src/os/file.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.

 // Package os provides a platform-independent interface to operating system
 // functionality. The design is Unix-like, although the error handling is
 // Go-like; failing calls return values of type error rather than error numbers.
 // Often, more information is available within the error. For example,
 // if a call that takes a file name fails, such as Open or Stat, the error
 // will include the failing file name when printed and will be of type
 // *PathError, which may be unpacked for more information.
 //
 // The os interface is intended to be uniform across all operating systems.
 // Features not generally available appear in the system-specific package syscall.
 //
 // Here is a simple example, opening a file and reading some of it.
 //
 //	file, err := os.Open("file.go") // For read access.
 //	if err != nil {
 //		log.Fatal(err)
 //	}
 //
 // If the open fails, the error string will be self-explanatory, like
 //
 //	open file.go: no such file or directory
 //
 // The file's data can then be read into a slice of bytes. Read and
 // Write take their byte counts from the length of the argument slice.
 //
 //	data := make([]byte, 100)
 //	count, err := file.Read(data)
 //	if err != nil {
 //		log.Fatal(err)
 //	}
 //	fmt.Printf("read %d bytes: %q\n", count, data[:count])
 //
 // Note: The maximum number of concurrent operations on a File may be limited by
 // the OS or the system. The number should be high, but exceeding it may degrade
 // performance or cause other issues.
 //
 package os

 import (
 	"errors"
 	"internal/poll"
 	"internal/testlog"
 	"io"
 	"runtime"
 	"syscall"
 	"time"
 )

 // Name returns the name of the file as presented to Open.
 func (f *File) Name() string { return f.name }

 // Stdin, Stdout, and Stderr are open Files pointing to the standard input,
 // standard output, and standard error file descriptors.
 //
 // Note that the Go runtime writes to standard error for panics and crashes;
 // closing Stderr may cause those messages to go elsewhere, perhaps
 // to a file opened later.
 var (
 	Stdin  = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
 	Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
 	Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
 )

 // Flags to OpenFile wrapping those of the underlying system. Not all
 // flags may be implemented on a given system.
 const (
 	// Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
 	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
 	O_WRONLY int = syscall.O_WRONLY // open the file write-only.
 	O_RDWR   int = syscall.O_RDWR   // open the file read-write.
 	// The remaining values may be or'ed in to control behavior.
 	O_APPEND int = syscall.O_APPEND // append data to the file when writing.
 	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
 	O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.
 	O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
 	O_TRUNC  int = syscall.O_TRUNC  // truncate regular writable file when opened.
 )

 // Seek whence values.
 //
 // Deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.
 const (
 	SEEK_SET int = 0 // seek relative to the origin of the file
 	SEEK_CUR int = 1 // seek relative to the current offset
 	SEEK_END int = 2 // seek relative to the end
 )

 // LinkError records an error during a link or symlink or rename
 // system call and the paths that caused it.
 type LinkError struct {
 	Op  string
 	Old string
 	New string
 	Err error
 }

 func (e *LinkError) Error() string {
 	return e.Op + " " + e.Old + " " + e.New + ": " + e.Err.Error()
 }

 func (e *LinkError) Unwrap() error {
 	return e.Err
 }

 // Read reads up to len(b) bytes from the File.
 // It returns the number of bytes read and any error encountered.
 // At end of file, Read returns 0, io.EOF.
 func (f *File) Read(b []byte) (n int, err error) {
 	if err := f.checkValid("read"); err != nil {
 		return 0, err
 	}
 	n, e := f.read(b)
 	return n, f.wrapErr("read", e)
 }

 // ReadAt reads len(b) bytes from the File starting at byte offset off.
 // It returns the number of bytes read and the error, if any.
 // ReadAt always returns a non-nil error when n < len(b).
 // At end of file, that error is io.EOF.
 func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
 	if err := f.checkValid("read"); err != nil {
 		return 0, err
 	}

 	if off < 0 {
 		return 0, &PathError{"readat", f.name, errors.New("negative offset")}
 	}

 	for len(b) > 0 {
 		m, e := f.pread(b, off)
 		if e != nil {
 			err = f.wrapErr("read", e)
 			break
 		}
 		n += m
 		b = b[m:]
 		off += int64(m)
 	}
 	return
 }

 // Write writes len(b) bytes to the File.
 // It returns the number of bytes written and an error, if any.
 // Write returns a non-nil error when n != len(b).
 func (f *File) Write(b []byte) (n int, err error) {
 	if err := f.checkValid("write"); err != nil {
 		return 0, err
 	}
 	n, e := f.write(b)
 	if n < 0 {
 		n = 0
 	}
 	if n != len(b) {
 		err = io.ErrShortWrite
 	}

 	epipecheck(f, e)

 	if e != nil {
 		err = f.wrapErr("write", e)
 	}

 	return n, err
 }

 var errWriteAtInAppendMode = errors.New("os: invalid use of WriteAt on file opened with O_APPEND")

 // WriteAt writes len(b) bytes to the File starting at byte offset off.
 // It returns the number of bytes written and an error, if any.
 // WriteAt returns a non-nil error when n != len(b).
 //
 // If file was opened with the O_APPEND flag, WriteAt returns an error.
 func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
 	if err := f.checkValid("write"); err != nil {
 		return 0, err
 	}
 	if f.appendMode {
 		return 0, errWriteAtInAppendMode
 	}

 	if off < 0 {
 		return 0, &PathError{"writeat", f.name, errors.New("negative offset")}
 	}

 	for len(b) > 0 {
 		m, e := f.pwrite(b, off)
 		if e != nil {
 			err = f.wrapErr("write", e)
 			break
 		}
 		n += m
 		b = b[m:]
 		off += int64(m)
 	}
 	return
 }

 // 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.
 // The behavior of Seek on a file opened with O_APPEND is not specified.
 func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 	if err := f.checkValid("seek"); err != nil {
 		return 0, err
 	}
 	r, e := f.seek(offset, whence)
 	if e == nil && f.dirinfo != nil && r != 0 {
 		e = syscall.EISDIR
 	}
 	if e != nil {
 		return 0, f.wrapErr("seek", e)
 	}
 	return r, nil
 }

 // WriteString is like Write, but writes the contents of string s rather than
 // a slice of bytes.
 func (f *File) WriteString(s string) (n int, err error) {
 	return f.Write([]byte(s))
 }

 // Mkdir creates a new directory with the specified name and permission
 // bits (before umask).
 // If there is an error, it will be of type *PathError.
 func Mkdir(name string, perm FileMode) error {
 	if runtime.GOOS == "windows" && isWindowsNulName(name) {
 		return &PathError{"mkdir", name, syscall.ENOTDIR}
 	}
 	e := syscall.Mkdir(fixLongPath(name), syscallMode(perm))

 	if e != nil {
 		return &PathError{"mkdir", name, e}
 	}

 	// mkdir(2) itself won't handle the sticky bit on *BSD and Solaris
 	if !supportsCreateWithStickyBit && perm&ModeSticky != 0 {
 		e = setStickyBit(name)

 		if e != nil {
 			Remove(name)
 			return e
 		}
 	}

 	return nil
 }

 // setStickyBit adds ModeSticky to the permission bits of path, non atomic.
 func setStickyBit(name string) error {
 	fi, err := Stat(name)
 	if err != nil {
 		return err
 	}
 	return Chmod(name, fi.Mode()|ModeSticky)
 }

 // Chdir changes the current working directory to the named directory.
 // If there is an error, it will be of type *PathError.
 func Chdir(dir string) error {
 	if e := syscall.Chdir(dir); e != nil {
 		testlog.Open(dir) // observe likely non-existent directory
 		return &PathError{"chdir", dir, e}
 	}
 	if log := testlog.Logger(); log != nil {
 		wd, err := Getwd()
 		if err == nil {
 			log.Chdir(wd)
 		}
 	}
 	return nil
 }

 // Open opens the named file for reading. If successful, methods on
 // the returned file can be used for reading; the associated file
 // descriptor has mode O_RDONLY.
 // If there is an error, it will be of type *PathError.
 func Open(name string) (*File, error) {
 	return OpenFile(name, O_RDONLY, 0)
 }

 // Create creates or truncates the named file. If the file already exists,
 // it is truncated. If the file does not exist, it is created with mode 0666
 // (before umask). If successful, methods on the returned File can
 // be used for I/O; the associated file descriptor has mode O_RDWR.
 // If there is an error, it will be of type *PathError.
 func Create(name string) (*File, error) {
 	return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
 }

 // OpenFile is the generalized open call; most users will use Open
 // or Create instead. It opens the named file with specified flag
 // (O_RDONLY etc.). If the file does not exist, and the O_CREATE flag
 // is passed, it is created with mode perm (before umask). If successful,
 // methods on the returned File can be used for I/O.
 // If there is an error, it will be of type *PathError.
 func OpenFile(name string, flag int, perm FileMode) (*File, error) {
 	testlog.Open(name)
 	f, err := openFileNolog(name, flag, perm)
 	if err != nil {
 		return nil, err
 	}
 	f.appendMode = flag&O_APPEND != 0

 	return f, nil
 }

 // lstat is overridden in tests.
 var lstat = Lstat

 // Rename renames (moves) oldpath to newpath.
 // If newpath already exists and is not a directory, Rename replaces it.
 // OS-specific restrictions may apply when oldpath and newpath are in different directories.
 // If there is an error, it will be of type *LinkError.
 func Rename(oldpath, newpath string) error {
 	return rename(oldpath, newpath)
 }

 // Many functions in package syscall return a count of -1 instead of 0.
 // Using fixCount(call()) instead of call() corrects the count.
 func fixCount(n int, err error) (int, error) {
 	if n < 0 {
 		n = 0
 	}
 	return n, err
 }

 // wrapErr wraps an error that occurred during an operation on an open file.
 // It passes io.EOF through unchanged, otherwise converts
 // poll.ErrFileClosing to ErrClosed and wraps the error in a PathError.
 func (f *File) wrapErr(op string, err error) error {
 	if err == nil || err == io.EOF {
 		return err
 	}
 	if err == poll.ErrFileClosing {
 		err = ErrClosed
 	}
 	return &PathError{op, f.name, err}
 }

 // TempDir returns the default directory to use for temporary files.
 //
 // On Unix systems, it returns $TMPDIR if non-empty, else /tmp.
 // On Windows, it uses GetTempPath, returning the first non-empty
 // value from %TMP%, %TEMP%, %USERPROFILE%, or the Windows directory.
 // On Plan 9, it returns /tmp.
 //
 // The directory is neither guaranteed to exist nor have accessible
 // permissions.
 func TempDir() string {
 	return tempDir()
 }

 // UserCacheDir returns the default root directory to use for user-specific
 // cached data. Users should create their own application-specific subdirectory
 // within this one and use that.
 //
 // On Unix systems, it returns $XDG_CACHE_HOME as specified by
 // https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html if
 // non-empty, else $HOME/.cache.
 // On Darwin, it returns $HOME/Library/Caches.
 // On Windows, it returns %LocalAppData%.
 // On Plan 9, it returns $home/lib/cache.
 //
 // If the location cannot be determined (for example, $HOME is not defined),
 // then it will return an error.
 func UserCacheDir() (string, error) {
 	var dir string

 	switch runtime.GOOS {
 	case "windows":
 		dir = Getenv("LocalAppData")
 		if dir == "" {
 			return "", errors.New("%LocalAppData% is not defined")
 		}

 	case "darwin":
 		dir = Getenv("HOME")
 		if dir == "" {
 			return "", errors.New("$HOME is not defined")
 		}
 		dir += "/Library/Caches"

 	case "plan9":
 		dir = Getenv("home")
 		if dir == "" {
 			return "", errors.New("$home is not defined")
 		}
 		dir += "/lib/cache"

 	default: // Unix
 		dir = Getenv("XDG_CACHE_HOME")
 		if dir == "" {
 			dir = Getenv("HOME")
 			if dir == "" {
 				return "", errors.New("neither $XDG_CACHE_HOME nor $HOME are defined")
 			}
 			dir += "/.cache"
 		}
 	}

 	return dir, nil
 }

 // UserConfigDir returns the default root directory to use for user-specific
 // configuration data. Users should create their own application-specific
 // subdirectory within this one and use that.
 //
 // On Unix systems, it returns $XDG_CONFIG_HOME as specified by
 // https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html if
 // non-empty, else $HOME/.config.
 // On Darwin, it returns $HOME/Library/Application Support.
 // On Windows, it returns %AppData%.
 // On Plan 9, it returns $home/lib.
 //
 // If the location cannot be determined (for example, $HOME is not defined),
 // then it will return an error.
 func UserConfigDir() (string, error) {
 	var dir string

 	switch runtime.GOOS {
 	case "windows":
 		dir = Getenv("AppData")
 		if dir == "" {
 			return "", errors.New("%AppData% is not defined")
 		}

 	case "darwin":
 		dir = Getenv("HOME")
 		if dir == "" {
 			return "", errors.New("$HOME is not defined")
 		}
 		dir += "/Library/Application Support"

 	case "plan9":
 		dir = Getenv("home")
 		if dir == "" {
 			return "", errors.New("$home is not defined")
 		}
 		dir += "/lib"

 	default: // Unix
 		dir = Getenv("XDG_CONFIG_HOME")
 		if dir == "" {
 			dir = Getenv("HOME")
 			if dir == "" {
 				return "", errors.New("neither $XDG_CONFIG_HOME nor $HOME are defined")
 			}
 			dir += "/.config"
 		}
 	}

 	return dir, nil
 }

 // UserHomeDir returns the current user's home directory.
 //
 // On Unix, including macOS, it returns the $HOME environment variable.
 // On Windows, it returns %USERPROFILE%.
 // On Plan 9, it returns the $home environment variable.
 func UserHomeDir() (string, error) {
 	env, enverr := "HOME", "$HOME"
 	switch runtime.GOOS {
 	case "windows":
 		env, enverr = "USERPROFILE", "%userprofile%"
 	case "plan9":
 		env, enverr = "home", "$home"
 	}
 	if v := Getenv(env); v != "" {
 		return v, nil
 	}
 	// On some geese the home directory is not always defined.
 	switch runtime.GOOS {
 	case "nacl":
 		return "/", nil
 	case "android":
 		return "/sdcard", nil
 	case "darwin":
 		if runtime.GOARCH == "arm" || runtime.GOARCH == "arm64" {
 			return "/", nil
 		}
 	}
 	return "", errors.New(enverr + " is not defined")
 }

 // Chmod changes the mode of the named file to mode.
 // If the file is a symbolic link, it changes the mode of the link's target.
 // If there is an error, it will be of type *PathError.
 //
 // A different subset of the mode bits are used, depending on the
 // operating system.
 //
 // On Unix, the mode's permission bits, ModeSetuid, ModeSetgid, and
 // ModeSticky are used.
 //
 // On Windows, only the 0200 bit (owner writable) of mode is used; it
 // controls whether the file's read-only attribute is set or cleared.
 // The other bits are currently unused. For compatibility with Go 1.12
 // and earlier, use a non-zero mode. Use mode 0400 for a read-only
 // file and 0600 for a readable+writable file.
 //
 // On Plan 9, the mode's permission bits, ModeAppend, ModeExclusive,
 // and ModeTemporary are used.
 func Chmod(name string, mode FileMode) error { return chmod(name, mode) }

 // Chmod changes the mode of the file to mode.
 // If there is an error, it will be of type *PathError.
 func (f *File) Chmod(mode FileMode) error { return f.chmod(mode) }

 // SetDeadline sets the read and write deadlines for a File.
 // It is equivalent to calling both SetReadDeadline and SetWriteDeadline.
 //
 // Only some kinds of files support setting a deadline. Calls to SetDeadline
 // for files that do not support deadlines will return ErrNoDeadline.
 // On most systems ordinary files do not support deadlines, but pipes do.
 //
 // A deadline is an absolute time after which I/O operations fail with an
 // error instead of blocking. The deadline applies to all future and pending
 // I/O, not just the immediately following call to Read or Write.
 // After a deadline has been exceeded, the connection can be refreshed
 // by setting a deadline in the future.
 //
 // An error returned after a timeout fails will implement the
 // Timeout method, and calling the Timeout method will return true.
 // The PathError and SyscallError types implement the Timeout method.
 // In general, call IsTimeout to test whether an error indicates a timeout.
 //
 // An idle timeout can be implemented by repeatedly extending
 // the deadline after successful Read or Write calls.
 //
 // A zero value for t means I/O operations will not time out.
 func (f *File) SetDeadline(t time.Time) error {
 	return f.setDeadline(t)
 }

 // SetReadDeadline sets the deadline for future Read calls and any
 // currently-blocked Read call.
 // A zero value for t means Read will not time out.
 // Not all files support setting deadlines; see SetDeadline.
 func (f *File) SetReadDeadline(t time.Time) error {
 	return f.setReadDeadline(t)
 }

 // SetWriteDeadline sets the deadline for any future Write calls and any
 // currently-blocked Write call.
 // Even if Write times out, it may return n > 0, indicating that
 // some of the data was successfully written.
 // A zero value for t means Write will not time out.
 // Not all files support setting deadlines; see SetDeadline.
 func (f *File) SetWriteDeadline(t time.Time) error {
 	return f.setWriteDeadline(t)
 }

 // SyscallConn returns a raw file.
 // This implements the syscall.Conn interface.
 func (f *File) SyscallConn() (syscall.RawConn, error) {
 	if err := f.checkValid("SyscallConn"); err != nil {
 		return nil, err
 	}
 	return newRawConn(f)
 }

 // isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
 // True is returned if name is 'NUL' whatever the case.
 func isWindowsNulName(name string) bool {
 	if len(name) != 3 {
 		return false
 	}
 	if name[0] != 'n' && name[0] != 'N' {
 		return false
 	}
 	if name[1] != 'u' && name[1] != 'U' {
 		return false
 	}
 	if name[2] != 'l' && name[2] != 'L' {
 		return false
 	}
 	return true
 }
	// 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 provides a platform-independent interface to operating system
	// functionality. The design is Unix-like, although the error handling is
	// Go-like; failing calls return values of type error rather than error numbers.
	// Often, more information is available within the error. For example,
	// if a call that takes a file name fails, such as Open or Stat, the error
	// will include the failing file name when printed and will be of type
	// *PathError, which may be unpacked for more information.
	//
	// The os interface is intended to be uniform across all operating systems.
	// Features not generally available appear in the system-specific package syscall.
	//
	// Here is a simple example, opening a file and reading some of it.
	//
	// file, err := os.Open("file.go") // For read access.
	// if err != nil {
	// log.Fatal(err)
	// }
	//
	// If the open fails, the error string will be self-explanatory, like
	//
	// open file.go: no such file or directory
	//
	// The file's data can then be read into a slice of bytes. Read and
	// Write take their byte counts from the length of the argument slice.
	//
	// data := make([]byte, 100)
	// count, err := file.Read(data)
	// if err != nil {
	// log.Fatal(err)
	// }
	// fmt.Printf("read %d bytes: %q\n", count, data[:count])
	//
	// Note: The maximum number of concurrent operations on a File may be limited by
	// the OS or the system. The number should be high, but exceeding it may degrade
	// performance or cause other issues.
	//
	package os

	import (
	"errors"
	"internal/poll"
	"internal/testlog"
	"io"
	"runtime"
	"syscall"
	"time"
	)

	// Name returns the name of the file as presented to Open.
	func (f *File) Name() string { return f.name }

	// Stdin, Stdout, and Stderr are open Files pointing to the standard input,
	// standard output, and standard error file descriptors.
	//
	// Note that the Go runtime writes to standard error for panics and crashes;
	// closing Stderr may cause those messages to go elsewhere, perhaps
	// to a file opened later.
	var (
	Stdin = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
	Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
	Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
	)

	// Flags to OpenFile wrapping those of the underlying system. Not all
	// flags may be implemented on a given system.
	const (
	// Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
	O_WRONLY int = syscall.O_WRONLY // open the file write-only.
	O_RDWR int = syscall.O_RDWR // open the file read-write.
	// The remaining values may be or'ed in to control behavior.
	O_APPEND int = syscall.O_APPEND // append data to the file when writing.
	O_CREATE int = syscall.O_CREAT // create a new file if none exists.
	O_EXCL int = syscall.O_EXCL // used with O_CREATE, file must not exist.
	O_SYNC int = syscall.O_SYNC // open for synchronous I/O.
	O_TRUNC int = syscall.O_TRUNC // truncate regular writable file when opened.
	)

	// Seek whence values.
	//
	// Deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.
	const (
	SEEK_SET int = 0 // seek relative to the origin of the file
	SEEK_CUR int = 1 // seek relative to the current offset
	SEEK_END int = 2 // seek relative to the end
	)

	// LinkError records an error during a link or symlink or rename
	// system call and the paths that caused it.
	type LinkError struct {
	Op string
	Old string
	New string
	Err error
	}

	func (e *LinkError) Error() string {
	return e.Op + " " + e.Old + " " + e.New + ": " + e.Err.Error()
	}

	func (e *LinkError) Unwrap() error {
	return e.Err
	}

	// Read reads up to len(b) bytes from the File.
	// It returns the number of bytes read and any error encountered.
	// At end of file, Read returns 0, io.EOF.
	func (f *File) Read(b []byte) (n int, err error) {
	if err := f.checkValid("read"); err != nil {
	return 0, err
	}
	n, e := f.read(b)
	return n, f.wrapErr("read", e)
	}

	// ReadAt reads len(b) bytes from the File starting at byte offset off.
	// It returns the number of bytes read and the error, if any.
	// ReadAt always returns a non-nil error when n < len(b).
	// At end of file, that error is io.EOF.
	func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
	if err := f.checkValid("read"); err != nil {
	return 0, err
	}

	if off < 0 {
	return 0, &PathError{"readat", f.name, errors.New("negative offset")}
	}

	for len(b) > 0 {
	m, e := f.pread(b, off)
	if e != nil {
	err = f.wrapErr("read", e)
	break
	}
	n += m
	b = b[m:]
	off += int64(m)
	}
	return
	}

	// Write writes len(b) bytes to the File.
	// It returns the number of bytes written and an error, if any.
	// Write returns a non-nil error when n != len(b).
	func (f *File) Write(b []byte) (n int, err error) {
	if err := f.checkValid("write"); err != nil {
	return 0, err
	}
	n, e := f.write(b)
	if n < 0 {
	n = 0
	}
	if n != len(b) {
	err = io.ErrShortWrite
	}

	epipecheck(f, e)

	if e != nil {
	err = f.wrapErr("write", e)
	}

	return n, err
	}

	var errWriteAtInAppendMode = errors.New("os: invalid use of WriteAt on file opened with O_APPEND")

	// WriteAt writes len(b) bytes to the File starting at byte offset off.
	// It returns the number of bytes written and an error, if any.
	// WriteAt returns a non-nil error when n != len(b).
	//
	// If file was opened with the O_APPEND flag, WriteAt returns an error.
	func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
	if err := f.checkValid("write"); err != nil {
	return 0, err
	}
	if f.appendMode {
	return 0, errWriteAtInAppendMode
	}

	if off < 0 {
	return 0, &PathError{"writeat", f.name, errors.New("negative offset")}
	}

	for len(b) > 0 {
	m, e := f.pwrite(b, off)
	if e != nil {
	err = f.wrapErr("write", e)
	break
	}
	n += m
	b = b[m:]
	off += int64(m)
	}
	return
	}

	// 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.
	// The behavior of Seek on a file opened with O_APPEND is not specified.
	func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
	if err := f.checkValid("seek"); err != nil {
	return 0, err
	}
	r, e := f.seek(offset, whence)
	if e == nil && f.dirinfo != nil && r != 0 {
	e = syscall.EISDIR
	}
	if e != nil {
	return 0, f.wrapErr("seek", e)
	}
	return r, nil
	}

	// WriteString is like Write, but writes the contents of string s rather than
	// a slice of bytes.
	func (f *File) WriteString(s string) (n int, err error) {
	return f.Write([]byte(s))
	}

	// Mkdir creates a new directory with the specified name and permission
	// bits (before umask).
	// If there is an error, it will be of type *PathError.
	func Mkdir(name string, perm FileMode) error {
	if runtime.GOOS == "windows" && isWindowsNulName(name) {
	return &PathError{"mkdir", name, syscall.ENOTDIR}
	}
	e := syscall.Mkdir(fixLongPath(name), syscallMode(perm))

	if e != nil {
	return &PathError{"mkdir", name, e}
	}

	// mkdir(2) itself won't handle the sticky bit on *BSD and Solaris
	if !supportsCreateWithStickyBit && perm&ModeSticky != 0 {
	e = setStickyBit(name)

	if e != nil {
	Remove(name)
	return e
	}
	}

	return nil
	}

	// setStickyBit adds ModeSticky to the permission bits of path, non atomic.
	func setStickyBit(name string) error {
	fi, err := Stat(name)
	if err != nil {
	return err
	}
	return Chmod(name, fi.Mode()\|ModeSticky)
	}

	// Chdir changes the current working directory to the named directory.
	// If there is an error, it will be of type *PathError.
	func Chdir(dir string) error {
	if e := syscall.Chdir(dir); e != nil {
	testlog.Open(dir) // observe likely non-existent directory
	return &PathError{"chdir", dir, e}
	}
	if log := testlog.Logger(); log != nil {
	wd, err := Getwd()
	if err == nil {
	log.Chdir(wd)
	}
	}
	return nil
	}

	// Open opens the named file for reading. If successful, methods on
	// the returned file can be used for reading; the associated file
	// descriptor has mode O_RDONLY.
	// If there is an error, it will be of type *PathError.
	func Open(name string) (*File, error) {
	return OpenFile(name, O_RDONLY, 0)
	}

	// Create creates or truncates the named file. If the file already exists,
	// it is truncated. If the file does not exist, it is created with mode 0666
	// (before umask). If successful, methods on the returned File can
	// be used for I/O; the associated file descriptor has mode O_RDWR.
	// If there is an error, it will be of type *PathError.
	func Create(name string) (*File, error) {
	return OpenFile(name, O_RDWR\|O_CREATE\|O_TRUNC, 0666)
	}

	// OpenFile is the generalized open call; most users will use Open
	// or Create instead. It opens the named file with specified flag
	// (O_RDONLY etc.). If the file does not exist, and the O_CREATE flag
	// is passed, it is created with mode perm (before umask). If successful,
	// methods on the returned File can be used for I/O.
	// If there is an error, it will be of type *PathError.
	func OpenFile(name string, flag int, perm FileMode) (*File, error) {
	testlog.Open(name)
	f, err := openFileNolog(name, flag, perm)
	if err != nil {
	return nil, err
	}
	f.appendMode = flag&O_APPEND != 0

	return f, nil
	}

	// lstat is overridden in tests.
	var lstat = Lstat

	// Rename renames (moves) oldpath to newpath.
	// If newpath already exists and is not a directory, Rename replaces it.
	// OS-specific restrictions may apply when oldpath and newpath are in different directories.
	// If there is an error, it will be of type *LinkError.
	func Rename(oldpath, newpath string) error {
	return rename(oldpath, newpath)
	}

	// Many functions in package syscall return a count of -1 instead of 0.
	// Using fixCount(call()) instead of call() corrects the count.
	func fixCount(n int, err error) (int, error) {
	if n < 0 {
	n = 0
	}
	return n, err
	}

	// wrapErr wraps an error that occurred during an operation on an open file.
	// It passes io.EOF through unchanged, otherwise converts
	// poll.ErrFileClosing to ErrClosed and wraps the error in a PathError.
	func (f *File) wrapErr(op string, err error) error {
	if err == nil \|\| err == io.EOF {
	return err
	}
	if err == poll.ErrFileClosing {
	err = ErrClosed
	}
	return &PathError{op, f.name, err}
	}

	// TempDir returns the default directory to use for temporary files.
	//
	// On Unix systems, it returns $TMPDIR if non-empty, else /tmp.
	// On Windows, it uses GetTempPath, returning the first non-empty
	// value from %TMP%, %TEMP%, %USERPROFILE%, or the Windows directory.
	// On Plan 9, it returns /tmp.
	//
	// The directory is neither guaranteed to exist nor have accessible
	// permissions.
	func TempDir() string {
	return tempDir()
	}

	// UserCacheDir returns the default root directory to use for user-specific
	// cached data. Users should create their own application-specific subdirectory
	// within this one and use that.
	//
	// On Unix systems, it returns $XDG_CACHE_HOME as specified by
	// https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html if
	// non-empty, else $HOME/.cache.
	// On Darwin, it returns $HOME/Library/Caches.
	// On Windows, it returns %LocalAppData%.
	// On Plan 9, it returns $home/lib/cache.
	//
	// If the location cannot be determined (for example, $HOME is not defined),
	// then it will return an error.
	func UserCacheDir() (string, error) {
	var dir string

	switch runtime.GOOS {
	case "windows":
	dir = Getenv("LocalAppData")
	if dir == "" {
	return "", errors.New("%LocalAppData% is not defined")
	}

	case "darwin":
	dir = Getenv("HOME")
	if dir == "" {
	return "", errors.New("$HOME is not defined")
	}
	dir += "/Library/Caches"

	case "plan9":
	dir = Getenv("home")
	if dir == "" {
	return "", errors.New("$home is not defined")
	}
	dir += "/lib/cache"

	default: // Unix
	dir = Getenv("XDG_CACHE_HOME")
	if dir == "" {
	dir = Getenv("HOME")
	if dir == "" {
	return "", errors.New("neither $XDG_CACHE_HOME nor $HOME are defined")
	}
	dir += "/.cache"
	}
	}

	return dir, nil
	}

	// UserConfigDir returns the default root directory to use for user-specific
	// configuration data. Users should create their own application-specific
	// subdirectory within this one and use that.
	//
	// On Unix systems, it returns $XDG_CONFIG_HOME as specified by
	// https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html if
	// non-empty, else $HOME/.config.
	// On Darwin, it returns $HOME/Library/Application Support.
	// On Windows, it returns %AppData%.
	// On Plan 9, it returns $home/lib.
	//
	// If the location cannot be determined (for example, $HOME is not defined),
	// then it will return an error.
	func UserConfigDir() (string, error) {
	var dir string

	switch runtime.GOOS {
	case "windows":
	dir = Getenv("AppData")
	if dir == "" {
	return "", errors.New("%AppData% is not defined")
	}

	case "darwin":
	dir = Getenv("HOME")
	if dir == "" {
	return "", errors.New("$HOME is not defined")
	}
	dir += "/Library/Application Support"

	case "plan9":
	dir = Getenv("home")
	if dir == "" {
	return "", errors.New("$home is not defined")
	}
	dir += "/lib"

	default: // Unix
	dir = Getenv("XDG_CONFIG_HOME")
	if dir == "" {
	dir = Getenv("HOME")
	if dir == "" {
	return "", errors.New("neither $XDG_CONFIG_HOME nor $HOME are defined")
	}
	dir += "/.config"
	}
	}

	return dir, nil
	}

	// UserHomeDir returns the current user's home directory.
	//
	// On Unix, including macOS, it returns the $HOME environment variable.
	// On Windows, it returns %USERPROFILE%.
	// On Plan 9, it returns the $home environment variable.
	func UserHomeDir() (string, error) {
	env, enverr := "HOME", "$HOME"
	switch runtime.GOOS {
	case "windows":
	env, enverr = "USERPROFILE", "%userprofile%"
	case "plan9":
	env, enverr = "home", "$home"
	}
	if v := Getenv(env); v != "" {
	return v, nil
	}
	// On some geese the home directory is not always defined.
	switch runtime.GOOS {
	case "nacl":
	return "/", nil
	case "android":
	return "/sdcard", nil
	case "darwin":
	if runtime.GOARCH == "arm" \|\| runtime.GOARCH == "arm64" {
	return "/", nil
	}
	}
	return "", errors.New(enverr + " is not defined")
	}

	// Chmod changes the mode of the named file to mode.
	// If the file is a symbolic link, it changes the mode of the link's target.
	// If there is an error, it will be of type *PathError.
	//
	// A different subset of the mode bits are used, depending on the
	// operating system.
	//
	// On Unix, the mode's permission bits, ModeSetuid, ModeSetgid, and
	// ModeSticky are used.
	//
	// On Windows, only the 0200 bit (owner writable) of mode is used; it
	// controls whether the file's read-only attribute is set or cleared.
	// The other bits are currently unused. For compatibility with Go 1.12
	// and earlier, use a non-zero mode. Use mode 0400 for a read-only
	// file and 0600 for a readable+writable file.
	//
	// On Plan 9, the mode's permission bits, ModeAppend, ModeExclusive,
	// and ModeTemporary are used.
	func Chmod(name string, mode FileMode) error { return chmod(name, mode) }

	// Chmod changes the mode of the file to mode.
	// If there is an error, it will be of type *PathError.
	func (f *File) Chmod(mode FileMode) error { return f.chmod(mode) }

	// SetDeadline sets the read and write deadlines for a File.
	// It is equivalent to calling both SetReadDeadline and SetWriteDeadline.
	//
	// Only some kinds of files support setting a deadline. Calls to SetDeadline
	// for files that do not support deadlines will return ErrNoDeadline.
	// On most systems ordinary files do not support deadlines, but pipes do.
	//
	// A deadline is an absolute time after which I/O operations fail with an
	// error instead of blocking. The deadline applies to all future and pending
	// I/O, not just the immediately following call to Read or Write.
	// After a deadline has been exceeded, the connection can be refreshed
	// by setting a deadline in the future.
	//
	// An error returned after a timeout fails will implement the
	// Timeout method, and calling the Timeout method will return true.
	// The PathError and SyscallError types implement the Timeout method.
	// In general, call IsTimeout to test whether an error indicates a timeout.
	//
	// An idle timeout can be implemented by repeatedly extending
	// the deadline after successful Read or Write calls.
	//
	// A zero value for t means I/O operations will not time out.
	func (f *File) SetDeadline(t time.Time) error {
	return f.setDeadline(t)
	}

	// SetReadDeadline sets the deadline for future Read calls and any
	// currently-blocked Read call.
	// A zero value for t means Read will not time out.
	// Not all files support setting deadlines; see SetDeadline.
	func (f *File) SetReadDeadline(t time.Time) error {
	return f.setReadDeadline(t)
	}

	// SetWriteDeadline sets the deadline for any future Write calls and any
	// currently-blocked Write call.
	// Even if Write times out, it may return n > 0, indicating that
	// some of the data was successfully written.
	// A zero value for t means Write will not time out.
	// Not all files support setting deadlines; see SetDeadline.
	func (f *File) SetWriteDeadline(t time.Time) error {
	return f.setWriteDeadline(t)
	}

	// SyscallConn returns a raw file.
	// This implements the syscall.Conn interface.
	func (f *File) SyscallConn() (syscall.RawConn, error) {
	if err := f.checkValid("SyscallConn"); err != nil {
	return nil, err
	}
	return newRawConn(f)
	}

	// isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
	// True is returned if name is 'NUL' whatever the case.
	func isWindowsNulName(name string) bool {
	if len(name) != 3 {
	return false
	}
	if name[0] != 'n' && name[0] != 'N' {
	return false
	}
	if name[1] != 'u' && name[1] != 'U' {
	return false
	}
	if name[2] != 'l' && name[2] != 'L' {
	return false
	}
	return true
	}