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 "io/fs" 49 "runtime" 50 "syscall" 51 "time" 52 ) 53 54 // Name returns the name of the file as presented to Open. 55 func (f *File) Name() string { return f.name } 56 57 // Stdin, Stdout, and Stderr are open Files pointing to the standard input, 58 // standard output, and standard error file descriptors. 59 // 60 // Note that the Go runtime writes to standard error for panics and crashes; 61 // closing Stderr may cause those messages to go elsewhere, perhaps 62 // to a file opened later. 63 var ( 64 Stdin = NewFile(uintptr(syscall.Stdin), "/dev/stdin") 65 Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout") 66 Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr") 67 ) 68 69 // Flags to OpenFile wrapping those of the underlying system. Not all 70 // flags may be implemented on a given system. 71 const ( 72 // Exactly one of O_RDONLY, O_WRONLY, or O_RDWR must be specified. 73 O_RDONLY int = syscall.O_RDONLY // open the file read-only. 74 O_WRONLY int = syscall.O_WRONLY // open the file write-only. 75 O_RDWR int = syscall.O_RDWR // open the file read-write. 76 // The remaining values may be or'ed in to control behavior. 77 O_APPEND int = syscall.O_APPEND // append data to the file when writing. 78 O_CREATE int = syscall.O_CREAT // create a new file if none exists. 79 O_EXCL int = syscall.O_EXCL // used with O_CREATE, file must not exist. 80 O_SYNC int = syscall.O_SYNC // open for synchronous I/O. 81 O_TRUNC int = syscall.O_TRUNC // truncate regular writable file when opened. 82 ) 83 84 // Seek whence values. 85 // 86 // Deprecated: Use io.SeekStart, io.SeekCurrent, and io.SeekEnd. 87 const ( 88 SEEK_SET int = 0 // seek relative to the origin of the file 89 SEEK_CUR int = 1 // seek relative to the current offset 90 SEEK_END int = 2 // seek relative to the end 91 ) 92 93 // LinkError records an error during a link or symlink or rename 94 // system call and the paths that caused it. 95 type LinkError struct { 96 Op string 97 Old string 98 New string 99 Err error 100 } 101 102 func (e *LinkError) Error() string { 103 return e.Op + " " + e.Old + " " + e.New + ": " + e.Err.Error() 104 } 105 106 func (e *LinkError) Unwrap() error { 107 return e.Err 108 } 109 110 // Read reads up to len(b) bytes from the File. 111 // It returns the number of bytes read and any error encountered. 112 // At end of file, Read returns 0, io.EOF. 113 func (f *File) Read(b []byte) (n int, err error) { 114 if err := f.checkValid("read"); err != nil { 115 return 0, err 116 } 117 n, e := f.read(b) 118 return n, f.wrapErr("read", e) 119 } 120 121 // ReadAt reads len(b) bytes from the File starting at byte offset off. 122 // It returns the number of bytes read and the error, if any. 123 // ReadAt always returns a non-nil error when n < len(b). 124 // At end of file, that error is io.EOF. 125 func (f *File) ReadAt(b []byte, off int64) (n int, err error) { 126 if err := f.checkValid("read"); err != nil { 127 return 0, err 128 } 129 130 if off < 0 { 131 return 0, &PathError{Op: "readat", Path: f.name, Err: errors.New("negative offset")} 132 } 133 134 for len(b) > 0 { 135 m, e := f.pread(b, off) 136 if e != nil { 137 err = f.wrapErr("read", e) 138 break 139 } 140 n += m 141 b = b[m:] 142 off += int64(m) 143 } 144 return 145 } 146 147 // ReadFrom implements io.ReaderFrom. 148 func (f *File) ReadFrom(r io.Reader) (n int64, err error) { 149 if err := f.checkValid("write"); err != nil { 150 return 0, err 151 } 152 n, handled, e := f.readFrom(r) 153 if !handled { 154 return genericReadFrom(f, r) // without wrapping 155 } 156 return n, f.wrapErr("write", e) 157 } 158 159 func genericReadFrom(f *File, r io.Reader) (int64, error) { 160 return io.Copy(onlyWriter{f}, r) 161 } 162 163 type onlyWriter struct { 164 io.Writer 165 } 166 167 // Write writes len(b) bytes to the File. 168 // It returns the number of bytes written and an error, if any. 169 // Write returns a non-nil error when n != len(b). 170 func (f *File) Write(b []byte) (n int, err error) { 171 if err := f.checkValid("write"); err != nil { 172 return 0, err 173 } 174 n, e := f.write(b) 175 if n < 0 { 176 n = 0 177 } 178 if n != len(b) { 179 err = io.ErrShortWrite 180 } 181 182 epipecheck(f, e) 183 184 if e != nil { 185 err = f.wrapErr("write", e) 186 } 187 188 return n, err 189 } 190 191 var errWriteAtInAppendMode = errors.New("os: invalid use of WriteAt on file opened with O_APPEND") 192 193 // WriteAt writes len(b) bytes to the File starting at byte offset off. 194 // It returns the number of bytes written and an error, if any. 195 // WriteAt returns a non-nil error when n != len(b). 196 // 197 // If file was opened with the O_APPEND flag, WriteAt returns an error. 198 func (f *File) WriteAt(b []byte, off int64) (n int, err error) { 199 if err := f.checkValid("write"); err != nil { 200 return 0, err 201 } 202 if f.appendMode { 203 return 0, errWriteAtInAppendMode 204 } 205 206 if off < 0 { 207 return 0, &PathError{Op: "writeat", Path: f.name, Err: errors.New("negative offset")} 208 } 209 210 for len(b) > 0 { 211 m, e := f.pwrite(b, off) 212 if e != nil { 213 err = f.wrapErr("write", e) 214 break 215 } 216 n += m 217 b = b[m:] 218 off += int64(m) 219 } 220 return 221 } 222 223 // Seek sets the offset for the next Read or Write on file to offset, interpreted 224 // according to whence: 0 means relative to the origin of the file, 1 means 225 // relative to the current offset, and 2 means relative to the end. 226 // It returns the new offset and an error, if any. 227 // The behavior of Seek on a file opened with O_APPEND is not specified. 228 // 229 // If f is a directory, the behavior of Seek varies by operating 230 // system; you can seek to the beginning of the directory on Unix-like 231 // operating systems, but not on Windows. 232 func (f *File) Seek(offset int64, whence int) (ret int64, err error) { 233 if err := f.checkValid("seek"); err != nil { 234 return 0, err 235 } 236 r, e := f.seek(offset, whence) 237 if e == nil && f.dirinfo != nil && r != 0 { 238 e = syscall.EISDIR 239 } 240 if e != nil { 241 return 0, f.wrapErr("seek", e) 242 } 243 return r, nil 244 } 245 246 // WriteString is like Write, but writes the contents of string s rather than 247 // a slice of bytes. 248 func (f *File) WriteString(s string) (n int, err error) { 249 return f.Write([]byte(s)) 250 } 251 252 // Mkdir creates a new directory with the specified name and permission 253 // bits (before umask). 254 // If there is an error, it will be of type *PathError. 255 func Mkdir(name string, perm FileMode) error { 256 if runtime.GOOS == "windows" && isWindowsNulName(name) { 257 return &PathError{Op: "mkdir", Path: name, Err: syscall.ENOTDIR} 258 } 259 longName := fixLongPath(name) 260 e := ignoringEINTR(func() error { 261 return syscall.Mkdir(longName, syscallMode(perm)) 262 }) 263 264 if e != nil { 265 return &PathError{Op: "mkdir", Path: name, Err: e} 266 } 267 268 // mkdir(2) itself won't handle the sticky bit on *BSD and Solaris 269 if !supportsCreateWithStickyBit && perm&ModeSticky != 0 { 270 e = setStickyBit(name) 271 272 if e != nil { 273 Remove(name) 274 return e 275 } 276 } 277 278 return nil 279 } 280 281 // setStickyBit adds ModeSticky to the permission bits of path, non atomic. 282 func setStickyBit(name string) error { 283 fi, err := Stat(name) 284 if err != nil { 285 return err 286 } 287 return Chmod(name, fi.Mode()|ModeSticky) 288 } 289 290 // Chdir changes the current working directory to the named directory. 291 // If there is an error, it will be of type *PathError. 292 func Chdir(dir string) error { 293 if e := syscall.Chdir(dir); e != nil { 294 testlog.Open(dir) // observe likely non-existent directory 295 return &PathError{Op: "chdir", Path: dir, Err: e} 296 } 297 if log := testlog.Logger(); log != nil { 298 wd, err := Getwd() 299 if err == nil { 300 log.Chdir(wd) 301 } 302 } 303 return nil 304 } 305 306 // Open opens the named file for reading. If successful, methods on 307 // the returned file can be used for reading; the associated file 308 // descriptor has mode O_RDONLY. 309 // If there is an error, it will be of type *PathError. 310 func Open(name string) (*File, error) { 311 return OpenFile(name, O_RDONLY, 0) 312 } 313 314 // Create creates or truncates the named file. If the file already exists, 315 // it is truncated. If the file does not exist, it is created with mode 0666 316 // (before umask). If successful, methods on the returned File can 317 // be used for I/O; the associated file descriptor has mode O_RDWR. 318 // If there is an error, it will be of type *PathError. 319 func Create(name string) (*File, error) { 320 return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666) 321 } 322 323 // OpenFile is the generalized open call; most users will use Open 324 // or Create instead. It opens the named file with specified flag 325 // (O_RDONLY etc.). If the file does not exist, and the O_CREATE flag 326 // is passed, it is created with mode perm (before umask). If successful, 327 // methods on the returned File can be used for I/O. 328 // If there is an error, it will be of type *PathError. 329 func OpenFile(name string, flag int, perm FileMode) (*File, error) { 330 testlog.Open(name) 331 f, err := openFileNolog(name, flag, perm) 332 if err != nil { 333 return nil, err 334 } 335 f.appendMode = flag&O_APPEND != 0 336 337 return f, nil 338 } 339 340 // lstat is overridden in tests. 341 var lstat = Lstat 342 343 // Rename renames (moves) oldpath to newpath. 344 // If newpath already exists and is not a directory, Rename replaces it. 345 // OS-specific restrictions may apply when oldpath and newpath are in different directories. 346 // If there is an error, it will be of type *LinkError. 347 func Rename(oldpath, newpath string) error { 348 return rename(oldpath, newpath) 349 } 350 351 // Many functions in package syscall return a count of -1 instead of 0. 352 // Using fixCount(call()) instead of call() corrects the count. 353 func fixCount(n int, err error) (int, error) { 354 if n < 0 { 355 n = 0 356 } 357 return n, err 358 } 359 360 // wrapErr wraps an error that occurred during an operation on an open file. 361 // It passes io.EOF through unchanged, otherwise converts 362 // poll.ErrFileClosing to ErrClosed and wraps the error in a PathError. 363 func (f *File) wrapErr(op string, err error) error { 364 if err == nil || err == io.EOF { 365 return err 366 } 367 if err == poll.ErrFileClosing { 368 err = ErrClosed 369 } 370 return &PathError{Op: op, Path: f.name, Err: err} 371 } 372 373 // TempDir returns the default directory to use for temporary files. 374 // 375 // On Unix systems, it returns $TMPDIR if non-empty, else /tmp. 376 // On Windows, it uses GetTempPath, returning the first non-empty 377 // value from %TMP%, %TEMP%, %USERPROFILE%, or the Windows directory. 378 // On Plan 9, it returns /tmp. 379 // 380 // The directory is neither guaranteed to exist nor have accessible 381 // permissions. 382 func TempDir() string { 383 return tempDir() 384 } 385 386 // UserCacheDir returns the default root directory to use for user-specific 387 // cached data. Users should create their own application-specific subdirectory 388 // within this one and use that. 389 // 390 // On Unix systems, it returns $XDG_CACHE_HOME as specified by 391 // https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if 392 // non-empty, else $HOME/.cache. 393 // On Darwin, it returns $HOME/Library/Caches. 394 // On Windows, it returns %LocalAppData%. 395 // On Plan 9, it returns $home/lib/cache. 396 // 397 // If the location cannot be determined (for example, $HOME is not defined), 398 // then it will return an error. 399 func UserCacheDir() (string, error) { 400 var dir string 401 402 switch runtime.GOOS { 403 case "windows": 404 dir = Getenv("LocalAppData") 405 if dir == "" { 406 return "", errors.New("%LocalAppData% is not defined") 407 } 408 409 case "darwin", "ios": 410 dir = Getenv("HOME") 411 if dir == "" { 412 return "", errors.New("$HOME is not defined") 413 } 414 dir += "/Library/Caches" 415 416 case "plan9": 417 dir = Getenv("home") 418 if dir == "" { 419 return "", errors.New("$home is not defined") 420 } 421 dir += "/lib/cache" 422 423 default: // Unix 424 dir = Getenv("XDG_CACHE_HOME") 425 if dir == "" { 426 dir = Getenv("HOME") 427 if dir == "" { 428 return "", errors.New("neither $XDG_CACHE_HOME nor $HOME are defined") 429 } 430 dir += "/.cache" 431 } 432 } 433 434 return dir, nil 435 } 436 437 // UserConfigDir returns the default root directory to use for user-specific 438 // configuration data. Users should create their own application-specific 439 // subdirectory within this one and use that. 440 // 441 // On Unix systems, it returns $XDG_CONFIG_HOME as specified by 442 // https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if 443 // non-empty, else $HOME/.config. 444 // On Darwin, it returns $HOME/Library/Application Support. 445 // On Windows, it returns %AppData%. 446 // On Plan 9, it returns $home/lib. 447 // 448 // If the location cannot be determined (for example, $HOME is not defined), 449 // then it will return an error. 450 func UserConfigDir() (string, error) { 451 var dir string 452 453 switch runtime.GOOS { 454 case "windows": 455 dir = Getenv("AppData") 456 if dir == "" { 457 return "", errors.New("%AppData% is not defined") 458 } 459 460 case "darwin", "ios": 461 dir = Getenv("HOME") 462 if dir == "" { 463 return "", errors.New("$HOME is not defined") 464 } 465 dir += "/Library/Application Support" 466 467 case "plan9": 468 dir = Getenv("home") 469 if dir == "" { 470 return "", errors.New("$home is not defined") 471 } 472 dir += "/lib" 473 474 default: // Unix 475 dir = Getenv("XDG_CONFIG_HOME") 476 if dir == "" { 477 dir = Getenv("HOME") 478 if dir == "" { 479 return "", errors.New("neither $XDG_CONFIG_HOME nor $HOME are defined") 480 } 481 dir += "/.config" 482 } 483 } 484 485 return dir, nil 486 } 487 488 // UserHomeDir returns the current user's home directory. 489 // 490 // On Unix, including macOS, it returns the $HOME environment variable. 491 // On Windows, it returns %USERPROFILE%. 492 // On Plan 9, it returns the $home environment variable. 493 func UserHomeDir() (string, error) { 494 env, enverr := "HOME", "$HOME" 495 switch runtime.GOOS { 496 case "windows": 497 env, enverr = "USERPROFILE", "%userprofile%" 498 case "plan9": 499 env, enverr = "home", "$home" 500 } 501 if v := Getenv(env); v != "" { 502 return v, nil 503 } 504 // On some geese the home directory is not always defined. 505 switch runtime.GOOS { 506 case "android": 507 return "/sdcard", nil 508 case "ios": 509 return "/", nil 510 } 511 return "", errors.New(enverr + " is not defined") 512 } 513 514 // Chmod changes the mode of the named file to mode. 515 // If the file is a symbolic link, it changes the mode of the link's target. 516 // If there is an error, it will be of type *PathError. 517 // 518 // A different subset of the mode bits are used, depending on the 519 // operating system. 520 // 521 // On Unix, the mode's permission bits, ModeSetuid, ModeSetgid, and 522 // ModeSticky are used. 523 // 524 // On Windows, only the 0200 bit (owner writable) of mode is used; it 525 // controls whether the file's read-only attribute is set or cleared. 526 // The other bits are currently unused. For compatibility with Go 1.12 527 // and earlier, use a non-zero mode. Use mode 0400 for a read-only 528 // file and 0600 for a readable+writable file. 529 // 530 // On Plan 9, the mode's permission bits, ModeAppend, ModeExclusive, 531 // and ModeTemporary are used. 532 func Chmod(name string, mode FileMode) error { return chmod(name, mode) } 533 534 // Chmod changes the mode of the file to mode. 535 // If there is an error, it will be of type *PathError. 536 func (f *File) Chmod(mode FileMode) error { return f.chmod(mode) } 537 538 // SetDeadline sets the read and write deadlines for a File. 539 // It is equivalent to calling both SetReadDeadline and SetWriteDeadline. 540 // 541 // Only some kinds of files support setting a deadline. Calls to SetDeadline 542 // for files that do not support deadlines will return ErrNoDeadline. 543 // On most systems ordinary files do not support deadlines, but pipes do. 544 // 545 // A deadline is an absolute time after which I/O operations fail with an 546 // error instead of blocking. The deadline applies to all future and pending 547 // I/O, not just the immediately following call to Read or Write. 548 // After a deadline has been exceeded, the connection can be refreshed 549 // by setting a deadline in the future. 550 // 551 // If the deadline is exceeded a call to Read or Write or to other I/O 552 // methods will return an error that wraps ErrDeadlineExceeded. 553 // This can be tested using errors.Is(err, os.ErrDeadlineExceeded). 554 // That error implements the Timeout method, and calling the Timeout 555 // method will return true, but there are other possible errors for which 556 // the Timeout will return true even if the deadline has not been exceeded. 557 // 558 // An idle timeout can be implemented by repeatedly extending 559 // the deadline after successful Read or Write calls. 560 // 561 // A zero value for t means I/O operations will not time out. 562 func (f *File) SetDeadline(t time.Time) error { 563 return f.setDeadline(t) 564 } 565 566 // SetReadDeadline sets the deadline for future Read calls and any 567 // currently-blocked Read call. 568 // A zero value for t means Read will not time out. 569 // Not all files support setting deadlines; see SetDeadline. 570 func (f *File) SetReadDeadline(t time.Time) error { 571 return f.setReadDeadline(t) 572 } 573 574 // SetWriteDeadline sets the deadline for any future Write calls and any 575 // currently-blocked Write call. 576 // Even if Write times out, it may return n > 0, indicating that 577 // some of the data was successfully written. 578 // A zero value for t means Write will not time out. 579 // Not all files support setting deadlines; see SetDeadline. 580 func (f *File) SetWriteDeadline(t time.Time) error { 581 return f.setWriteDeadline(t) 582 } 583 584 // SyscallConn returns a raw file. 585 // This implements the syscall.Conn interface. 586 func (f *File) SyscallConn() (syscall.RawConn, error) { 587 if err := f.checkValid("SyscallConn"); err != nil { 588 return nil, err 589 } 590 return newRawConn(f) 591 } 592 593 // isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows. 594 // True is returned if name is 'NUL' whatever the case. 595 func isWindowsNulName(name string) bool { 596 if len(name) != 3 { 597 return false 598 } 599 if name[0] != 'n' && name[0] != 'N' { 600 return false 601 } 602 if name[1] != 'u' && name[1] != 'U' { 603 return false 604 } 605 if name[2] != 'l' && name[2] != 'L' { 606 return false 607 } 608 return true 609 } 610 611 // DirFS returns a file system (an fs.FS) for the tree of files rooted at the directory dir. 612 // 613 // Note that DirFS("/prefix") only guarantees that the Open calls it makes to the 614 // operating system will begin with "/prefix": DirFS("/prefix").Open("file") is the 615 // same as os.Open("/prefix/file"). So if /prefix/file is a symbolic link pointing outside 616 // the /prefix tree, then using DirFS does not stop the access any more than using 617 // os.Open does. DirFS is therefore not a general substitute for a chroot-style security 618 // mechanism when the directory tree contains arbitrary content. 619 func DirFS(dir string) fs.FS { 620 return dirFS(dir) 621 } 622 623 func containsAny(s, chars string) bool { 624 for i := 0; i < len(s); i++ { 625 for j := 0; j < len(chars); j++ { 626 if s[i] == chars[j] { 627 return true 628 } 629 } 630 } 631 return false 632 } 633 634 type dirFS string 635 636 func (dir dirFS) Open(name string) (fs.File, error) { 637 if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) { 638 return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid} 639 } 640 f, err := Open(string(dir) + "/" + name) 641 if err != nil { 642 return nil, err // nil fs.File 643 } 644 return f, nil 645 } 646 647 // ReadFile reads the named file and returns the contents. 648 // A successful call returns err == nil, not err == EOF. 649 // Because ReadFile reads the whole file, it does not treat an EOF from Read 650 // as an error to be reported. 651 func ReadFile(name string) ([]byte, error) { 652 f, err := Open(name) 653 if err != nil { 654 return nil, err 655 } 656 defer f.Close() 657 658 var size int 659 if info, err := f.Stat(); err == nil { 660 size64 := info.Size() 661 if int64(int(size64)) == size64 { 662 size = int(size64) 663 } 664 } 665 size++ // one byte for final read at EOF 666 667 // If a file claims a small size, read at least 512 bytes. 668 // In particular, files in Linux's /proc claim size 0 but 669 // then do not work right if read in small pieces, 670 // so an initial read of 1 byte would not work correctly. 671 if size < 512 { 672 size = 512 673 } 674 675 data := make([]byte, 0, size) 676 for { 677 if len(data) >= cap(data) { 678 d := append(data[:cap(data)], 0) 679 data = d[:len(data)] 680 } 681 n, err := f.Read(data[len(data):cap(data)]) 682 data = data[:len(data)+n] 683 if err != nil { 684 if err == io.EOF { 685 err = nil 686 } 687 return data, err 688 } 689 } 690 } 691 692 // WriteFile writes data to the named file, creating it if necessary. 693 // If the file does not exist, WriteFile creates it with permissions perm (before umask); 694 // otherwise WriteFile truncates it before writing, without changing permissions. 695 func WriteFile(name string, data []byte, perm FileMode) error { 696 f, err := OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, perm) 697 if err != nil { 698 return err 699 } 700 _, err = f.Write(data) 701 if err1 := f.Close(); err1 != nil && err == nil { 702 err = err1 703 } 704 return err 705 } 706