src/os/file_windows.go - go.git - Git at Google

 // Copyright 2009 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 os

 import (
 	"errors"
 	"internal/godebug"
 	"internal/poll"
 	"internal/syscall/windows"
 	"runtime"
 	"sync"
 	"sync/atomic"
 	"syscall"
 	"unsafe"
 )

 // This matches the value in syscall/syscall_windows.go.
 const _UTIME_OMIT = -1

 // file is the real representation of *File.
 // The extra level of indirection ensures that no clients of os
 // can overwrite this data, which could cause the finalizer
 // to close the wrong file descriptor.
 type file struct {
 	pfd        poll.FD
 	name       string
 	dirinfo    atomic.Pointer[dirInfo] // nil unless directory being read
 	appendMode bool                    // whether file is opened for appending
 }

 // Fd returns the Windows handle referencing the open file.
 // If f is closed, the file descriptor becomes invalid.
 // If f is garbage collected, a finalizer may close the file descriptor,
 // making it invalid; see [runtime.SetFinalizer] for more information on when
 // a finalizer might be run. On Unix systems this will cause the [File.SetDeadline]
 // methods to stop working.
 func (file *File) Fd() uintptr {
 	if file == nil {
 		return uintptr(syscall.InvalidHandle)
 	}
 	return uintptr(file.pfd.Sysfd)
 }

 // newFile returns a new File with the given file handle and name.
 // Unlike NewFile, it does not check that h is syscall.InvalidHandle.
 func newFile(h syscall.Handle, name string, kind string) *File {
 	if kind == "file" {
 		var m uint32
 		if syscall.GetConsoleMode(h, &m) == nil {
 			kind = "console"
 		}
 		if t, err := syscall.GetFileType(h); err == nil && t == syscall.FILE_TYPE_PIPE {
 			kind = "pipe"
 		}
 	}

 	f := &File{&file{
 		pfd: poll.FD{
 			Sysfd:         h,
 			IsStream:      true,
 			ZeroReadIsEOF: true,
 		},
 		name: name,
 	}}
 	runtime.SetFinalizer(f.file, (*file).close)

 	// Ignore initialization errors.
 	// Assume any problems will show up in later I/O.
 	f.pfd.Init(kind, false)

 	return f
 }

 // newConsoleFile creates new File that will be used as console.
 func newConsoleFile(h syscall.Handle, name string) *File {
 	return newFile(h, name, "console")
 }

 // NewFile returns a new File with the given file descriptor and
 // name. The returned value will be nil if fd is not a valid file
 // descriptor.
 func NewFile(fd uintptr, name string) *File {
 	h := syscall.Handle(fd)
 	if h == syscall.InvalidHandle {
 		return nil
 	}
 	return newFile(h, name, "file")
 }

 func epipecheck(file *File, e error) {
 }

 // DevNull is the name of the operating system's “null device.”
 // On Unix-like systems, it is "/dev/null"; on Windows, "NUL".
 const DevNull = "NUL"

 // openFileNolog is the Windows implementation of OpenFile.
 func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	if name == "" {
 		return nil, &PathError{Op: "open", Path: name, Err: syscall.ENOENT}
 	}
 	path := fixLongPath(name)
 	r, e := syscall.Open(path, flag|syscall.O_CLOEXEC, syscallMode(perm))
 	if e != nil {
 		// We should return EISDIR when we are trying to open a directory with write access.
 		if e == syscall.ERROR_ACCESS_DENIED && (flag&O_WRONLY != 0 || flag&O_RDWR != 0) {
 			pathp, e1 := syscall.UTF16PtrFromString(path)
 			if e1 == nil {
 				var fa syscall.Win32FileAttributeData
 				e1 = syscall.GetFileAttributesEx(pathp, syscall.GetFileExInfoStandard, (*byte)(unsafe.Pointer(&fa)))
 				if e1 == nil && fa.FileAttributes&syscall.FILE_ATTRIBUTE_DIRECTORY != 0 {
 					e = syscall.EISDIR
 				}
 			}
 		}
 		return nil, &PathError{Op: "open", Path: name, Err: e}
 	}
 	return newFile(r, name, "file"), nil
 }

 func openDirNolog(name string) (*File, error) {
 	return openFileNolog(name, O_RDONLY, 0)
 }

 func (file *file) close() error {
 	if file == nil {
 		return syscall.EINVAL
 	}
 	if info := file.dirinfo.Swap(nil); info != nil {
 		info.close()
 	}
 	var err error
 	if e := file.pfd.Close(); e != nil {
 		if e == poll.ErrFileClosing {
 			e = ErrClosed
 		}
 		err = &PathError{Op: "close", Path: file.name, Err: e}
 	}

 	// no need for a finalizer anymore
 	runtime.SetFinalizer(file, nil)
 	return err
 }

 // seek sets the offset for the next Read or Write on file to offset, interpreted
 // according to whence: 0 means relative to the origin of the file, 1 means
 // relative to the current offset, and 2 means relative to the end.
 // It returns the new offset and an error, if any.
 func (f *File) seek(offset int64, whence int) (ret int64, err error) {
 	if info := f.dirinfo.Swap(nil); info != nil {
 		// Free cached dirinfo, so we allocate a new one if we
 		// access this file as a directory again. See #35767 and #37161.
 		info.close()
 	}
 	ret, err = f.pfd.Seek(offset, whence)
 	runtime.KeepAlive(f)
 	return ret, err
 }

 // Truncate changes the size of the named file.
 // If the file is a symbolic link, it changes the size of the link's target.
 func Truncate(name string, size int64) error {
 	f, e := OpenFile(name, O_WRONLY, 0666)
 	if e != nil {
 		return e
 	}
 	defer f.Close()
 	e1 := f.Truncate(size)
 	if e1 != nil {
 		return e1
 	}
 	return nil
 }

 // Remove removes the named file or directory.
 // If there is an error, it will be of type *PathError.
 func Remove(name string) error {
 	p, e := syscall.UTF16PtrFromString(fixLongPath(name))
 	if e != nil {
 		return &PathError{Op: "remove", Path: name, Err: e}
 	}

 	// Go file interface forces us to know whether
 	// name is a file or directory. Try both.
 	e = syscall.DeleteFile(p)
 	if e == nil {
 		return nil
 	}
 	e1 := syscall.RemoveDirectory(p)
 	if e1 == nil {
 		return nil
 	}

 	// Both failed: figure out which error to return.
 	if e1 != e {
 		a, e2 := syscall.GetFileAttributes(p)
 		if e2 != nil {
 			e = e2
 		} else {
 			if a&syscall.FILE_ATTRIBUTE_DIRECTORY != 0 {
 				e = e1
 			} else if a&syscall.FILE_ATTRIBUTE_READONLY != 0 {
 				if e1 = syscall.SetFileAttributes(p, a&^syscall.FILE_ATTRIBUTE_READONLY); e1 == nil {
 					if e = syscall.DeleteFile(p); e == nil {
 						return nil
 					}
 				}
 			}
 		}
 	}
 	return &PathError{Op: "remove", Path: name, Err: e}
 }

 func rename(oldname, newname string) error {
 	e := windows.Rename(fixLongPath(oldname), fixLongPath(newname))
 	if e != nil {
 		return &LinkError{"rename", oldname, newname, e}
 	}
 	return nil
 }

 // Pipe returns a connected pair of Files; reads from r return bytes written to w.
 // It returns the files and an error, if any. The Windows handles underlying
 // the returned files are marked as inheritable by child processes.
 func Pipe() (r *File, w *File, err error) {
 	var p [2]syscall.Handle
 	e := syscall.Pipe(p[:])
 	if e != nil {
 		return nil, nil, NewSyscallError("pipe", e)
 	}
 	return newFile(p[0], "|0", "pipe"), newFile(p[1], "|1", "pipe"), nil
 }

 var (
 	useGetTempPath2Once sync.Once
 	useGetTempPath2     bool
 )

 func tempDir() string {
 	useGetTempPath2Once.Do(func() {
 		useGetTempPath2 = (windows.ErrorLoadingGetTempPath2() == nil)
 	})
 	getTempPath := syscall.GetTempPath
 	if useGetTempPath2 {
 		getTempPath = windows.GetTempPath2
 	}
 	n := uint32(syscall.MAX_PATH)
 	for {
 		b := make([]uint16, n)
 		n, _ = getTempPath(uint32(len(b)), &b[0])
 		if n > uint32(len(b)) {
 			continue
 		}
 		if n == 3 && b[1] == ':' && b[2] == '\\' {
 			// Do nothing for path, like C:\.
 		} else if n > 0 && b[n-1] == '\\' {
 			// Otherwise remove terminating \.
 			n--
 		}
 		return syscall.UTF16ToString(b[:n])
 	}
 }

 // Link creates newname as a hard link to the oldname file.
 // If there is an error, it will be of type *LinkError.
 func Link(oldname, newname string) error {
 	n, err := syscall.UTF16PtrFromString(fixLongPath(newname))
 	if err != nil {
 		return &LinkError{"link", oldname, newname, err}
 	}
 	o, err := syscall.UTF16PtrFromString(fixLongPath(oldname))
 	if err != nil {
 		return &LinkError{"link", oldname, newname, err}
 	}
 	err = syscall.CreateHardLink(n, o, 0)
 	if err != nil {
 		return &LinkError{"link", oldname, newname, err}
 	}
 	return nil
 }

 // Symlink creates newname as a symbolic link to oldname.
 // On Windows, a symlink to a non-existent oldname creates a file symlink;
 // if oldname is later created as a directory the symlink will not work.
 // If there is an error, it will be of type *LinkError.
 func Symlink(oldname, newname string) error {
 	// '/' does not work in link's content
 	oldname = fromSlash(oldname)

 	// need the exact location of the oldname when it's relative to determine if it's a directory
 	destpath := oldname
 	if v := volumeName(oldname); v == "" {
 		if len(oldname) > 0 && IsPathSeparator(oldname[0]) {
 			// oldname is relative to the volume containing newname.
 			if v = volumeName(newname); v != "" {
 				// Prepend the volume explicitly, because it may be different from the
 				// volume of the current working directory.
 				destpath = v + oldname
 			}
 		} else {
 			// oldname is relative to newname.
 			destpath = dirname(newname) + `\` + oldname
 		}
 	}

 	fi, err := Stat(destpath)
 	isdir := err == nil && fi.IsDir()

 	n, err := syscall.UTF16PtrFromString(fixLongPath(newname))
 	if err != nil {
 		return &LinkError{"symlink", oldname, newname, err}
 	}
 	var o *uint16
 	if isAbs(oldname) {
 		o, err = syscall.UTF16PtrFromString(fixLongPath(oldname))
 	} else {
 		// Do not use fixLongPath on oldname for relative symlinks,
 		// as it would turn the name into an absolute path thus making
 		// an absolute symlink instead.
 		// Notice that CreateSymbolicLinkW does not fail for relative
 		// symlinks beyond MAX_PATH, so this does not prevent the
 		// creation of an arbitrary long path name.
 		o, err = syscall.UTF16PtrFromString(oldname)
 	}
 	if err != nil {
 		return &LinkError{"symlink", oldname, newname, err}
 	}

 	var flags uint32 = windows.SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
 	if isdir {
 		flags |= syscall.SYMBOLIC_LINK_FLAG_DIRECTORY
 	}
 	err = syscall.CreateSymbolicLink(n, o, flags)
 	if err != nil {
 		// the unprivileged create flag is unsupported
 		// below Windows 10 (1703, v10.0.14972). retry without it.
 		flags &^= windows.SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
 		err = syscall.CreateSymbolicLink(n, o, flags)
 		if err != nil {
 			return &LinkError{"symlink", oldname, newname, err}
 		}
 	}
 	return nil
 }

 // openSymlink calls CreateFile Windows API with FILE_FLAG_OPEN_REPARSE_POINT
 // parameter, so that Windows does not follow symlink, if path is a symlink.
 // openSymlink returns opened file handle.
 func openSymlink(path string) (syscall.Handle, error) {
 	p, err := syscall.UTF16PtrFromString(path)
 	if err != nil {
 		return 0, err
 	}
 	attrs := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS)
 	// Use FILE_FLAG_OPEN_REPARSE_POINT, otherwise CreateFile will follow symlink.
 	// See https://docs.microsoft.com/en-us/windows/desktop/FileIO/symbolic-link-effects-on-file-systems-functions#createfile-and-createfiletransacted
 	attrs |= syscall.FILE_FLAG_OPEN_REPARSE_POINT
 	h, err := syscall.CreateFile(p, 0, 0, nil, syscall.OPEN_EXISTING, attrs, 0)
 	if err != nil {
 		return 0, err
 	}
 	return h, nil
 }

 var winreadlinkvolume = godebug.New("winreadlinkvolume")

 // normaliseLinkPath converts absolute paths returned by
 // DeviceIoControl(h, FSCTL_GET_REPARSE_POINT, ...)
 // into paths acceptable by all Windows APIs.
 // For example, it converts
 //
 //	\??\C:\foo\bar into C:\foo\bar
 //	\??\UNC\foo\bar into \\foo\bar
 //	\??\Volume{abc}\ into \\?\Volume{abc}\
 func normaliseLinkPath(path string) (string, error) {
 	if len(path) < 4 || path[:4] != `\??\` {
 		// unexpected path, return it as is
 		return path, nil
 	}
 	// we have path that start with \??\
 	s := path[4:]
 	switch {
 	case len(s) >= 2 && s[1] == ':': // \??\C:\foo\bar
 		return s, nil
 	case len(s) >= 4 && s[:4] == `UNC\`: // \??\UNC\foo\bar
 		return `\\` + s[4:], nil
 	}

 	// \??\Volume{abc}\
 	if winreadlinkvolume.Value() != "0" {
 		return `\\?\` + path[4:], nil
 	}
 	winreadlinkvolume.IncNonDefault()

 	h, err := openSymlink(path)
 	if err != nil {
 		return "", err
 	}
 	defer syscall.CloseHandle(h)

 	buf := make([]uint16, 100)
 	for {
 		n, err := windows.GetFinalPathNameByHandle(h, &buf[0], uint32(len(buf)), windows.VOLUME_NAME_DOS)
 		if err != nil {
 			return "", err
 		}
 		if n < uint32(len(buf)) {
 			break
 		}
 		buf = make([]uint16, n)
 	}
 	s = syscall.UTF16ToString(buf)
 	if len(s) > 4 && s[:4] == `\\?\` {
 		s = s[4:]
 		if len(s) > 3 && s[:3] == `UNC` {
 			// return path like \\server\share\...
 			return `\` + s[3:], nil
 		}
 		return s, nil
 	}
 	return "", errors.New("GetFinalPathNameByHandle returned unexpected path: " + s)
 }

 func readReparseLink(path string) (string, error) {
 	h, err := openSymlink(path)
 	if err != nil {
 		return "", err
 	}
 	defer syscall.CloseHandle(h)

 	rdbbuf := make([]byte, syscall.MAXIMUM_REPARSE_DATA_BUFFER_SIZE)
 	var bytesReturned uint32
 	err = syscall.DeviceIoControl(h, syscall.FSCTL_GET_REPARSE_POINT, nil, 0, &rdbbuf[0], uint32(len(rdbbuf)), &bytesReturned, nil)
 	if err != nil {
 		return "", err
 	}

 	rdb := (*windows.REPARSE_DATA_BUFFER)(unsafe.Pointer(&rdbbuf[0]))
 	switch rdb.ReparseTag {
 	case syscall.IO_REPARSE_TAG_SYMLINK:
 		rb := (*windows.SymbolicLinkReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME))
 		s := rb.Path()
 		if rb.Flags&windows.SYMLINK_FLAG_RELATIVE != 0 {
 			return s, nil
 		}
 		return normaliseLinkPath(s)
 	case windows.IO_REPARSE_TAG_MOUNT_POINT:
 		return normaliseLinkPath((*windows.MountPointReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME)).Path())
 	default:
 		// the path is not a symlink or junction but another type of reparse
 		// point
 		return "", syscall.ENOENT
 	}
 }

 func readlink(name string) (string, error) {
 	s, err := readReparseLink(fixLongPath(name))
 	if err != nil {
 		return "", &PathError{Op: "readlink", Path: name, Err: err}
 	}
 	return s, nil
 }
	// Copyright 2009 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 os

	import (
	"errors"
	"internal/godebug"
	"internal/poll"
	"internal/syscall/windows"
	"runtime"
	"sync"
	"sync/atomic"
	"syscall"
	"unsafe"
	)

	// This matches the value in syscall/syscall_windows.go.
	const _UTIME_OMIT = -1

	// file is the real representation of *File.
	// The extra level of indirection ensures that no clients of os
	// can overwrite this data, which could cause the finalizer
	// to close the wrong file descriptor.
	type file struct {
	pfd poll.FD
	name string
	dirinfo atomic.Pointer[dirInfo] // nil unless directory being read
	appendMode bool // whether file is opened for appending
	}

	// Fd returns the Windows handle referencing the open file.
	// If f is closed, the file descriptor becomes invalid.
	// If f is garbage collected, a finalizer may close the file descriptor,
	// making it invalid; see [runtime.SetFinalizer] for more information on when
	// a finalizer might be run. On Unix systems this will cause the [File.SetDeadline]
	// methods to stop working.
	func (file *File) Fd() uintptr {
	if file == nil {
	return uintptr(syscall.InvalidHandle)
	}
	return uintptr(file.pfd.Sysfd)
	}

	// newFile returns a new File with the given file handle and name.
	// Unlike NewFile, it does not check that h is syscall.InvalidHandle.
	func newFile(h syscall.Handle, name string, kind string) *File {
	if kind == "file" {
	var m uint32
	if syscall.GetConsoleMode(h, &m) == nil {
	kind = "console"
	}
	if t, err := syscall.GetFileType(h); err == nil && t == syscall.FILE_TYPE_PIPE {
	kind = "pipe"
	}
	}

	f := &File{&file{
	pfd: poll.FD{
	Sysfd: h,
	IsStream: true,
	ZeroReadIsEOF: true,
	},
	name: name,
	}}
	runtime.SetFinalizer(f.file, (*file).close)

	// Ignore initialization errors.
	// Assume any problems will show up in later I/O.
	f.pfd.Init(kind, false)

	return f
	}

	// newConsoleFile creates new File that will be used as console.
	func newConsoleFile(h syscall.Handle, name string) *File {
	return newFile(h, name, "console")
	}

	// NewFile returns a new File with the given file descriptor and
	// name. The returned value will be nil if fd is not a valid file
	// descriptor.
	func NewFile(fd uintptr, name string) *File {
	h := syscall.Handle(fd)
	if h == syscall.InvalidHandle {
	return nil
	}
	return newFile(h, name, "file")
	}

	func epipecheck(file *File, e error) {
	}

	// DevNull is the name of the operating system's “null device.”
	// On Unix-like systems, it is "/dev/null"; on Windows, "NUL".
	const DevNull = "NUL"

	// openFileNolog is the Windows implementation of OpenFile.
	func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
	if name == "" {
	return nil, &PathError{Op: "open", Path: name, Err: syscall.ENOENT}
	}
	path := fixLongPath(name)
	r, e := syscall.Open(path, flag\|syscall.O_CLOEXEC, syscallMode(perm))
	if e != nil {
	// We should return EISDIR when we are trying to open a directory with write access.
	if e == syscall.ERROR_ACCESS_DENIED && (flag&O_WRONLY != 0 \|\| flag&O_RDWR != 0) {
	pathp, e1 := syscall.UTF16PtrFromString(path)
	if e1 == nil {
	var fa syscall.Win32FileAttributeData
	e1 = syscall.GetFileAttributesEx(pathp, syscall.GetFileExInfoStandard, (*byte)(unsafe.Pointer(&fa)))
	if e1 == nil && fa.FileAttributes&syscall.FILE_ATTRIBUTE_DIRECTORY != 0 {
	e = syscall.EISDIR
	}
	}
	}
	return nil, &PathError{Op: "open", Path: name, Err: e}
	}
	return newFile(r, name, "file"), nil
	}

	func openDirNolog(name string) (*File, error) {
	return openFileNolog(name, O_RDONLY, 0)
	}

	func (file *file) close() error {
	if file == nil {
	return syscall.EINVAL
	}
	if info := file.dirinfo.Swap(nil); info != nil {
	info.close()
	}
	var err error
	if e := file.pfd.Close(); e != nil {
	if e == poll.ErrFileClosing {
	e = ErrClosed
	}
	err = &PathError{Op: "close", Path: file.name, Err: e}
	}

	// no need for a finalizer anymore
	runtime.SetFinalizer(file, nil)
	return err
	}

	// seek sets the offset for the next Read or Write on file to offset, interpreted
	// according to whence: 0 means relative to the origin of the file, 1 means
	// relative to the current offset, and 2 means relative to the end.
	// It returns the new offset and an error, if any.
	func (f *File) seek(offset int64, whence int) (ret int64, err error) {
	if info := f.dirinfo.Swap(nil); info != nil {
	// Free cached dirinfo, so we allocate a new one if we
	// access this file as a directory again. See #35767 and #37161.
	info.close()
	}
	ret, err = f.pfd.Seek(offset, whence)
	runtime.KeepAlive(f)
	return ret, err
	}

	// Truncate changes the size of the named file.
	// If the file is a symbolic link, it changes the size of the link's target.
	func Truncate(name string, size int64) error {
	f, e := OpenFile(name, O_WRONLY, 0666)
	if e != nil {
	return e
	}
	defer f.Close()
	e1 := f.Truncate(size)
	if e1 != nil {
	return e1
	}
	return nil
	}

	// Remove removes the named file or directory.
	// If there is an error, it will be of type *PathError.
	func Remove(name string) error {
	p, e := syscall.UTF16PtrFromString(fixLongPath(name))
	if e != nil {
	return &PathError{Op: "remove", Path: name, Err: e}
	}

	// Go file interface forces us to know whether
	// name is a file or directory. Try both.
	e = syscall.DeleteFile(p)
	if e == nil {
	return nil
	}
	e1 := syscall.RemoveDirectory(p)
	if e1 == nil {
	return nil
	}

	// Both failed: figure out which error to return.
	if e1 != e {
	a, e2 := syscall.GetFileAttributes(p)
	if e2 != nil {
	e = e2
	} else {
	if a&syscall.FILE_ATTRIBUTE_DIRECTORY != 0 {
	e = e1
	} else if a&syscall.FILE_ATTRIBUTE_READONLY != 0 {
	if e1 = syscall.SetFileAttributes(p, a&^syscall.FILE_ATTRIBUTE_READONLY); e1 == nil {
	if e = syscall.DeleteFile(p); e == nil {
	return nil
	}
	}
	}
	}
	}
	return &PathError{Op: "remove", Path: name, Err: e}
	}

	func rename(oldname, newname string) error {
	e := windows.Rename(fixLongPath(oldname), fixLongPath(newname))
	if e != nil {
	return &LinkError{"rename", oldname, newname, e}
	}
	return nil
	}

	// Pipe returns a connected pair of Files; reads from r return bytes written to w.
	// It returns the files and an error, if any. The Windows handles underlying
	// the returned files are marked as inheritable by child processes.
	func Pipe() (r File, w File, err error) {
	var p [2]syscall.Handle
	e := syscall.Pipe(p[:])
	if e != nil {
	return nil, nil, NewSyscallError("pipe", e)
	}
	return newFile(p[0], "\|0", "pipe"), newFile(p[1], "\|1", "pipe"), nil
	}

	var (
	useGetTempPath2Once sync.Once
	useGetTempPath2 bool
	)

	func tempDir() string {
	useGetTempPath2Once.Do(func() {
	useGetTempPath2 = (windows.ErrorLoadingGetTempPath2() == nil)
	})
	getTempPath := syscall.GetTempPath
	if useGetTempPath2 {
	getTempPath = windows.GetTempPath2
	}
	n := uint32(syscall.MAX_PATH)
	for {
	b := make([]uint16, n)
	n, _ = getTempPath(uint32(len(b)), &b[0])
	if n > uint32(len(b)) {
	continue
	}
	if n == 3 && b[1] == ':' && b[2] == '\\' {
	// Do nothing for path, like C:\.
	} else if n > 0 && b[n-1] == '\\' {
	// Otherwise remove terminating \.
	n--
	}
	return syscall.UTF16ToString(b[:n])
	}
	}

	// Link creates newname as a hard link to the oldname file.
	// If there is an error, it will be of type *LinkError.
	func Link(oldname, newname string) error {
	n, err := syscall.UTF16PtrFromString(fixLongPath(newname))
	if err != nil {
	return &LinkError{"link", oldname, newname, err}
	}
	o, err := syscall.UTF16PtrFromString(fixLongPath(oldname))
	if err != nil {
	return &LinkError{"link", oldname, newname, err}
	}
	err = syscall.CreateHardLink(n, o, 0)
	if err != nil {
	return &LinkError{"link", oldname, newname, err}
	}
	return nil
	}

	// Symlink creates newname as a symbolic link to oldname.
	// On Windows, a symlink to a non-existent oldname creates a file symlink;
	// if oldname is later created as a directory the symlink will not work.
	// If there is an error, it will be of type *LinkError.
	func Symlink(oldname, newname string) error {
	// '/' does not work in link's content
	oldname = fromSlash(oldname)

	// need the exact location of the oldname when it's relative to determine if it's a directory
	destpath := oldname
	if v := volumeName(oldname); v == "" {
	if len(oldname) > 0 && IsPathSeparator(oldname[0]) {
	// oldname is relative to the volume containing newname.
	if v = volumeName(newname); v != "" {
	// Prepend the volume explicitly, because it may be different from the
	// volume of the current working directory.
	destpath = v + oldname
	}
	} else {
	// oldname is relative to newname.
	destpath = dirname(newname) + `\` + oldname
	}
	}

	fi, err := Stat(destpath)
	isdir := err == nil && fi.IsDir()

	n, err := syscall.UTF16PtrFromString(fixLongPath(newname))
	if err != nil {
	return &LinkError{"symlink", oldname, newname, err}
	}
	var o *uint16
	if isAbs(oldname) {
	o, err = syscall.UTF16PtrFromString(fixLongPath(oldname))
	} else {
	// Do not use fixLongPath on oldname for relative symlinks,
	// as it would turn the name into an absolute path thus making
	// an absolute symlink instead.
	// Notice that CreateSymbolicLinkW does not fail for relative
	// symlinks beyond MAX_PATH, so this does not prevent the
	// creation of an arbitrary long path name.
	o, err = syscall.UTF16PtrFromString(oldname)
	}
	if err != nil {
	return &LinkError{"symlink", oldname, newname, err}
	}

	var flags uint32 = windows.SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
	if isdir {
	flags \|= syscall.SYMBOLIC_LINK_FLAG_DIRECTORY
	}
	err = syscall.CreateSymbolicLink(n, o, flags)
	if err != nil {
	// the unprivileged create flag is unsupported
	// below Windows 10 (1703, v10.0.14972). retry without it.
	flags &^= windows.SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
	err = syscall.CreateSymbolicLink(n, o, flags)
	if err != nil {
	return &LinkError{"symlink", oldname, newname, err}
	}
	}
	return nil
	}

	// openSymlink calls CreateFile Windows API with FILE_FLAG_OPEN_REPARSE_POINT
	// parameter, so that Windows does not follow symlink, if path is a symlink.
	// openSymlink returns opened file handle.
	func openSymlink(path string) (syscall.Handle, error) {
	p, err := syscall.UTF16PtrFromString(path)
	if err != nil {
	return 0, err
	}
	attrs := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS)
	// Use FILE_FLAG_OPEN_REPARSE_POINT, otherwise CreateFile will follow symlink.
	// See https://docs.microsoft.com/en-us/windows/desktop/FileIO/symbolic-link-effects-on-file-systems-functions#createfile-and-createfiletransacted
	attrs \|= syscall.FILE_FLAG_OPEN_REPARSE_POINT
	h, err := syscall.CreateFile(p, 0, 0, nil, syscall.OPEN_EXISTING, attrs, 0)
	if err != nil {
	return 0, err
	}
	return h, nil
	}

	var winreadlinkvolume = godebug.New("winreadlinkvolume")

	// normaliseLinkPath converts absolute paths returned by
	// DeviceIoControl(h, FSCTL_GET_REPARSE_POINT, ...)
	// into paths acceptable by all Windows APIs.
	// For example, it converts
	//
	// \??\C:\foo\bar into C:\foo\bar
	// \??\UNC\foo\bar into \\foo\bar
	// \??\Volume{abc}\ into \\?\Volume{abc}\
	func normaliseLinkPath(path string) (string, error) {
	if len(path) < 4 \|\| path[:4] != `\??\` {
	// unexpected path, return it as is
	return path, nil
	}
	// we have path that start with \??\
	s := path[4:]
	switch {
	case len(s) >= 2 && s[1] == ':': // \??\C:\foo\bar
	return s, nil
	case len(s) >= 4 && s[:4] == `UNC\`: // \??\UNC\foo\bar
	return `\\` + s[4:], nil
	}

	// \??\Volume{abc}\
	if winreadlinkvolume.Value() != "0" {
	return `\\?\` + path[4:], nil
	}
	winreadlinkvolume.IncNonDefault()

	h, err := openSymlink(path)
	if err != nil {
	return "", err
	}
	defer syscall.CloseHandle(h)

	buf := make([]uint16, 100)
	for {
	n, err := windows.GetFinalPathNameByHandle(h, &buf[0], uint32(len(buf)), windows.VOLUME_NAME_DOS)
	if err != nil {
	return "", err
	}
	if n < uint32(len(buf)) {
	break
	}
	buf = make([]uint16, n)
	}
	s = syscall.UTF16ToString(buf)
	if len(s) > 4 && s[:4] == `\\?\` {
	s = s[4:]
	if len(s) > 3 && s[:3] == `UNC` {
	// return path like \\server\share\...
	return `\` + s[3:], nil
	}
	return s, nil
	}
	return "", errors.New("GetFinalPathNameByHandle returned unexpected path: " + s)
	}

	func readReparseLink(path string) (string, error) {
	h, err := openSymlink(path)
	if err != nil {
	return "", err
	}
	defer syscall.CloseHandle(h)

	rdbbuf := make([]byte, syscall.MAXIMUM_REPARSE_DATA_BUFFER_SIZE)
	var bytesReturned uint32
	err = syscall.DeviceIoControl(h, syscall.FSCTL_GET_REPARSE_POINT, nil, 0, &rdbbuf[0], uint32(len(rdbbuf)), &bytesReturned, nil)
	if err != nil {
	return "", err
	}

	rdb := (*windows.REPARSE_DATA_BUFFER)(unsafe.Pointer(&rdbbuf[0]))
	switch rdb.ReparseTag {
	case syscall.IO_REPARSE_TAG_SYMLINK:
	rb := (*windows.SymbolicLinkReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME))
	s := rb.Path()
	if rb.Flags&windows.SYMLINK_FLAG_RELATIVE != 0 {
	return s, nil
	}
	return normaliseLinkPath(s)
	case windows.IO_REPARSE_TAG_MOUNT_POINT:
	return normaliseLinkPath((*windows.MountPointReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME)).Path())
	default:
	// the path is not a symlink or junction but another type of reparse
	// point
	return "", syscall.ENOENT
	}
	}

	func readlink(name string) (string, error) {
	s, err := readReparseLink(fixLongPath(name))
	if err != nil {
	return "", &PathError{Op: "readlink", Path: name, Err: err}
	}
	return s, nil
	}