Source file src/os/file.go

Documentation: os

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package os provides a platform-independent interface to operating system
     6  // functionality. The design is Unix-like, although the error handling is
     7  // Go-like; failing calls return values of type error rather than error numbers.
     8  // Often, more information is available within the error. For example,
     9  // if a call that takes a file name fails, such as Open or Stat, the error
    10  // will include the failing file name when printed and will be of type
    11  // *PathError, which may be unpacked for more information.
    12  //
    13  // The os interface is intended to be uniform across all operating systems.
    14  // Features not generally available appear in the system-specific package syscall.
    15  //
    16  // Here is a simple example, opening a file and reading some of it.
    17  //
    18  //	file, err := os.Open("file.go") // For read access.
    19  //	if err != nil {
    20  //		log.Fatal(err)
    21  //	}
    22  //
    23  // If the open fails, the error string will be self-explanatory, like
    24  //
    25  //	open file.go: no such file or directory
    26  //
    27  // The file's data can then be read into a slice of bytes. Read and
    28  // Write take their byte counts from the length of the argument slice.
    29  //
    30  //	data := make([]byte, 100)
    31  //	count, err := file.Read(data)
    32  //	if err != nil {
    33  //		log.Fatal(err)
    34  //	}
    35  //	fmt.Printf("read %d bytes: %q\n", count, data[:count])
    36  //
    37  // Note: The maximum number of concurrent operations on a File may be limited by
    38  // the OS or the system. The number should be high, but exceeding it may degrade
    39  // performance or cause other issues.
    40  //
    41  package os
    42  
    43  import (
    44  	"errors"
    45  	"internal/poll"
    46  	"internal/testlog"
    47  	"io"
    48  	"runtime"
    49  	"syscall"
    50  	"time"
    51  )
    52  
    53  // Name returns the name of the file as presented to Open.
    54  func (f *File) Name() string { return f.name }
    55  
    56  // Stdin, Stdout, and Stderr are open Files pointing to the standard input,
    57  // standard output, and standard error file descriptors.
    58  //
    59  // Note that the Go runtime writes to standard error for panics and crashes;
    60  // closing Stderr may cause those messages to go elsewhere, perhaps
    61  // to a file opened later.
    62  var (
    63  	Stdin  = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
    64  	Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
    65  	Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
    66  )
    67  
    68  // Flags to OpenFile wrapping those of the underlying system. Not all
    69  // flags may be implemented on a given system.
    70  const (
    71  	// Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified.
    72  	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
    73  	O_WRONLY int = syscall.O_WRONLY // open the file write-only.
    74  	O_RDWR   int = syscall.O_RDWR   // open the file read-write.
    75  	// The remaining values may be or'ed in to control behavior.
    76  	O_APPEND int = syscall.O_APPEND // append data to the file when writing.
    77  	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
    78  	O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist.
    79  	O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
    80  	O_TRUNC  int = syscall.O_TRUNC  // truncate regular writable file when opened.
    81  )
    82  
    83  // Seek whence values.
    84  //
    85  // Deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd.
    86  const (
    87  	SEEK_SET int = 0 // seek relative to the origin of the file
    88  	SEEK_CUR int = 1 // seek relative to the current offset
    89  	SEEK_END int = 2 // seek relative to the end
    90  )
    91  
    92  // LinkError records an error during a link or symlink or rename
    93  // system call and the paths that caused it.
    94  type LinkError struct {
    95  	Op  string
    96  	Old string
    97  	New string
    98  	Err error
    99  }
   100  
   101  func (e *LinkError) Error() string {
   102  	return e.Op + " " + e.Old + " " + e.New + ": " + e.Err.Error()
   103  }
   104  
   105  func (e *LinkError) Unwrap() error {
   106  	return e.Err
   107  }
   108  
   109  // Read reads up to len(b) bytes from the File.
   110  // It returns the number of bytes read and any error encountered.
   111  // At end of file, Read returns 0, io.EOF.
   112  func (f *File) Read(b []byte) (n int, err error) {
   113  	if err := f.checkValid("read"); err != nil {
   114  		return 0, err
   115  	}
   116  	n, e := f.read(b)
   117  	return n, f.wrapErr("read", e)
   118  }
   119  
   120  // ReadAt reads len(b) bytes from the File starting at byte offset off.
   121  // It returns the number of bytes read and the error, if any.
   122  // ReadAt always returns a non-nil error when n < len(b).
   123  // At end of file, that error is io.EOF.
   124  func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
   125  	if err := f.checkValid("read"); err != nil {
   126  		return 0, err
   127  	}
   128  
   129  	if off < 0 {
   130  		return 0, &PathError{"readat", f.name, errors.New("negative offset")}
   131  	}
   132  
   133  	for len(b) > 0 {
   134  		m, e := f.pread(b, off)
   135  		if e != nil {
   136  			err = f.wrapErr("read", e)
   137  			break
   138  		}
   139  		n += m
   140  		b = b[m:]
   141  		off += int64(m)
   142  	}
   143  	return
   144  }
   145  
   146  // Write writes len(b) bytes to the File.
   147  // It returns the number of bytes written and an error, if any.
   148  // Write returns a non-nil error when n != len(b).
   149  func (f *File) Write(b []byte) (n int, err error) {
   150  	if err := f.checkValid("write"); err != nil {
   151  		return 0, err
   152  	}
   153  	n, e := f.write(b)
   154  	if n < 0 {
   155  		n = 0
   156  	}
   157  	if n != len(b) {
   158  		err = io.ErrShortWrite
   159  	}
   160  
   161  	epipecheck(f, e)
   162  
   163  	if e != nil {
   164  		err = f.wrapErr("write", e)
   165  	}
   166  
   167  	return n, err
   168  }
   169  
   170  var errWriteAtInAppendMode = errors.New("os: invalid use of WriteAt on file opened with O_APPEND")
   171  
   172  // WriteAt writes len(b) bytes to the File starting at byte offset off.
   173  // It returns the number of bytes written and an error, if any.
   174  // WriteAt returns a non-nil error when n != len(b).
   175  //
   176  // If file was opened with the O_APPEND flag, WriteAt returns an error.
   177  func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
   178  	if err := f.checkValid("write"); err != nil {
   179  		return 0, err
   180  	}
   181  	if f.appendMode {
   182  		return 0, errWriteAtInAppendMode
   183  	}
   184  
   185  	if off < 0 {
   186  		return 0, &PathError{"writeat", f.name, errors.New("negative offset")}
   187  	}
   188  
   189  	for len(b) > 0 {
   190  		m, e := f.pwrite(b, off)
   191  		if e != nil {
   192  			err = f.wrapErr("write", e)
   193  			break
   194  		}
   195  		n += m
   196  		b = b[m:]
   197  		off += int64(m)
   198  	}
   199  	return
   200  }
   201  
   202  // Seek sets the offset for the next Read or Write on file to offset, interpreted
   203  // according to whence: 0 means relative to the origin of the file, 1 means
   204  // relative to the current offset, and 2 means relative to the end.
   205  // It returns the new offset and an error, if any.
   206  // The behavior of Seek on a file opened with O_APPEND is not specified.
   207  //
   208  // If f is a directory, the behavior of Seek varies by operating
   209  // system; you can seek to the beginning of the directory on Unix-like
   210  // operating systems, but not on Windows.
   211  func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
   212  	if err := f.checkValid("seek"); err != nil {
   213  		return 0, err
   214  	}
   215  	r, e := f.seek(offset, whence)
   216  	if e == nil && f.dirinfo != nil && r != 0 {
   217  		e = syscall.EISDIR
   218  	}
   219  	if e != nil {
   220  		return 0, f.wrapErr("seek", e)
   221  	}
   222  	return r, nil
   223  }
   224  
   225  // WriteString is like Write, but writes the contents of string s rather than
   226  // a slice of bytes.
   227  func (f *File) WriteString(s string) (n int, err error) {
   228  	return f.Write([]byte(s))
   229  }
   230  
   231  // Mkdir creates a new directory with the specified name and permission
   232  // bits (before umask).
   233  // If there is an error, it will be of type *PathError.
   234  func Mkdir(name string, perm FileMode) error {
   235  	if runtime.GOOS == "windows" && isWindowsNulName(name) {
   236  		return &PathError{"mkdir", name, syscall.ENOTDIR}
   237  	}
   238  	e := syscall.Mkdir(fixLongPath(name), syscallMode(perm))
   239  
   240  	if e != nil {
   241  		return &PathError{"mkdir", name, e}
   242  	}
   243  
   244  	// mkdir(2) itself won't handle the sticky bit on *BSD and Solaris
   245  	if !supportsCreateWithStickyBit && perm&ModeSticky != 0 {
   246  		e = setStickyBit(name)
   247  
   248  		if e != nil {
   249  			Remove(name)
   250  			return e
   251  		}
   252  	}
   253  
   254  	return nil
   255  }
   256  
   257  // setStickyBit adds ModeSticky to the permission bits of path, non atomic.
   258  func setStickyBit(name string) error {
   259  	fi, err := Stat(name)
   260  	if err != nil {
   261  		return err
   262  	}
   263  	return Chmod(name, fi.Mode()|ModeSticky)
   264  }
   265  
   266  // Chdir changes the current working directory to the named directory.
   267  // If there is an error, it will be of type *PathError.
   268  func Chdir(dir string) error {
   269  	if e := syscall.Chdir(dir); e != nil {
   270  		testlog.Open(dir) // observe likely non-existent directory
   271  		return &PathError{"chdir", dir, e}
   272  	}
   273  	if log := testlog.Logger(); log != nil {
   274  		wd, err := Getwd()
   275  		if err == nil {
   276  			log.Chdir(wd)
   277  		}
   278  	}
   279  	return nil
   280  }
   281  
   282  // Open opens the named file for reading. If successful, methods on
   283  // the returned file can be used for reading; the associated file
   284  // descriptor has mode O_RDONLY.
   285  // If there is an error, it will be of type *PathError.
   286  func Open(name string) (*File, error) {
   287  	return OpenFile(name, O_RDONLY, 0)
   288  }
   289  
   290  // Create creates or truncates the named file. If the file already exists,
   291  // it is truncated. If the file does not exist, it is created with mode 0666
   292  // (before umask). If successful, methods on the returned File can
   293  // be used for I/O; the associated file descriptor has mode O_RDWR.
   294  // If there is an error, it will be of type *PathError.
   295  func Create(name string) (*File, error) {
   296  	return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
   297  }
   298  
   299  // OpenFile is the generalized open call; most users will use Open
   300  // or Create instead. It opens the named file with specified flag
   301  // (O_RDONLY etc.). If the file does not exist, and the O_CREATE flag
   302  // is passed, it is created with mode perm (before umask). If successful,
   303  // methods on the returned File can be used for I/O.
   304  // If there is an error, it will be of type *PathError.
   305  func OpenFile(name string, flag int, perm FileMode) (*File, error) {
   306  	testlog.Open(name)
   307  	f, err := openFileNolog(name, flag, perm)
   308  	if err != nil {
   309  		return nil, err
   310  	}
   311  	f.appendMode = flag&O_APPEND != 0
   312  
   313  	return f, nil
   314  }
   315  
   316  // lstat is overridden in tests.
   317  var lstat = Lstat
   318  
   319  // Rename renames (moves) oldpath to newpath.
   320  // If newpath already exists and is not a directory, Rename replaces it.
   321  // OS-specific restrictions may apply when oldpath and newpath are in different directories.
   322  // If there is an error, it will be of type *LinkError.
   323  func Rename(oldpath, newpath string) error {
   324  	return rename(oldpath, newpath)
   325  }
   326  
   327  // Many functions in package syscall return a count of -1 instead of 0.
   328  // Using fixCount(call()) instead of call() corrects the count.
   329  func fixCount(n int, err error) (int, error) {
   330  	if n < 0 {
   331  		n = 0
   332  	}
   333  	return n, err
   334  }
   335  
   336  // wrapErr wraps an error that occurred during an operation on an open file.
   337  // It passes io.EOF through unchanged, otherwise converts
   338  // poll.ErrFileClosing to ErrClosed and wraps the error in a PathError.
   339  func (f *File) wrapErr(op string, err error) error {
   340  	if err == nil || err == io.EOF {
   341  		return err
   342  	}
   343  	if err == poll.ErrFileClosing {
   344  		err = ErrClosed
   345  	}
   346  	return &PathError{op, f.name, err}
   347  }
   348  
   349  // TempDir returns the default directory to use for temporary files.
   350  //
   351  // On Unix systems, it returns $TMPDIR if non-empty, else /tmp.
   352  // On Windows, it uses GetTempPath, returning the first non-empty
   353  // value from %TMP%, %TEMP%, %USERPROFILE%, or the Windows directory.
   354  // On Plan 9, it returns /tmp.
   355  //
   356  // The directory is neither guaranteed to exist nor have accessible
   357  // permissions.
   358  func TempDir() string {
   359  	return tempDir()
   360  }
   361  
   362  // UserCacheDir returns the default root directory to use for user-specific
   363  // cached data. Users should create their own application-specific subdirectory
   364  // within this one and use that.
   365  //
   366  // On Unix systems, it returns $XDG_CACHE_HOME as specified by
   367  // https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html if
   368  // non-empty, else $HOME/.cache.
   369  // On Darwin, it returns $HOME/Library/Caches.
   370  // On Windows, it returns %LocalAppData%.
   371  // On Plan 9, it returns $home/lib/cache.
   372  //
   373  // If the location cannot be determined (for example, $HOME is not defined),
   374  // then it will return an error.
   375  func UserCacheDir() (string, error) {
   376  	var dir string
   377  
   378  	switch runtime.GOOS {
   379  	case "windows":
   380  		dir = Getenv("LocalAppData")
   381  		if dir == "" {
   382  			return "", errors.New("%LocalAppData% is not defined")
   383  		}
   384  
   385  	case "darwin":
   386  		dir = Getenv("HOME")
   387  		if dir == "" {
   388  			return "", errors.New("$HOME is not defined")
   389  		}
   390  		dir += "/Library/Caches"
   391  
   392  	case "plan9":
   393  		dir = Getenv("home")
   394  		if dir == "" {
   395  			return "", errors.New("$home is not defined")
   396  		}
   397  		dir += "/lib/cache"
   398  
   399  	default: // Unix
   400  		dir = Getenv("XDG_CACHE_HOME")
   401  		if dir == "" {
   402  			dir = Getenv("HOME")
   403  			if dir == "" {
   404  				return "", errors.New("neither $XDG_CACHE_HOME nor $HOME are defined")
   405  			}
   406  			dir += "/.cache"
   407  		}
   408  	}
   409  
   410  	return dir, nil
   411  }
   412  
   413  // UserConfigDir returns the default root directory to use for user-specific
   414  // configuration data. Users should create their own application-specific
   415  // subdirectory within this one and use that.
   416  //
   417  // On Unix systems, it returns $XDG_CONFIG_HOME as specified by
   418  // https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if
   419  // non-empty, else $HOME/.config.
   420  // On Darwin, it returns $HOME/Library/Application Support.
   421  // On Windows, it returns %AppData%.
   422  // On Plan 9, it returns $home/lib.
   423  //
   424  // If the location cannot be determined (for example, $HOME is not defined),
   425  // then it will return an error.
   426  func UserConfigDir() (string, error) {
   427  	var dir string
   428  
   429  	switch runtime.GOOS {
   430  	case "windows":
   431  		dir = Getenv("AppData")
   432  		if dir == "" {
   433  			return "", errors.New("%AppData% is not defined")
   434  		}
   435  
   436  	case "darwin":
   437  		dir = Getenv("HOME")
   438  		if dir == "" {
   439  			return "", errors.New("$HOME is not defined")
   440  		}
   441  		dir += "/Library/Application Support"
   442  
   443  	case "plan9":
   444  		dir = Getenv("home")
   445  		if dir == "" {
   446  			return "", errors.New("$home is not defined")
   447  		}
   448  		dir += "/lib"
   449  
   450  	default: // Unix
   451  		dir = Getenv("XDG_CONFIG_HOME")
   452  		if dir == "" {
   453  			dir = Getenv("HOME")
   454  			if dir == "" {
   455  				return "", errors.New("neither $XDG_CONFIG_HOME nor $HOME are defined")
   456  			}
   457  			dir += "/.config"
   458  		}
   459  	}
   460  
   461  	return dir, nil
   462  }
   463  
   464  // UserHomeDir returns the current user's home directory.
   465  //
   466  // On Unix, including macOS, it returns the $HOME environment variable.
   467  // On Windows, it returns %USERPROFILE%.
   468  // On Plan 9, it returns the $home environment variable.
   469  func UserHomeDir() (string, error) {
   470  	env, enverr := "HOME", "$HOME"
   471  	switch runtime.GOOS {
   472  	case "windows":
   473  		env, enverr = "USERPROFILE", "%userprofile%"
   474  	case "plan9":
   475  		env, enverr = "home", "$home"
   476  	}
   477  	if v := Getenv(env); v != "" {
   478  		return v, nil
   479  	}
   480  	// On some geese the home directory is not always defined.
   481  	switch runtime.GOOS {
   482  	case "android":
   483  		return "/sdcard", nil
   484  	case "darwin":
   485  		if runtime.GOARCH == "arm" || runtime.GOARCH == "arm64" {
   486  			return "/", nil
   487  		}
   488  	}
   489  	return "", errors.New(enverr + " is not defined")
   490  }
   491  
   492  // Chmod changes the mode of the named file to mode.
   493  // If the file is a symbolic link, it changes the mode of the link's target.
   494  // If there is an error, it will be of type *PathError.
   495  //
   496  // A different subset of the mode bits are used, depending on the
   497  // operating system.
   498  //
   499  // On Unix, the mode's permission bits, ModeSetuid, ModeSetgid, and
   500  // ModeSticky are used.
   501  //
   502  // On Windows, only the 0200 bit (owner writable) of mode is used; it
   503  // controls whether the file's read-only attribute is set or cleared.
   504  // The other bits are currently unused. For compatibility with Go 1.12
   505  // and earlier, use a non-zero mode. Use mode 0400 for a read-only
   506  // file and 0600 for a readable+writable file.
   507  //
   508  // On Plan 9, the mode's permission bits, ModeAppend, ModeExclusive,
   509  // and ModeTemporary are used.
   510  func Chmod(name string, mode FileMode) error { return chmod(name, mode) }
   511  
   512  // Chmod changes the mode of the file to mode.
   513  // If there is an error, it will be of type *PathError.
   514  func (f *File) Chmod(mode FileMode) error { return f.chmod(mode) }
   515  
   516  // SetDeadline sets the read and write deadlines for a File.
   517  // It is equivalent to calling both SetReadDeadline and SetWriteDeadline.
   518  //
   519  // Only some kinds of files support setting a deadline. Calls to SetDeadline
   520  // for files that do not support deadlines will return ErrNoDeadline.
   521  // On most systems ordinary files do not support deadlines, but pipes do.
   522  //
   523  // A deadline is an absolute time after which I/O operations fail with an
   524  // error instead of blocking. The deadline applies to all future and pending
   525  // I/O, not just the immediately following call to Read or Write.
   526  // After a deadline has been exceeded, the connection can be refreshed
   527  // by setting a deadline in the future.
   528  //
   529  // An error returned after a timeout fails will implement the
   530  // Timeout method, and calling the Timeout method will return true.
   531  // The PathError and SyscallError types implement the Timeout method.
   532  // In general, call IsTimeout to test whether an error indicates a timeout.
   533  //
   534  // An idle timeout can be implemented by repeatedly extending
   535  // the deadline after successful Read or Write calls.
   536  //
   537  // A zero value for t means I/O operations will not time out.
   538  func (f *File) SetDeadline(t time.Time) error {
   539  	return f.setDeadline(t)
   540  }
   541  
   542  // SetReadDeadline sets the deadline for future Read calls and any
   543  // currently-blocked Read call.
   544  // A zero value for t means Read will not time out.
   545  // Not all files support setting deadlines; see SetDeadline.
   546  func (f *File) SetReadDeadline(t time.Time) error {
   547  	return f.setReadDeadline(t)
   548  }
   549  
   550  // SetWriteDeadline sets the deadline for any future Write calls and any
   551  // currently-blocked Write call.
   552  // Even if Write times out, it may return n > 0, indicating that
   553  // some of the data was successfully written.
   554  // A zero value for t means Write will not time out.
   555  // Not all files support setting deadlines; see SetDeadline.
   556  func (f *File) SetWriteDeadline(t time.Time) error {
   557  	return f.setWriteDeadline(t)
   558  }
   559  
   560  // SyscallConn returns a raw file.
   561  // This implements the syscall.Conn interface.
   562  func (f *File) SyscallConn() (syscall.RawConn, error) {
   563  	if err := f.checkValid("SyscallConn"); err != nil {
   564  		return nil, err
   565  	}
   566  	return newRawConn(f)
   567  }
   568  
   569  // isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
   570  // True is returned if name is 'NUL' whatever the case.
   571  func isWindowsNulName(name string) bool {
   572  	if len(name) != 3 {
   573  		return false
   574  	}
   575  	if name[0] != 'n' && name[0] != 'N' {
   576  		return false
   577  	}
   578  	if name[1] != 'u' && name[1] != 'U' {
   579  		return false
   580  	}
   581  	if name[2] != 'l' && name[2] != 'L' {
   582  		return false
   583  	}
   584  	return true
   585  }
   586  

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