Source File
	file.go

Belonging Package
	os

// 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])
//
// # Concurrency
//
// The methods of [File] correspond to file system operations. All are
// safe for concurrent use. 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/filepathlite"
	"internal/poll"
	"internal/testlog"
	"io"
	"io/fs"
	"runtime"
	"slices"
	"syscall"
	"time"
	"unsafe"
)

// Name returns the name of the file as presented to Open.
//
// It is safe to call Name after [Close].
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
}

// 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.
//
// NewFile's behavior differs on some platforms:
//
//   - On Unix, if fd is in non-blocking mode, NewFile will attempt to return a pollable file.
//   - On Windows, if fd is opened for asynchronous I/O (that is, [syscall.FILE_FLAG_OVERLAPPED]
//     has been specified in the [syscall.CreateFile] call), NewFile will attempt to return a pollable
//     file by associating fd with the Go runtime I/O completion port.
//     The I/O operations will be performed synchronously if the association fails.
//
// Only pollable files support [File.SetDeadline], [File.SetReadDeadline], and [File.SetWriteDeadline].
//
// After passing it to NewFile, fd may become invalid under the same conditions described
// in the comments of [File.Fd], and the same constraints apply.
func NewFile(fd uintptr, name string) *File {
	return newFileFromNewFile(fd, name)
}

// Read reads up to len(b) bytes from the File and stores them in b.
// 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{Op: "readat", Path: f.name, Err: 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
}

// ReadFrom implements io.ReaderFrom.
func (f *File) ReadFrom(r io.Reader) (n int64, err error) {
	if err := f.checkValid("write"); err != nil {
		return 0, err
	}
	n, handled, e := f.readFrom(r)
	if !handled {
		return genericReadFrom(f, r) // without wrapping
	}
	return n, f.wrapErr("write", e)
}

// noReadFrom can be embedded alongside another type to
// hide the ReadFrom method of that other type.
type noReadFrom struct{}

// ReadFrom hides another ReadFrom method.
// It should never be called.
func (noReadFrom) ReadFrom(io.Reader) (int64, error) {
	panic("can't happen")
}

// fileWithoutReadFrom implements all the methods of *File other
// than ReadFrom. This is used to permit ReadFrom to call io.Copy
// without leading to a recursive call to ReadFrom.
type fileWithoutReadFrom struct {
	noReadFrom
	*File
}

func genericReadFrom(f *File, r io.Reader) (int64, error) {
	return io.Copy(fileWithoutReadFrom{File: f}, r)
}

// Write writes len(b) bytes from b 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{Op: "writeat", Path: f.name, Err: 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
}

// WriteTo implements io.WriterTo.
func (f *File) WriteTo(w io.Writer) (n int64, err error) {
	if err := f.checkValid("read"); err != nil {
		return 0, err
	}
	n, handled, e := f.writeTo(w)
	if handled {
		return n, f.wrapErr("read", e)
	}
	return genericWriteTo(f, w) // without wrapping
}

// noWriteTo can be embedded alongside another type to
// hide the WriteTo method of that other type.
type noWriteTo struct{}

// WriteTo hides another WriteTo method.
// It should never be called.
func (noWriteTo) WriteTo(io.Writer) (int64, error) {
	panic("can't happen")
}

// fileWithoutWriteTo implements all the methods of *File other
// than WriteTo. This is used to permit WriteTo to call io.Copy
// without leading to a recursive call to WriteTo.
type fileWithoutWriteTo struct {
	noWriteTo
	*File
}

func genericWriteTo(f *File, w io.Writer) (int64, error) {
	return io.Copy(w, fileWithoutWriteTo{File: f})
}

// 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.Load() != 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) {
	b := unsafe.Slice(unsafe.StringData(s), len(s))
	return f.Write(b)
}

// 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 {
	longName := fixLongPath(name)
	e := ignoringEINTR(func() error {
		return syscall.Mkdir(longName, syscallMode(perm))
	})

	if e != nil {
		return &PathError{Op: "mkdir", Path: name, Err: 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{Op: "chdir", Path: dir, Err: e}
	}
	if runtime.GOOS == "windows" {
		abs := filepathlite.IsAbs(dir)
		getwdCache.Lock()
		if abs {
			getwdCache.dir = dir
		} else {
			getwdCache.dir = ""
		}
		getwdCache.Unlock()
	}
	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 0o666
// (before umask). If successful, methods on the returned File can
// be used for I/O; the associated file descriptor has mode [O_RDWR].
// The directory containing the file must already exist.
// 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);
// the containing directory must exist. 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
}

var errPathEscapes = errors.New("path escapes from parent")

// openDir opens a file which is assumed to be a directory. As such, it skips
// the syscalls that make the file descriptor non-blocking as these take time
// and will fail on file descriptors for directories.
func openDir(name string) (*File, error) {
	testlog.Open(name)
	return openDirNolog(name)
}

// 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.
// If newpath already exists and is a directory, Rename returns an error.
// OS-specific restrictions may apply when oldpath and newpath are in different directories.
// Even within the same directory, on non-Unix platforms Rename is not an atomic operation.
// If there is an error, it will be of type *LinkError.
func Rename(oldpath, newpath string) error {
	return rename(oldpath, newpath)
}

// Readlink returns the destination of the named symbolic link.
// If there is an error, it will be of type [*PathError].
//
// If the link destination is relative, Readlink returns the relative path
// without resolving it to an absolute one.
func Readlink(name string) (string, error) {
	return readlink(name)
}

// 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
}

// checkWrapErr is the test hook to enable checking unexpected wrapped errors of poll.ErrFileClosing.
// It is set to true in the export_test.go for tests (including fuzz tests).
var checkWrapErr = false

// 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
	} else if checkWrapErr && errors.Is(err, poll.ErrFileClosing) {
		panic("unexpected error wrapping poll.ErrFileClosing: " + err.Error())
	}
	return &PathError{Op: op, Path: f.name, Err: 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://specifications.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) or
// the path in $XDG_CACHE_HOME is relative, 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", "ios":
		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"
		} else if !filepathlite.IsAbs(dir) {
			return "", errors.New("path in $XDG_CACHE_HOME is relative")
		}
	}

	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://specifications.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) or
// the path in $XDG_CONFIG_HOME is relative, 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", "ios":
		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"
		} else if !filepathlite.IsAbs(dir) {
			return "", errors.New("path in $XDG_CONFIG_HOME is relative")
		}
	}

	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 $home environment variable.
//
// If the expected variable is not set in the environment, UserHomeDir
// returns either a platform-specific default value or a non-nil error.
func UserHomeDir() (string, error) {
	env, enverr := "HOME", "$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 operating systems the home directory is not always defined.
	switch runtime.GOOS {
	case "android":
		return "/sdcard", nil
	case "ios":
		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 0o200 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 0o400 for a read-only
// file and 0o600 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.
//
// If the deadline is exceeded a call to Read or Write or to other I/O
// methods will return an error that wraps ErrDeadlineExceeded.
// This can be tested using errors.Is(err, os.ErrDeadlineExceeded).
// That error implements the Timeout method, and calling the Timeout
// method will return true, but there are other possible errors for which
// the Timeout will return true even if the deadline has not been exceeded.
//
// 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)
}

// Fd returns the system file descriptor or handle referencing the open file.
// If f is closed, the descriptor becomes invalid.
// If f is garbage collected, a finalizer may close the descriptor,
// making it invalid; see [runtime.SetFinalizer] for more information on when
// a finalizer might be run.
//
// Do not close the returned descriptor; that could cause a later
// close of f to close an unrelated descriptor.
//
// Fd's behavior differs on some platforms:
//
//   - On Unix and Windows, [File.SetDeadline] methods will stop working.
//   - On Windows, the file descriptor will be disassociated from the
//     Go runtime I/O completion port if there are no concurrent I/O
//     operations on the file.
//
// For most uses prefer the f.SyscallConn method.
func (f *File) Fd() uintptr {
	return f.fd()
}

// DirFS returns a file system (an fs.FS) for the tree of files rooted at the directory dir.
//
// Note that DirFS("/prefix") only guarantees that the Open calls it makes to the
// operating system will begin with "/prefix": DirFS("/prefix").Open("file") is the
// same as os.Open("/prefix/file"). So if /prefix/file is a symbolic link pointing outside
// the /prefix tree, then using DirFS does not stop the access any more than using
// os.Open does. Additionally, the root of the fs.FS returned for a relative path,
// DirFS("prefix"), will be affected by later calls to Chdir. DirFS is therefore not
// a general substitute for a chroot-style security mechanism when the directory tree
// contains arbitrary content.
//
// Use [Root.FS] to obtain a fs.FS that prevents escapes from the tree via symbolic links.
//
// The directory dir must not be "".
//
// The result implements [io/fs.StatFS], [io/fs.ReadFileFS], [io/fs.ReadDirFS], and
// [io/fs.ReadLinkFS].
func DirFS(dir string) fs.FS {
	return dirFS(dir)
}

var _ fs.StatFS = dirFS("")
var _ fs.ReadFileFS = dirFS("")
var _ fs.ReadDirFS = dirFS("")
var _ fs.ReadLinkFS = dirFS("")

type dirFS string

func (dir dirFS) Open(name string) (fs.File, error) {
	fullname, err := dir.join(name)
	if err != nil {
		return nil, &PathError{Op: "open", Path: name, Err: err}
	}
	f, err := Open(fullname)
	if err != nil {
		// DirFS takes a string appropriate for GOOS,
		// while the name argument here is always slash separated.
		// dir.join will have mixed the two; undo that for
		// error reporting.
		err.(*PathError).Path = name
		return nil, err
	}
	return f, nil
}

// The ReadFile method calls the [ReadFile] function for the file
// with the given name in the directory. The function provides
// robust handling for small files and special file systems.
// Through this method, dirFS implements [io/fs.ReadFileFS].
func (dir dirFS) ReadFile(name string) ([]byte, error) {
	fullname, err := dir.join(name)
	if err != nil {
		return nil, &PathError{Op: "readfile", Path: name, Err: err}
	}
	b, err := ReadFile(fullname)
	if err != nil {
		if e, ok := err.(*PathError); ok {
			// See comment in dirFS.Open.
			e.Path = name
		}
		return nil, err
	}
	return b, nil
}

// ReadDir reads the named directory, returning all its directory entries sorted
// by filename. Through this method, dirFS implements [io/fs.ReadDirFS].
func (dir dirFS) ReadDir(name string) ([]DirEntry, error) {
	fullname, err := dir.join(name)
	if err != nil {
		return nil, &PathError{Op: "readdir", Path: name, Err: err}
	}
	entries, err := ReadDir(fullname)
	if err != nil {
		if e, ok := err.(*PathError); ok {
			// See comment in dirFS.Open.
			e.Path = name
		}
		return nil, err
	}
	return entries, nil
}

func (dir dirFS) Stat(name string) (fs.FileInfo, error) {
	fullname, err := dir.join(name)
	if err != nil {
		return nil, &PathError{Op: "stat", Path: name, Err: err}
	}
	f, err := Stat(fullname)
	if err != nil {
		// See comment in dirFS.Open.
		err.(*PathError).Path = name
		return nil, err
	}
	return f, nil
}

func (dir dirFS) Lstat(name string) (fs.FileInfo, error) {
	fullname, err := dir.join(name)
	if err != nil {
		return nil, &PathError{Op: "lstat", Path: name, Err: err}
	}
	f, err := Lstat(fullname)
	if err != nil {
		// See comment in dirFS.Open.
		err.(*PathError).Path = name
		return nil, err
	}
	return f, nil
}

func (dir dirFS) ReadLink(name string) (string, error) {
	fullname, err := dir.join(name)
	if err != nil {
		return "", &PathError{Op: "readlink", Path: name, Err: err}
	}
	return Readlink(fullname)
}

// join returns the path for name in dir.
func (dir dirFS) join(name string) (string, error) {
	if dir == "" {
		return "", errors.New("os: DirFS with empty root")
	}
	name, err := filepathlite.Localize(name)
	if err != nil {
		return "", ErrInvalid
	}
	if IsPathSeparator(dir[len(dir)-1]) {
		return string(dir) + name, nil
	}
	return string(dir) + string(PathSeparator) + name, nil
}

// ReadFile reads the named file and returns the contents.
// A successful call returns err == nil, not err == EOF.
// Because ReadFile reads the whole file, it does not treat an EOF from Read
// as an error to be reported.
func ReadFile(name string) ([]byte, error) {
	f, err := Open(name)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	return readFileContents(statOrZero(f), f.Read)
}

func statOrZero(f *File) int64 {
	if fi, err := f.Stat(); err == nil {
		return fi.Size()
	}
	return 0
}

// readFileContents reads the contents of a file using the provided read function
// (*os.File.Read, except in tests) one or more times, until an error is seen.
//
// The provided size is the stat size of the file, which might be 0 for a
// /proc-like file that doesn't report a size.
func readFileContents(statSize int64, read func([]byte) (int, error)) ([]byte, error) {
	zeroSize := statSize == 0

	// Figure out how big to make the initial slice. For files with known size
	// that fit in memory, use that size + 1. Otherwise, use a small buffer and
	// we'll grow.
	var size int
	if int64(int(statSize)) == statSize {
		size = int(statSize)
	}
	size++ // one byte for final read at EOF

	const minBuf = 512
	// If a file claims a small size, read at least 512 bytes. In particular,
	// files in Linux's /proc claim size 0 but then do not work right if read in
	// small pieces, so an initial read of 1 byte would not work correctly.
	if size < minBuf {
		size = minBuf
	}

	data := make([]byte, 0, size)
	for {
		n, err := read(data[len(data):cap(data)])
		data = data[:len(data)+n]
		if err != nil {
			if err == io.EOF {
				err = nil
			}
			return data, err
		}

		// If we're either out of capacity or if the file was a /proc-like zero
		// sized file, grow the buffer. Per Issue 72080, we always want to issue
		// Read calls on zero-length files with a non-tiny buffer size.
		capRemain := cap(data) - len(data)
		if capRemain == 0 || (zeroSize && capRemain < minBuf) {
			data = slices.Grow(data, minBuf)
		}
	}
}

// WriteFile writes data to the named file, creating it if necessary.
// If the file does not exist, WriteFile creates it with permissions perm (before umask);
// otherwise WriteFile truncates it before writing, without changing permissions.
// Since WriteFile requires multiple system calls to complete, a failure mid-operation
// can leave the file in a partially written state.
func WriteFile(name string, data []byte, perm FileMode) error {
	f, err := OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, perm)
	if err != nil {
		return err
	}
	_, err = f.Write(data)
	if err1 := f.Close(); err1 != nil && err == nil {
		err = err1
	}
	return err
}

The pages are generated with Golds v0.7.9-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
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