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// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package fs defines basic interfaces to a file system.
// A file system can be provided by the host operating system
// but also by other packages.
package fs
import (
// An FS provides access to a hierarchical file system.
// The FS interface is the minimum implementation required of the file system.
// A file system may implement additional interfaces,
// such as ReadFileFS, to provide additional or optimized functionality.
type FS interface {
// Open opens the named file.
// When Open returns an error, it should be of type *PathError
// with the Op field set to "open", the Path field set to name,
// and the Err field describing the problem.
// Open should reject attempts to open names that do not satisfy
// ValidPath(name), returning a *PathError with Err set to
// ErrInvalid or ErrNotExist.
Open(name string) (File, error)
// ValidPath reports whether the given path name
// is valid for use in a call to Open.
// Path names passed to open are UTF-8-encoded,
// unrooted, slash-separated sequences of path elements, like “x/y/z”.
// Path names must not contain an element that is “.” or “..” or the empty string,
// except for the special case that the root directory is named “.”.
// Paths must not start or end with a slash: “/x” and “x/” are invalid.
// Note that paths are slash-separated on all systems, even Windows.
// Paths containing other characters such as backslash and colon
// are accepted as valid, but those characters must never be
// interpreted by an FS implementation as path element separators.
func ValidPath(name string) bool {
if !utf8.ValidString(name) {
return false
if name == "." {
// special case
return true
// Iterate over elements in name, checking each.
for {
i := 0
for i < len(name) && name[i] != '/' {
elem := name[:i]
if elem == "" || elem == "." || elem == ".." {
return false
if i == len(name) {
return true // reached clean ending
name = name[i+1:]
// A File provides access to a single file.
// The File interface is the minimum implementation required of the file.
// Directory files should also implement ReadDirFile.
// A file may implement io.ReaderAt or io.Seeker as optimizations.
type File interface {
Stat() (FileInfo, error)
Read([]byte) (int, error)
Close() error
// A DirEntry is an entry read from a directory
// (using the ReadDir function or a ReadDirFile's ReadDir method).
type DirEntry interface {
// Name returns the name of the file (or subdirectory) described by the entry.
// This name is only the final element of the path (the base name), not the entire path.
// For example, Name would return "hello.go" not "home/gopher/hello.go".
Name() string
// IsDir reports whether the entry describes a directory.
IsDir() bool
// Type returns the type bits for the entry.
// The type bits are a subset of the usual FileMode bits, those returned by the FileMode.Type method.
Type() FileMode
// Info returns the FileInfo for the file or subdirectory described by the entry.
// The returned FileInfo may be from the time of the original directory read
// or from the time of the call to Info. If the file has been removed or renamed
// since the directory read, Info may return an error satisfying errors.Is(err, ErrNotExist).
// If the entry denotes a symbolic link, Info reports the information about the link itself,
// not the link's target.
Info() (FileInfo, error)
// A ReadDirFile is a directory file whose entries can be read with the ReadDir method.
// Every directory file should implement this interface.
// (It is permissible for any file to implement this interface,
// but if so ReadDir should return an error for non-directories.)
type ReadDirFile interface {
// ReadDir reads the contents of the directory and returns
// a slice of up to n DirEntry values in directory order.
// Subsequent calls on the same file will yield further DirEntry values.
// If n > 0, ReadDir returns at most n DirEntry structures.
// In this case, if ReadDir returns an empty slice, it will return
// a non-nil error explaining why.
// At the end of a directory, the error is io.EOF.
// (ReadDir must return io.EOF itself, not an error wrapping io.EOF.)
// If n <= 0, ReadDir returns all the DirEntry values from the directory
// in a single slice. In this case, if ReadDir succeeds (reads all the way
// to the end of the directory), it returns the slice and a nil error.
// If it encounters an error before the end of the directory,
// ReadDir returns the DirEntry list read until that point and a non-nil error.
ReadDir(n int) ([]DirEntry, error)
// Generic file system errors.
// Errors returned by file systems can be tested against these errors
// using errors.Is.
var (
ErrInvalid = errInvalid() // "invalid argument"
ErrPermission = errPermission() // "permission denied"
ErrExist = errExist() // "file already exists"
ErrNotExist = errNotExist() // "file does not exist"
ErrClosed = errClosed() // "file already closed"
func errInvalid() error { return oserror.ErrInvalid }
func errPermission() error { return oserror.ErrPermission }
func errExist() error { return oserror.ErrExist }
func errNotExist() error { return oserror.ErrNotExist }
func errClosed() error { return oserror.ErrClosed }
// A FileInfo describes a file and is returned by Stat.
type FileInfo interface {
Name() string // base name of the file
Size() int64 // length in bytes for regular files; system-dependent for others
Mode() FileMode // file mode bits
ModTime() time.Time // modification time
IsDir() bool // abbreviation for Mode().IsDir()
Sys() any // underlying data source (can return nil)
// A FileMode represents a file's mode and permission bits.
// The bits have the same definition on all systems, so that
// information about files can be moved from one system
// to another portably. Not all bits apply to all systems.
// The only required bit is ModeDir for directories.
type FileMode uint32
// The defined file mode bits are the most significant bits of the FileMode.
// The nine least-significant bits are the standard Unix rwxrwxrwx permissions.
// The values of these bits should be considered part of the public API and
// may be used in wire protocols or disk representations: they must not be
// changed, although new bits might be added.
const (
// The single letters are the abbreviations
// used by the String method's formatting.
ModeDir FileMode = 1 << (32 - 1 - iota) // d: is a directory
ModeAppend // a: append-only
ModeExclusive // l: exclusive use
ModeTemporary // T: temporary file; Plan 9 only
ModeSymlink // L: symbolic link
ModeDevice // D: device file
ModeNamedPipe // p: named pipe (FIFO)
ModeSocket // S: Unix domain socket
ModeSetuid // u: setuid
ModeSetgid // g: setgid
ModeCharDevice // c: Unix character device, when ModeDevice is set
ModeSticky // t: sticky
ModeIrregular // ?: non-regular file; nothing else is known about this file
// Mask for the type bits. For regular files, none will be set.
ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice | ModeCharDevice | ModeIrregular
ModePerm FileMode = 0777 // Unix permission bits
func (m FileMode) String() string {
const str = "dalTLDpSugct?"
var buf [32]byte // Mode is uint32.
w := 0
for i, c := range str {
if m&(1<<uint(32-1-i)) != 0 {
buf[w] = byte(c)
if w == 0 {
buf[w] = '-'
const rwx = "rwxrwxrwx"
for i, c := range rwx {
if m&(1<<uint(9-1-i)) != 0 {
buf[w] = byte(c)
} else {
buf[w] = '-'
return string(buf[:w])
// IsDir reports whether m describes a directory.
// That is, it tests for the ModeDir bit being set in m.
func (m FileMode) IsDir() bool {
return m&ModeDir != 0
// IsRegular reports whether m describes a regular file.
// That is, it tests that no mode type bits are set.
func (m FileMode) IsRegular() bool {
return m&ModeType == 0
// Perm returns the Unix permission bits in m (m & ModePerm).
func (m FileMode) Perm() FileMode {
return m & ModePerm
// Type returns type bits in m (m & ModeType).
func (m FileMode) Type() FileMode {
return m & ModeType
// PathError records an error and the operation and file path that caused it.
type PathError struct {
Op string
Path string
Err error
func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }
func (e *PathError) Unwrap() error { return e.Err }
// Timeout reports whether this error represents a timeout.
func (e *PathError) Timeout() bool {
t, ok := e.Err.(interface{ Timeout() bool })
return ok && t.Timeout()