`]: Read::read
25 /// [`write`]: Write::write
32 /// fn main() -> io::Result<()> {
33 /// let mut reader: &[u8] = b"hello";
34 /// let mut writer: Vec<u8> = vec![];
36 /// io::copy(&mut reader, &mut writer)?;
38 /// assert_eq!(&b"hello"[..], &writer[..]);
42 #[stable(feature = "rust1", since = "1.0.0")]
43 pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
49 if #[cfg(any(target_os = "linux", target_os = "android"))] {
50 crate::sys::kernel_copy::copy_spec(reader, writer)
52 generic_copy(reader, writer)
57 /// The userspace read-write-loop implementation of `io::copy` that is used when
58 /// OS-specific specializations for copy offloading are not available or not applicable.
59 pub(crate) fn generic_copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
64 BufferedCopySpec::copy_to(reader, writer)
67 /// Specialization of the read-write loop that either uses a stack buffer
68 /// or reuses the internal buffer of a BufWriter
69 trait BufferedCopySpec: Write {
70 fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64>;
73 impl<W: Write + ?Sized> BufferedCopySpec for W {
74 default fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
75 stack_buffer_copy(reader, writer)
79 impl<I: Write> BufferedCopySpec for BufWriter<I> {
80 fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
81 if writer.capacity() < DEFAULT_BUF_SIZE {
82 return stack_buffer_copy(reader, writer);
87 // - This creates a (mut) reference to a slice of
88 // _uninitialized_ integers, which is **undefined behavior**
90 // - Only the standard library gets to soundly "ignore" this,
91 // based on its privileged knowledge of unstable rustc
94 let spare_cap = writer.buffer_mut().spare_capacity_mut();
95 reader.initializer().initialize(MaybeUninit::slice_assume_init_mut(spare_cap));
101 let buf = writer.buffer_mut();
102 let spare_cap = buf.spare_capacity_mut();
104 if spare_cap.len() >= DEFAULT_BUF_SIZE {
105 match reader.read(unsafe { MaybeUninit::slice_assume_init_mut(spare_cap) }) {
106 Ok(0) => return Ok(len), // EOF reached
108 assert!(bytes_read <= spare_cap.len());
109 // Safety: The initializer contract guarantees that either it or `read`
110 // will have initialized these bytes. And we just checked that the number
111 // of bytes is within the buffer capacity.
112 unsafe { buf.set_len(buf.len() + bytes_read) };
113 len += bytes_read as u64;
114 // Read again if the buffer still has enough capacity, as BufWriter itself would do
115 // This will occur if the reader returns short reads
118 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
119 Err(e) => return Err(e),
128 fn stack_buffer_copy<R: Read + ?Sized, W: Write + ?Sized>(
132 let mut buf = MaybeUninit::<[u8; DEFAULT_BUF_SIZE]>::uninit();
135 // - This creates a (mut) reference to a slice of
136 // _uninitialized_ integers, which is **undefined behavior**
138 // - Only the standard library gets to soundly "ignore" this,
139 // based on its privileged knowledge of unstable rustc
142 reader.initializer().initialize(buf.assume_init_mut());
147 let len = match reader.read(unsafe { buf.assume_init_mut() }) {
148 Ok(0) => return Ok(written),
150 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
151 Err(e) => return Err(e),
153 writer.write_all(unsafe { &buf.assume_init_ref()[..len] })?;
154 written += len as u64;