src/os/file_unix.go - go - 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.

 //go:build aix || darwin || dragonfly || freebsd || (js && wasm) || linux || netbsd || openbsd || solaris
 // +build aix darwin dragonfly freebsd js,wasm linux netbsd openbsd solaris

 package os

 import (
 	"internal/poll"
 	"internal/syscall/unix"
 	"runtime"
 	"syscall"
 )

 // fixLongPath is a noop on non-Windows platforms.
 func fixLongPath(path string) string {
 	return path
 }

 func rename(oldname, newname string) error {
 	fi, err := Lstat(newname)
 	if err == nil && fi.IsDir() {
 		// There are two independent errors this function can return:
 		// one for a bad oldname, and one for a bad newname.
 		// At this point we've determined the newname is bad.
 		// But just in case oldname is also bad, prioritize returning
 		// the oldname error because that's what we did historically.
 		// However, if the old name and new name are not the same, yet
 		// they refer to the same file, it implies a case-only
 		// rename on a case-insensitive filesystem, which is ok.
 		if ofi, err := Lstat(oldname); err != nil {
 			if pe, ok := err.(*PathError); ok {
 				err = pe.Err
 			}
 			return &LinkError{"rename", oldname, newname, err}
 		} else if newname == oldname || !SameFile(fi, ofi) {
 			return &LinkError{"rename", oldname, newname, syscall.EEXIST}
 		}
 	}
 	err = ignoringEINTR(func() error {
 		return syscall.Rename(oldname, newname)
 	})
 	if err != nil {
 		return &LinkError{"rename", oldname, newname, err}
 	}
 	return nil
 }

 // 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     *dirInfo // nil unless directory being read
 	nonblock    bool     // whether we set nonblocking mode
 	stdoutOrErr bool     // whether this is stdout or stderr
 	appendMode  bool     // whether file is opened for appending
 }

 // Fd returns the integer Unix file descriptor 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 SetDeadline
 // methods to stop working.
 // Because file descriptors can be reused, the returned file descriptor may
 // only be closed through the Close method of f, or by its finalizer during
 // garbage collection. Otherwise, during garbage collection the finalizer
 // may close an unrelated file descriptor with the same (reused) number.
 //
 // As an alternative, see the f.SyscallConn method.
 func (f *File) Fd() uintptr {
 	if f == nil {
 		return ^(uintptr(0))
 	}

 	// If we put the file descriptor into nonblocking mode,
 	// then set it to blocking mode before we return it,
 	// because historically we have always returned a descriptor
 	// opened in blocking mode. The File will continue to work,
 	// but any blocking operation will tie up a thread.
 	if f.nonblock {
 		f.pfd.SetBlocking()
 	}

 	return uintptr(f.pfd.Sysfd)
 }

 // 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. On Unix systems, if the file descriptor is in
 // non-blocking mode, NewFile will attempt to return a pollable File
 // (one for which the SetDeadline methods work).
 //
 // After passing it to NewFile, fd may become invalid under the same
 // conditions described in the comments of the Fd method, and the same
 // constraints apply.
 func NewFile(fd uintptr, name string) *File {
 	kind := kindNewFile
 	if nb, err := unix.IsNonblock(int(fd)); err == nil && nb {
 		kind = kindNonBlock
 	}
 	return newFile(fd, name, kind)
 }

 // newFileKind describes the kind of file to newFile.
 type newFileKind int

 const (
 	kindNewFile newFileKind = iota
 	kindOpenFile
 	kindPipe
 	kindNonBlock
 )

 // newFile is like NewFile, but if called from OpenFile or Pipe
 // (as passed in the kind parameter) it tries to add the file to
 // the runtime poller.
 func newFile(fd uintptr, name string, kind newFileKind) *File {
 	fdi := int(fd)
 	if fdi < 0 {
 		return nil
 	}
 	f := &File{&file{
 		pfd: poll.FD{
 			Sysfd:         fdi,
 			IsStream:      true,
 			ZeroReadIsEOF: true,
 		},
 		name:        name,
 		stdoutOrErr: fdi == 1 || fdi == 2,
 	}}

 	pollable := kind == kindOpenFile || kind == kindPipe || kind == kindNonBlock

 	// If the caller passed a non-blocking filedes (kindNonBlock),
 	// we assume they know what they are doing so we allow it to be
 	// used with kqueue.
 	if kind == kindOpenFile {
 		switch runtime.GOOS {
 		case "darwin", "ios", "dragonfly", "freebsd", "netbsd", "openbsd":
 			var st syscall.Stat_t
 			err := ignoringEINTR(func() error {
 				return syscall.Fstat(fdi, &st)
 			})
 			typ := st.Mode & syscall.S_IFMT
 			// Don't try to use kqueue with regular files on *BSDs.
 			// On FreeBSD a regular file is always
 			// reported as ready for writing.
 			// On Dragonfly, NetBSD and OpenBSD the fd is signaled
 			// only once as ready (both read and write).
 			// Issue 19093.
 			// Also don't add directories to the netpoller.
 			if err == nil && (typ == syscall.S_IFREG || typ == syscall.S_IFDIR) {
 				pollable = false
 			}

 			// In addition to the behavior described above for regular files,
 			// on Darwin, kqueue does not work properly with fifos:
 			// closing the last writer does not cause a kqueue event
 			// for any readers. See issue #24164.
 			if (runtime.GOOS == "darwin" || runtime.GOOS == "ios") && typ == syscall.S_IFIFO {
 				pollable = false
 			}
 		}
 	}

 	if err := f.pfd.Init("file", pollable); err != nil {
 		// An error here indicates a failure to register
 		// with the netpoll system. That can happen for
 		// a file descriptor that is not supported by
 		// epoll/kqueue; for example, disk files on
 		// GNU/Linux systems. We assume that any real error
 		// will show up in later I/O.
 	} else if pollable {
 		// We successfully registered with netpoll, so put
 		// the file into nonblocking mode.
 		if err := syscall.SetNonblock(fdi, true); err == nil {
 			f.nonblock = true
 		}
 	}

 	runtime.SetFinalizer(f.file, (*file).close)
 	return f
 }

 // epipecheck raises SIGPIPE if we get an EPIPE error on standard
 // output or standard error. See the SIGPIPE docs in os/signal, and
 // issue 11845.
 func epipecheck(file *File, e error) {
 	if e == syscall.EPIPE && file.stdoutOrErr {
 		sigpipe()
 	}
 }

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

 // openFileNolog is the Unix implementation of OpenFile.
 // Changes here should be reflected in openFdAt, if relevant.
 func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	setSticky := false
 	if !supportsCreateWithStickyBit && flag&O_CREATE != 0 && perm&ModeSticky != 0 {
 		if _, err := Stat(name); IsNotExist(err) {
 			setSticky = true
 		}
 	}

 	var r int
 	for {
 		var e error
 		r, e = syscall.Open(name, flag|syscall.O_CLOEXEC, syscallMode(perm))
 		if e == nil {
 			break
 		}

 		// We have to check EINTR here, per issues 11180 and 39237.
 		if e == syscall.EINTR {
 			continue
 		}

 		return nil, &PathError{Op: "open", Path: name, Err: e}
 	}

 	// open(2) itself won't handle the sticky bit on *BSD and Solaris
 	if setSticky {
 		setStickyBit(name)
 	}

 	// There's a race here with fork/exec, which we are
 	// content to live with. See ../syscall/exec_unix.go.
 	if !supportsCloseOnExec {
 		syscall.CloseOnExec(r)
 	}

 	return newFile(uintptr(r), name, kindOpenFile), nil
 }

 func (file *file) close() error {
 	if file == nil {
 		return syscall.EINVAL
 	}
 	if file.dirinfo != nil {
 		file.dirinfo.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 f.dirinfo != nil {
 		// Free cached dirinfo, so we allocate a new one if we
 		// access this file as a directory again. See #35767 and #37161.
 		f.dirinfo.close()
 		f.dirinfo = nil
 	}
 	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.
 // If there is an error, it will be of type *PathError.
 func Truncate(name string, size int64) error {
 	e := ignoringEINTR(func() error {
 		return syscall.Truncate(name, size)
 	})
 	if e != nil {
 		return &PathError{Op: "truncate", Path: name, Err: e}
 	}
 	return nil
 }

 // Remove removes the named file or (empty) directory.
 // If there is an error, it will be of type *PathError.
 func Remove(name string) error {
 	// System call interface forces us to know
 	// whether name is a file or directory.
 	// Try both: it is cheaper on average than
 	// doing a Stat plus the right one.
 	e := ignoringEINTR(func() error {
 		return syscall.Unlink(name)
 	})
 	if e == nil {
 		return nil
 	}
 	e1 := ignoringEINTR(func() error {
 		return syscall.Rmdir(name)
 	})
 	if e1 == nil {
 		return nil
 	}

 	// Both failed: figure out which error to return.
 	// OS X and Linux differ on whether unlink(dir)
 	// returns EISDIR, so can't use that. However,
 	// both agree that rmdir(file) returns ENOTDIR,
 	// so we can use that to decide which error is real.
 	// Rmdir might also return ENOTDIR if given a bad
 	// file path, like /etc/passwd/foo, but in that case,
 	// both errors will be ENOTDIR, so it's okay to
 	// use the error from unlink.
 	if e1 != syscall.ENOTDIR {
 		e = e1
 	}
 	return &PathError{Op: "remove", Path: name, Err: e}
 }

 func tempDir() string {
 	dir := Getenv("TMPDIR")
 	if dir == "" {
 		if runtime.GOOS == "android" {
 			dir = "/data/local/tmp"
 		} else {
 			dir = "/tmp"
 		}
 	}
 	return dir
 }

 // 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 {
 	e := ignoringEINTR(func() error {
 		return syscall.Link(oldname, newname)
 	})
 	if e != nil {
 		return &LinkError{"link", oldname, newname, e}
 	}
 	return nil
 }

 // Symlink creates newname as a symbolic link to oldname.
 // If there is an error, it will be of type *LinkError.
 func Symlink(oldname, newname string) error {
 	e := ignoringEINTR(func() error {
 		return syscall.Symlink(oldname, newname)
 	})
 	if e != nil {
 		return &LinkError{"symlink", oldname, newname, e}
 	}
 	return nil
 }

 // Readlink returns the destination of the named symbolic link.
 // If there is an error, it will be of type *PathError.
 func Readlink(name string) (string, error) {
 	for len := 128; ; len *= 2 {
 		b := make([]byte, len)
 		var (
 			n int
 			e error
 		)
 		for {
 			n, e = fixCount(syscall.Readlink(name, b))
 			if e != syscall.EINTR {
 				break
 			}
 		}
 		// buffer too small
 		if runtime.GOOS == "aix" && e == syscall.ERANGE {
 			continue
 		}
 		if e != nil {
 			return "", &PathError{Op: "readlink", Path: name, Err: e}
 		}
 		if n < len {
 			return string(b[0:n]), nil
 		}
 	}
 }

 type unixDirent struct {
 	parent string
 	name   string
 	typ    FileMode
 	info   FileInfo
 }

 func (d *unixDirent) Name() string   { return d.name }
 func (d *unixDirent) IsDir() bool    { return d.typ.IsDir() }
 func (d *unixDirent) Type() FileMode { return d.typ }

 func (d *unixDirent) Info() (FileInfo, error) {
 	if d.info != nil {
 		return d.info, nil
 	}
 	return lstat(d.parent + "/" + d.name)
 }

 func newUnixDirent(parent, name string, typ FileMode) (DirEntry, error) {
 	ude := &unixDirent{
 		parent: parent,
 		name:   name,
 		typ:    typ,
 	}
 	if typ != ^FileMode(0) && !testingForceReadDirLstat {
 		return ude, nil
 	}

 	info, err := lstat(parent + "/" + name)
 	if err != nil {
 		return nil, err
 	}

 	ude.typ = info.Mode().Type()
 	ude.info = info
 	return ude, 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.

	//go:build aix \|\| darwin \|\| dragonfly \|\| freebsd \|\| (js && wasm) \|\| linux \|\| netbsd \|\| openbsd \|\| solaris
	// +build aix darwin dragonfly freebsd js,wasm linux netbsd openbsd solaris

	package os

	import (
	"internal/poll"
	"internal/syscall/unix"
	"runtime"
	"syscall"
	)

	// fixLongPath is a noop on non-Windows platforms.
	func fixLongPath(path string) string {
	return path
	}

	func rename(oldname, newname string) error {
	fi, err := Lstat(newname)
	if err == nil && fi.IsDir() {
	// There are two independent errors this function can return:
	// one for a bad oldname, and one for a bad newname.
	// At this point we've determined the newname is bad.
	// But just in case oldname is also bad, prioritize returning
	// the oldname error because that's what we did historically.
	// However, if the old name and new name are not the same, yet
	// they refer to the same file, it implies a case-only
	// rename on a case-insensitive filesystem, which is ok.
	if ofi, err := Lstat(oldname); err != nil {
	if pe, ok := err.(*PathError); ok {
	err = pe.Err
	}
	return &LinkError{"rename", oldname, newname, err}
	} else if newname == oldname \|\| !SameFile(fi, ofi) {
	return &LinkError{"rename", oldname, newname, syscall.EEXIST}
	}
	}
	err = ignoringEINTR(func() error {
	return syscall.Rename(oldname, newname)
	})
	if err != nil {
	return &LinkError{"rename", oldname, newname, err}
	}
	return nil
	}

	// 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 *dirInfo // nil unless directory being read
	nonblock bool // whether we set nonblocking mode
	stdoutOrErr bool // whether this is stdout or stderr
	appendMode bool // whether file is opened for appending
	}

	// Fd returns the integer Unix file descriptor 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 SetDeadline
	// methods to stop working.
	// Because file descriptors can be reused, the returned file descriptor may
	// only be closed through the Close method of f, or by its finalizer during
	// garbage collection. Otherwise, during garbage collection the finalizer
	// may close an unrelated file descriptor with the same (reused) number.
	//
	// As an alternative, see the f.SyscallConn method.
	func (f *File) Fd() uintptr {
	if f == nil {
	return ^(uintptr(0))
	}

	// If we put the file descriptor into nonblocking mode,
	// then set it to blocking mode before we return it,
	// because historically we have always returned a descriptor
	// opened in blocking mode. The File will continue to work,
	// but any blocking operation will tie up a thread.
	if f.nonblock {
	f.pfd.SetBlocking()
	}

	return uintptr(f.pfd.Sysfd)
	}

	// 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. On Unix systems, if the file descriptor is in
	// non-blocking mode, NewFile will attempt to return a pollable File
	// (one for which the SetDeadline methods work).
	//
	// After passing it to NewFile, fd may become invalid under the same
	// conditions described in the comments of the Fd method, and the same
	// constraints apply.
	func NewFile(fd uintptr, name string) *File {
	kind := kindNewFile
	if nb, err := unix.IsNonblock(int(fd)); err == nil && nb {
	kind = kindNonBlock
	}
	return newFile(fd, name, kind)
	}

	// newFileKind describes the kind of file to newFile.
	type newFileKind int

	const (
	kindNewFile newFileKind = iota
	kindOpenFile
	kindPipe
	kindNonBlock
	)

	// newFile is like NewFile, but if called from OpenFile or Pipe
	// (as passed in the kind parameter) it tries to add the file to
	// the runtime poller.
	func newFile(fd uintptr, name string, kind newFileKind) *File {
	fdi := int(fd)
	if fdi < 0 {
	return nil
	}
	f := &File{&file{
	pfd: poll.FD{
	Sysfd: fdi,
	IsStream: true,
	ZeroReadIsEOF: true,
	},
	name: name,
	stdoutOrErr: fdi == 1 \|\| fdi == 2,
	}}

	pollable := kind == kindOpenFile \|\| kind == kindPipe \|\| kind == kindNonBlock

	// If the caller passed a non-blocking filedes (kindNonBlock),
	// we assume they know what they are doing so we allow it to be
	// used with kqueue.
	if kind == kindOpenFile {
	switch runtime.GOOS {
	case "darwin", "ios", "dragonfly", "freebsd", "netbsd", "openbsd":
	var st syscall.Stat_t
	err := ignoringEINTR(func() error {
	return syscall.Fstat(fdi, &st)
	})
	typ := st.Mode & syscall.S_IFMT
	// Don't try to use kqueue with regular files on *BSDs.
	// On FreeBSD a regular file is always
	// reported as ready for writing.
	// On Dragonfly, NetBSD and OpenBSD the fd is signaled
	// only once as ready (both read and write).
	// Issue 19093.
	// Also don't add directories to the netpoller.
	if err == nil && (typ == syscall.S_IFREG \|\| typ == syscall.S_IFDIR) {
	pollable = false
	}

	// In addition to the behavior described above for regular files,
	// on Darwin, kqueue does not work properly with fifos:
	// closing the last writer does not cause a kqueue event
	// for any readers. See issue #24164.
	if (runtime.GOOS == "darwin" \|\| runtime.GOOS == "ios") && typ == syscall.S_IFIFO {
	pollable = false
	}
	}
	}

	if err := f.pfd.Init("file", pollable); err != nil {
	// An error here indicates a failure to register
	// with the netpoll system. That can happen for
	// a file descriptor that is not supported by
	// epoll/kqueue; for example, disk files on
	// GNU/Linux systems. We assume that any real error
	// will show up in later I/O.
	} else if pollable {
	// We successfully registered with netpoll, so put
	// the file into nonblocking mode.
	if err := syscall.SetNonblock(fdi, true); err == nil {
	f.nonblock = true
	}
	}

	runtime.SetFinalizer(f.file, (*file).close)
	return f
	}

	// epipecheck raises SIGPIPE if we get an EPIPE error on standard
	// output or standard error. See the SIGPIPE docs in os/signal, and
	// issue 11845.
	func epipecheck(file *File, e error) {
	if e == syscall.EPIPE && file.stdoutOrErr {
	sigpipe()
	}
	}

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

	// openFileNolog is the Unix implementation of OpenFile.
	// Changes here should be reflected in openFdAt, if relevant.
	func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
	setSticky := false
	if !supportsCreateWithStickyBit && flag&O_CREATE != 0 && perm&ModeSticky != 0 {
	if _, err := Stat(name); IsNotExist(err) {
	setSticky = true
	}
	}

	var r int
	for {
	var e error
	r, e = syscall.Open(name, flag\|syscall.O_CLOEXEC, syscallMode(perm))
	if e == nil {
	break
	}

	// We have to check EINTR here, per issues 11180 and 39237.
	if e == syscall.EINTR {
	continue
	}

	return nil, &PathError{Op: "open", Path: name, Err: e}
	}

	// open(2) itself won't handle the sticky bit on *BSD and Solaris
	if setSticky {
	setStickyBit(name)
	}

	// There's a race here with fork/exec, which we are
	// content to live with. See ../syscall/exec_unix.go.
	if !supportsCloseOnExec {
	syscall.CloseOnExec(r)
	}

	return newFile(uintptr(r), name, kindOpenFile), nil
	}

	func (file *file) close() error {
	if file == nil {
	return syscall.EINVAL
	}
	if file.dirinfo != nil {
	file.dirinfo.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 f.dirinfo != nil {
	// Free cached dirinfo, so we allocate a new one if we
	// access this file as a directory again. See #35767 and #37161.
	f.dirinfo.close()
	f.dirinfo = nil
	}
	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.
	// If there is an error, it will be of type *PathError.
	func Truncate(name string, size int64) error {
	e := ignoringEINTR(func() error {
	return syscall.Truncate(name, size)
	})
	if e != nil {
	return &PathError{Op: "truncate", Path: name, Err: e}
	}
	return nil
	}

	// Remove removes the named file or (empty) directory.
	// If there is an error, it will be of type *PathError.
	func Remove(name string) error {
	// System call interface forces us to know
	// whether name is a file or directory.
	// Try both: it is cheaper on average than
	// doing a Stat plus the right one.
	e := ignoringEINTR(func() error {
	return syscall.Unlink(name)
	})
	if e == nil {
	return nil
	}
	e1 := ignoringEINTR(func() error {
	return syscall.Rmdir(name)
	})
	if e1 == nil {
	return nil
	}

	// Both failed: figure out which error to return.
	// OS X and Linux differ on whether unlink(dir)
	// returns EISDIR, so can't use that. However,
	// both agree that rmdir(file) returns ENOTDIR,
	// so we can use that to decide which error is real.
	// Rmdir might also return ENOTDIR if given a bad
	// file path, like /etc/passwd/foo, but in that case,
	// both errors will be ENOTDIR, so it's okay to
	// use the error from unlink.
	if e1 != syscall.ENOTDIR {
	e = e1
	}
	return &PathError{Op: "remove", Path: name, Err: e}
	}

	func tempDir() string {
	dir := Getenv("TMPDIR")
	if dir == "" {
	if runtime.GOOS == "android" {
	dir = "/data/local/tmp"
	} else {
	dir = "/tmp"
	}
	}
	return dir
	}

	// 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 {
	e := ignoringEINTR(func() error {
	return syscall.Link(oldname, newname)
	})
	if e != nil {
	return &LinkError{"link", oldname, newname, e}
	}
	return nil
	}

	// Symlink creates newname as a symbolic link to oldname.
	// If there is an error, it will be of type *LinkError.
	func Symlink(oldname, newname string) error {
	e := ignoringEINTR(func() error {
	return syscall.Symlink(oldname, newname)
	})
	if e != nil {
	return &LinkError{"symlink", oldname, newname, e}
	}
	return nil
	}

	// Readlink returns the destination of the named symbolic link.
	// If there is an error, it will be of type *PathError.
	func Readlink(name string) (string, error) {
	for len := 128; ; len *= 2 {
	b := make([]byte, len)
	var (
	n int
	e error
	)
	for {
	n, e = fixCount(syscall.Readlink(name, b))
	if e != syscall.EINTR {
	break
	}
	}
	// buffer too small
	if runtime.GOOS == "aix" && e == syscall.ERANGE {
	continue
	}
	if e != nil {
	return "", &PathError{Op: "readlink", Path: name, Err: e}
	}
	if n < len {
	return string(b[0:n]), nil
	}
	}
	}

	type unixDirent struct {
	parent string
	name string
	typ FileMode
	info FileInfo
	}

	func (d *unixDirent) Name() string { return d.name }
	func (d *unixDirent) IsDir() bool { return d.typ.IsDir() }
	func (d *unixDirent) Type() FileMode { return d.typ }

	func (d *unixDirent) Info() (FileInfo, error) {
	if d.info != nil {
	return d.info, nil
	}
	return lstat(d.parent + "/" + d.name)
	}

	func newUnixDirent(parent, name string, typ FileMode) (DirEntry, error) {
	ude := &unixDirent{
	parent: parent,
	name: name,
	typ: typ,
	}
	if typ != ^FileMode(0) && !testingForceReadDirLstat {
	return ude, nil
	}

	info, err := lstat(parent + "/" + name)
	if err != nil {
	return nil, err
	}

	ude.typ = info.Mode().Type()
	ude.info = info
	return ude, nil
	}