Source file src/io/io.go
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 io provides basic interfaces to I/O primitives. 6 // Its primary job is to wrap existing implementations of such primitives, 7 // such as those in package os, into shared public interfaces that 8 // abstract the functionality, plus some other related primitives. 9 // 10 // Because these interfaces and primitives wrap lower-level operations with 11 // various implementations, unless otherwise informed clients should not 12 // assume they are safe for parallel execution. 13 package io 14 15 import ( 16 "errors" 17 "sync" 18 ) 19 20 // Seek whence values. 21 const ( 22 SeekStart = 0 // seek relative to the origin of the file 23 SeekCurrent = 1 // seek relative to the current offset 24 SeekEnd = 2 // seek relative to the end 25 ) 26 27 // ErrShortWrite means that a write accepted fewer bytes than requested 28 // but failed to return an explicit error. 29 var ErrShortWrite = errors.New("short write") 30 31 // errInvalidWrite means that a write returned an impossible count. 32 var errInvalidWrite = errors.New("invalid write result") 33 34 // ErrShortBuffer means that a read required a longer buffer than was provided. 35 var ErrShortBuffer = errors.New("short buffer") 36 37 // EOF is the error returned by Read when no more input is available. 38 // (Read must return EOF itself, not an error wrapping EOF, 39 // because callers will test for EOF using ==.) 40 // Functions should return EOF only to signal a graceful end of input. 41 // If the EOF occurs unexpectedly in a structured data stream, 42 // the appropriate error is either ErrUnexpectedEOF or some other error 43 // giving more detail. 44 var EOF = errors.New("EOF") 45 46 // ErrUnexpectedEOF means that EOF was encountered in the 47 // middle of reading a fixed-size block or data structure. 48 var ErrUnexpectedEOF = errors.New("unexpected EOF") 49 50 // ErrNoProgress is returned by some clients of a Reader when 51 // many calls to Read have failed to return any data or error, 52 // usually the sign of a broken Reader implementation. 53 var ErrNoProgress = errors.New("multiple Read calls return no data or error") 54 55 // Reader is the interface that wraps the basic Read method. 56 // 57 // Read reads up to len(p) bytes into p. It returns the number of bytes 58 // read (0 <= n <= len(p)) and any error encountered. Even if Read 59 // returns n < len(p), it may use all of p as scratch space during the call. 60 // If some data is available but not len(p) bytes, Read conventionally 61 // returns what is available instead of waiting for more. 62 // 63 // When Read encounters an error or end-of-file condition after 64 // successfully reading n > 0 bytes, it returns the number of 65 // bytes read. It may return the (non-nil) error from the same call 66 // or return the error (and n == 0) from a subsequent call. 67 // An instance of this general case is that a Reader returning 68 // a non-zero number of bytes at the end of the input stream may 69 // return either err == EOF or err == nil. The next Read should 70 // return 0, EOF. 71 // 72 // Callers should always process the n > 0 bytes returned before 73 // considering the error err. Doing so correctly handles I/O errors 74 // that happen after reading some bytes and also both of the 75 // allowed EOF behaviors. 76 // 77 // Implementations of Read are discouraged from returning a 78 // zero byte count with a nil error, except when len(p) == 0. 79 // Callers should treat a return of 0 and nil as indicating that 80 // nothing happened; in particular it does not indicate EOF. 81 // 82 // Implementations must not retain p. 83 type Reader interface { 84 Read(p []byte) (n int, err error) 85 } 86 87 // Writer is the interface that wraps the basic Write method. 88 // 89 // Write writes len(p) bytes from p to the underlying data stream. 90 // It returns the number of bytes written from p (0 <= n <= len(p)) 91 // and any error encountered that caused the write to stop early. 92 // Write must return a non-nil error if it returns n < len(p). 93 // Write must not modify the slice data, even temporarily. 94 // 95 // Implementations must not retain p. 96 type Writer interface { 97 Write(p []byte) (n int, err error) 98 } 99 100 // Closer is the interface that wraps the basic Close method. 101 // 102 // The behavior of Close after the first call is undefined. 103 // Specific implementations may document their own behavior. 104 type Closer interface { 105 Close() error 106 } 107 108 // Seeker is the interface that wraps the basic Seek method. 109 // 110 // Seek sets the offset for the next Read or Write to offset, 111 // interpreted according to whence: 112 // SeekStart means relative to the start of the file, 113 // SeekCurrent means relative to the current offset, and 114 // SeekEnd means relative to the end. 115 // Seek returns the new offset relative to the start of the 116 // file or an error, if any. 117 // 118 // Seeking to an offset before the start of the file is an error. 119 // Seeking to any positive offset may be allowed, but if the new offset exceeds 120 // the size of the underlying object the behavior of subsequent I/O operations 121 // is implementation-dependent. 122 type Seeker interface { 123 Seek(offset int64, whence int) (int64, error) 124 } 125 126 // ReadWriter is the interface that groups the basic Read and Write methods. 127 type ReadWriter interface { 128 Reader 129 Writer 130 } 131 132 // ReadCloser is the interface that groups the basic Read and Close methods. 133 type ReadCloser interface { 134 Reader 135 Closer 136 } 137 138 // WriteCloser is the interface that groups the basic Write and Close methods. 139 type WriteCloser interface { 140 Writer 141 Closer 142 } 143 144 // ReadWriteCloser is the interface that groups the basic Read, Write and Close methods. 145 type ReadWriteCloser interface { 146 Reader 147 Writer 148 Closer 149 } 150 151 // ReadSeeker is the interface that groups the basic Read and Seek methods. 152 type ReadSeeker interface { 153 Reader 154 Seeker 155 } 156 157 // ReadSeekCloser is the interface that groups the basic Read, Seek and Close 158 // methods. 159 type ReadSeekCloser interface { 160 Reader 161 Seeker 162 Closer 163 } 164 165 // WriteSeeker is the interface that groups the basic Write and Seek methods. 166 type WriteSeeker interface { 167 Writer 168 Seeker 169 } 170 171 // ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods. 172 type ReadWriteSeeker interface { 173 Reader 174 Writer 175 Seeker 176 } 177 178 // ReaderFrom is the interface that wraps the ReadFrom method. 179 // 180 // ReadFrom reads data from r until EOF or error. 181 // The return value n is the number of bytes read. 182 // Any error except EOF encountered during the read is also returned. 183 // 184 // The Copy function uses ReaderFrom if available. 185 type ReaderFrom interface { 186 ReadFrom(r Reader) (n int64, err error) 187 } 188 189 // WriterTo is the interface that wraps the WriteTo method. 190 // 191 // WriteTo writes data to w until there's no more data to write or 192 // when an error occurs. The return value n is the number of bytes 193 // written. Any error encountered during the write is also returned. 194 // 195 // The Copy function uses WriterTo if available. 196 type WriterTo interface { 197 WriteTo(w Writer) (n int64, err error) 198 } 199 200 // ReaderAt is the interface that wraps the basic ReadAt method. 201 // 202 // ReadAt reads len(p) bytes into p starting at offset off in the 203 // underlying input source. It returns the number of bytes 204 // read (0 <= n <= len(p)) and any error encountered. 205 // 206 // When ReadAt returns n < len(p), it returns a non-nil error 207 // explaining why more bytes were not returned. In this respect, 208 // ReadAt is stricter than Read. 209 // 210 // Even if ReadAt returns n < len(p), it may use all of p as scratch 211 // space during the call. If some data is available but not len(p) bytes, 212 // ReadAt blocks until either all the data is available or an error occurs. 213 // In this respect ReadAt is different from Read. 214 // 215 // If the n = len(p) bytes returned by ReadAt are at the end of the 216 // input source, ReadAt may return either err == EOF or err == nil. 217 // 218 // If ReadAt is reading from an input source with a seek offset, 219 // ReadAt should not affect nor be affected by the underlying 220 // seek offset. 221 // 222 // Clients of ReadAt can execute parallel ReadAt calls on the 223 // same input source. 224 // 225 // Implementations must not retain p. 226 type ReaderAt interface { 227 ReadAt(p []byte, off int64) (n int, err error) 228 } 229 230 // WriterAt is the interface that wraps the basic WriteAt method. 231 // 232 // WriteAt writes len(p) bytes from p to the underlying data stream 233 // at offset off. It returns the number of bytes written from p (0 <= n <= len(p)) 234 // and any error encountered that caused the write to stop early. 235 // WriteAt must return a non-nil error if it returns n < len(p). 236 // 237 // If WriteAt is writing to a destination with a seek offset, 238 // WriteAt should not affect nor be affected by the underlying 239 // seek offset. 240 // 241 // Clients of WriteAt can execute parallel WriteAt calls on the same 242 // destination if the ranges do not overlap. 243 // 244 // Implementations must not retain p. 245 type WriterAt interface { 246 WriteAt(p []byte, off int64) (n int, err error) 247 } 248 249 // ByteReader is the interface that wraps the ReadByte method. 250 // 251 // ReadByte reads and returns the next byte from the input or 252 // any error encountered. If ReadByte returns an error, no input 253 // byte was consumed, and the returned byte value is undefined. 254 // 255 // ReadByte provides an efficient interface for byte-at-time 256 // processing. A Reader that does not implement ByteReader 257 // can be wrapped using bufio.NewReader to add this method. 258 type ByteReader interface { 259 ReadByte() (byte, error) 260 } 261 262 // ByteScanner is the interface that adds the UnreadByte method to the 263 // basic ReadByte method. 264 // 265 // UnreadByte causes the next call to ReadByte to return the last byte read. 266 // If the last operation was not a successful call to ReadByte, UnreadByte may 267 // return an error, unread the last byte read (or the byte prior to the 268 // last-unread byte), or (in implementations that support the Seeker interface) 269 // seek to one byte before the current offset. 270 type ByteScanner interface { 271 ByteReader 272 UnreadByte() error 273 } 274 275 // ByteWriter is the interface that wraps the WriteByte method. 276 type ByteWriter interface { 277 WriteByte(c byte) error 278 } 279 280 // RuneReader is the interface that wraps the ReadRune method. 281 // 282 // ReadRune reads a single encoded Unicode character 283 // and returns the rune and its size in bytes. If no character is 284 // available, err will be set. 285 type RuneReader interface { 286 ReadRune() (r rune, size int, err error) 287 } 288 289 // RuneScanner is the interface that adds the UnreadRune method to the 290 // basic ReadRune method. 291 // 292 // UnreadRune causes the next call to ReadRune to return the last rune read. 293 // If the last operation was not a successful call to ReadRune, UnreadRune may 294 // return an error, unread the last rune read (or the rune prior to the 295 // last-unread rune), or (in implementations that support the Seeker interface) 296 // seek to the start of the rune before the current offset. 297 type RuneScanner interface { 298 RuneReader 299 UnreadRune() error 300 } 301 302 // StringWriter is the interface that wraps the WriteString method. 303 type StringWriter interface { 304 WriteString(s string) (n int, err error) 305 } 306 307 // WriteString writes the contents of the string s to w, which accepts a slice of bytes. 308 // If w implements StringWriter, its WriteString method is invoked directly. 309 // Otherwise, w.Write is called exactly once. 310 func WriteString(w Writer, s string) (n int, err error) { 311 if sw, ok := w.(StringWriter); ok { 312 return sw.WriteString(s) 313 } 314 return w.Write([]byte(s)) 315 } 316 317 // ReadAtLeast reads from r into buf until it has read at least min bytes. 318 // It returns the number of bytes copied and an error if fewer bytes were read. 319 // The error is EOF only if no bytes were read. 320 // If an EOF happens after reading fewer than min bytes, 321 // ReadAtLeast returns ErrUnexpectedEOF. 322 // If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer. 323 // On return, n >= min if and only if err == nil. 324 // If r returns an error having read at least min bytes, the error is dropped. 325 func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) { 326 if len(buf) < min { 327 return 0, ErrShortBuffer 328 } 329 for n < min && err == nil { 330 var nn int 331 nn, err = r.Read(buf[n:]) 332 n += nn 333 } 334 if n >= min { 335 err = nil 336 } else if n > 0 && err == EOF { 337 err = ErrUnexpectedEOF 338 } 339 return 340 } 341 342 // ReadFull reads exactly len(buf) bytes from r into buf. 343 // It returns the number of bytes copied and an error if fewer bytes were read. 344 // The error is EOF only if no bytes were read. 345 // If an EOF happens after reading some but not all the bytes, 346 // ReadFull returns ErrUnexpectedEOF. 347 // On return, n == len(buf) if and only if err == nil. 348 // If r returns an error having read at least len(buf) bytes, the error is dropped. 349 func ReadFull(r Reader, buf []byte) (n int, err error) { 350 return ReadAtLeast(r, buf, len(buf)) 351 } 352 353 // CopyN copies n bytes (or until an error) from src to dst. 354 // It returns the number of bytes copied and the earliest 355 // error encountered while copying. 356 // On return, written == n if and only if err == nil. 357 // 358 // If dst implements the ReaderFrom interface, 359 // the copy is implemented using it. 360 func CopyN(dst Writer, src Reader, n int64) (written int64, err error) { 361 written, err = Copy(dst, LimitReader(src, n)) 362 if written == n { 363 return n, nil 364 } 365 if written < n && err == nil { 366 // src stopped early; must have been EOF. 367 err = EOF 368 } 369 return 370 } 371 372 // Copy copies from src to dst until either EOF is reached 373 // on src or an error occurs. It returns the number of bytes 374 // copied and the first error encountered while copying, if any. 375 // 376 // A successful Copy returns err == nil, not err == EOF. 377 // Because Copy is defined to read from src until EOF, it does 378 // not treat an EOF from Read as an error to be reported. 379 // 380 // If src implements the WriterTo interface, 381 // the copy is implemented by calling src.WriteTo(dst). 382 // Otherwise, if dst implements the ReaderFrom interface, 383 // the copy is implemented by calling dst.ReadFrom(src). 384 func Copy(dst Writer, src Reader) (written int64, err error) { 385 return copyBuffer(dst, src, nil) 386 } 387 388 // CopyBuffer is identical to Copy except that it stages through the 389 // provided buffer (if one is required) rather than allocating a 390 // temporary one. If buf is nil, one is allocated; otherwise if it has 391 // zero length, CopyBuffer panics. 392 // 393 // If either src implements WriterTo or dst implements ReaderFrom, 394 // buf will not be used to perform the copy. 395 func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) { 396 if buf != nil && len(buf) == 0 { 397 panic("empty buffer in CopyBuffer") 398 } 399 return copyBuffer(dst, src, buf) 400 } 401 402 // copyBuffer is the actual implementation of Copy and CopyBuffer. 403 // if buf is nil, one is allocated. 404 func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) { 405 // If the reader has a WriteTo method, use it to do the copy. 406 // Avoids an allocation and a copy. 407 if wt, ok := src.(WriterTo); ok { 408 return wt.WriteTo(dst) 409 } 410 // Similarly, if the writer has a ReadFrom method, use it to do the copy. 411 if rt, ok := dst.(ReaderFrom); ok { 412 return rt.ReadFrom(src) 413 } 414 if buf == nil { 415 size := 32 * 1024 416 if l, ok := src.(*LimitedReader); ok && int64(size) > l.N { 417 if l.N < 1 { 418 size = 1 419 } else { 420 size = int(l.N) 421 } 422 } 423 buf = make([]byte, size) 424 } 425 for { 426 nr, er := src.Read(buf) 427 if nr > 0 { 428 nw, ew := dst.Write(buf[0:nr]) 429 if nw < 0 || nr < nw { 430 nw = 0 431 if ew == nil { 432 ew = errInvalidWrite 433 } 434 } 435 written += int64(nw) 436 if ew != nil { 437 err = ew 438 break 439 } 440 if nr != nw { 441 err = ErrShortWrite 442 break 443 } 444 } 445 if er != nil { 446 if er != EOF { 447 err = er 448 } 449 break 450 } 451 } 452 return written, err 453 } 454 455 // LimitReader returns a Reader that reads from r 456 // but stops with EOF after n bytes. 457 // The underlying implementation is a *LimitedReader. 458 func LimitReader(r Reader, n int64) Reader { return &LimitedReader{r, n} } 459 460 // A LimitedReader reads from R but limits the amount of 461 // data returned to just N bytes. Each call to Read 462 // updates N to reflect the new amount remaining. 463 // Read returns EOF when N <= 0 or when the underlying R returns EOF. 464 type LimitedReader struct { 465 R Reader // underlying reader 466 N int64 // max bytes remaining 467 } 468 469 func (l *LimitedReader) Read(p []byte) (n int, err error) { 470 if l.N <= 0 { 471 return 0, EOF 472 } 473 if int64(len(p)) > l.N { 474 p = p[0:l.N] 475 } 476 n, err = l.R.Read(p) 477 l.N -= int64(n) 478 return 479 } 480 481 // NewSectionReader returns a SectionReader that reads from r 482 // starting at offset off and stops with EOF after n bytes. 483 func NewSectionReader(r ReaderAt, off int64, n int64) *SectionReader { 484 var remaining int64 485 const maxint64 = 1<<63 - 1 486 if off <= maxint64-n { 487 remaining = n + off 488 } else { 489 // Overflow, with no way to return error. 490 // Assume we can read up to an offset of 1<<63 - 1. 491 remaining = maxint64 492 } 493 return &SectionReader{r, off, off, remaining} 494 } 495 496 // SectionReader implements Read, Seek, and ReadAt on a section 497 // of an underlying ReaderAt. 498 type SectionReader struct { 499 r ReaderAt 500 base int64 501 off int64 502 limit int64 503 } 504 505 func (s *SectionReader) Read(p []byte) (n int, err error) { 506 if s.off >= s.limit { 507 return 0, EOF 508 } 509 if max := s.limit - s.off; int64(len(p)) > max { 510 p = p[0:max] 511 } 512 n, err = s.r.ReadAt(p, s.off) 513 s.off += int64(n) 514 return 515 } 516 517 var errWhence = errors.New("Seek: invalid whence") 518 var errOffset = errors.New("Seek: invalid offset") 519 520 func (s *SectionReader) Seek(offset int64, whence int) (int64, error) { 521 switch whence { 522 default: 523 return 0, errWhence 524 case SeekStart: 525 offset += s.base 526 case SeekCurrent: 527 offset += s.off 528 case SeekEnd: 529 offset += s.limit 530 } 531 if offset < s.base { 532 return 0, errOffset 533 } 534 s.off = offset 535 return offset - s.base, nil 536 } 537 538 func (s *SectionReader) ReadAt(p []byte, off int64) (n int, err error) { 539 if off < 0 || off >= s.limit-s.base { 540 return 0, EOF 541 } 542 off += s.base 543 if max := s.limit - off; int64(len(p)) > max { 544 p = p[0:max] 545 n, err = s.r.ReadAt(p, off) 546 if err == nil { 547 err = EOF 548 } 549 return n, err 550 } 551 return s.r.ReadAt(p, off) 552 } 553 554 // Size returns the size of the section in bytes. 555 func (s *SectionReader) Size() int64 { return s.limit - s.base } 556 557 // TeeReader returns a Reader that writes to w what it reads from r. 558 // All reads from r performed through it are matched with 559 // corresponding writes to w. There is no internal buffering - 560 // the write must complete before the read completes. 561 // Any error encountered while writing is reported as a read error. 562 func TeeReader(r Reader, w Writer) Reader { 563 return &teeReader{r, w} 564 } 565 566 type teeReader struct { 567 r Reader 568 w Writer 569 } 570 571 func (t *teeReader) Read(p []byte) (n int, err error) { 572 n, err = t.r.Read(p) 573 if n > 0 { 574 if n, err := t.w.Write(p[:n]); err != nil { 575 return n, err 576 } 577 } 578 return 579 } 580 581 // Discard is a Writer on which all Write calls succeed 582 // without doing anything. 583 var Discard Writer = discard{} 584 585 type discard struct{} 586 587 // discard implements ReaderFrom as an optimization so Copy to 588 // io.Discard can avoid doing unnecessary work. 589 var _ ReaderFrom = discard{} 590 591 func (discard) Write(p []byte) (int, error) { 592 return len(p), nil 593 } 594 595 func (discard) WriteString(s string) (int, error) { 596 return len(s), nil 597 } 598 599 var blackHolePool = sync.Pool{ 600 New: func() any { 601 b := make([]byte, 8192) 602 return &b 603 }, 604 } 605 606 func (discard) ReadFrom(r Reader) (n int64, err error) { 607 bufp := blackHolePool.Get().(*[]byte) 608 readSize := 0 609 for { 610 readSize, err = r.Read(*bufp) 611 n += int64(readSize) 612 if err != nil { 613 blackHolePool.Put(bufp) 614 if err == EOF { 615 return n, nil 616 } 617 return 618 } 619 } 620 } 621 622 // NopCloser returns a ReadCloser with a no-op Close method wrapping 623 // the provided Reader r. 624 func NopCloser(r Reader) ReadCloser { 625 return nopCloser{r} 626 } 627 628 type nopCloser struct { 629 Reader 630 } 631 632 func (nopCloser) Close() error { return nil } 633 634 // ReadAll reads from r until an error or EOF and returns the data it read. 635 // A successful call returns err == nil, not err == EOF. Because ReadAll is 636 // defined to read from src until EOF, it does not treat an EOF from Read 637 // as an error to be reported. 638 func ReadAll(r Reader) ([]byte, error) { 639 b := make([]byte, 0, 512) 640 for { 641 if len(b) == cap(b) { 642 // Add more capacity (let append pick how much). 643 b = append(b, 0)[:len(b)] 644 } 645 n, err := r.Read(b[len(b):cap(b)]) 646 b = b[:len(b)+n] 647 if err != nil { 648 if err == EOF { 649 err = nil 650 } 651 return b, err 652 } 653 } 654 } 655