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// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package poll
import (
var copyFileRangeSupported int32 = 1 // accessed atomically
const maxCopyFileRangeRound = 1 << 30
// CopyFileRange copies at most remain bytes of data from src to dst, using
// the copy_file_range system call. dst and src must refer to regular files.
func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) {
if atomic.LoadInt32(&copyFileRangeSupported) == 0 {
return 0, false, nil
for remain > 0 {
max := remain
if max > maxCopyFileRangeRound {
max = maxCopyFileRangeRound
n, err := copyFileRange(dst, src, int(max))
switch err {
case syscall.ENOSYS:
// copy_file_range(2) was introduced in Linux 4.5.
// Go supports Linux >= 2.6.33, so the system call
// may not be present.
// If we see ENOSYS, we have certainly not transfered
// any data, so we can tell the caller that we
// couldn't handle the transfer and let them fall
// back to more generic code.
// Seeing ENOSYS also means that we will not try to
// use copy_file_range(2) again.
atomic.StoreInt32(&copyFileRangeSupported, 0)
return 0, false, nil
case syscall.EXDEV, syscall.EINVAL:
// Prior to Linux 5.3, it was not possible to
// copy_file_range across file systems. Similarly to
// the ENOSYS case above, if we see EXDEV, we have
// not transfered any data, and we can let the caller
// fall back to generic code.
// As for EINVAL, that is what we see if, for example,
// dst or src refer to a pipe rather than a regular
// file. This is another case where no data has been
// transfered, so we consider it unhandled.
return 0, false, nil
case nil:
if n == 0 {
// src is at EOF, which means we are done.
return written, true, nil
remain -= n
written += n
return written, true, err
return written, true, nil
// copyFileRange performs one round of copy_file_range(2).
func copyFileRange(dst, src *FD, max int) (written int64, err error) {
// The signature of copy_file_range(2) is:
// ssize_t copy_file_range(int fd_in, loff_t *off_in,
// int fd_out, loff_t *off_out,
// size_t len, unsigned int flags);
// Note that in the call to unix.CopyFileRange below, we use nil
// values for off_in and off_out. For the system call, this means
// "use and update the file offsets". That is why we must acquire
// locks for both file descriptors (and why this whole machinery is
// in the internal/poll package to begin with).
if err := dst.writeLock(); err != nil {
return 0, err
defer dst.writeUnlock()
if err := src.readLock(); err != nil {
return 0, err
defer src.readUnlock()
var n int
for {
n, err = unix.CopyFileRange(src.Sysfd, nil, dst.Sysfd, nil, max, 0)
if err != syscall.EINTR {
return int64(n), err