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Source file src/io/io.go

Documentation: io

     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 an 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 and 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 is legal, but the behavior of subsequent
   120  // I/O operations on the underlying object is implementation-dependent.
   121  type Seeker interface {
   122  	Seek(offset int64, whence int) (int64, error)
   123  }
   124  
   125  // ReadWriter is the interface that groups the basic Read and Write methods.
   126  type ReadWriter interface {
   127  	Reader
   128  	Writer
   129  }
   130  
   131  // ReadCloser is the interface that groups the basic Read and Close methods.
   132  type ReadCloser interface {
   133  	Reader
   134  	Closer
   135  }
   136  
   137  // WriteCloser is the interface that groups the basic Write and Close methods.
   138  type WriteCloser interface {
   139  	Writer
   140  	Closer
   141  }
   142  
   143  // ReadWriteCloser is the interface that groups the basic Read, Write and Close methods.
   144  type ReadWriteCloser interface {
   145  	Reader
   146  	Writer
   147  	Closer
   148  }
   149  
   150  // ReadSeeker is the interface that groups the basic Read and Seek methods.
   151  type ReadSeeker interface {
   152  	Reader
   153  	Seeker
   154  }
   155  
   156  // ReadSeekCloser is the interface that groups the basic Read, Seek and Close
   157  // methods.
   158  type ReadSeekCloser interface {
   159  	Reader
   160  	Seeker
   161  	Closer
   162  }
   163  
   164  // WriteSeeker is the interface that groups the basic Write and Seek methods.
   165  type WriteSeeker interface {
   166  	Writer
   167  	Seeker
   168  }
   169  
   170  // ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods.
   171  type ReadWriteSeeker interface {
   172  	Reader
   173  	Writer
   174  	Seeker
   175  }
   176  
   177  // ReaderFrom is the interface that wraps the ReadFrom method.
   178  //
   179  // ReadFrom reads data from r until EOF or error.
   180  // The return value n is the number of bytes read.
   181  // Any error except EOF encountered during the read is also returned.
   182  //
   183  // The Copy function uses ReaderFrom if available.
   184  type ReaderFrom interface {
   185  	ReadFrom(r Reader) (n int64, err error)
   186  }
   187  
   188  // WriterTo is the interface that wraps the WriteTo method.
   189  //
   190  // WriteTo writes data to w until there's no more data to write or
   191  // when an error occurs. The return value n is the number of bytes
   192  // written. Any error encountered during the write is also returned.
   193  //
   194  // The Copy function uses WriterTo if available.
   195  type WriterTo interface {
   196  	WriteTo(w Writer) (n int64, err error)
   197  }
   198  
   199  // ReaderAt is the interface that wraps the basic ReadAt method.
   200  //
   201  // ReadAt reads len(p) bytes into p starting at offset off in the
   202  // underlying input source. It returns the number of bytes
   203  // read (0 <= n <= len(p)) and any error encountered.
   204  //
   205  // When ReadAt returns n < len(p), it returns a non-nil error
   206  // explaining why more bytes were not returned. In this respect,
   207  // ReadAt is stricter than Read.
   208  //
   209  // Even if ReadAt returns n < len(p), it may use all of p as scratch
   210  // space during the call. If some data is available but not len(p) bytes,
   211  // ReadAt blocks until either all the data is available or an error occurs.
   212  // In this respect ReadAt is different from Read.
   213  //
   214  // If the n = len(p) bytes returned by ReadAt are at the end of the
   215  // input source, ReadAt may return either err == EOF or err == nil.
   216  //
   217  // If ReadAt is reading from an input source with a seek offset,
   218  // ReadAt should not affect nor be affected by the underlying
   219  // seek offset.
   220  //
   221  // Clients of ReadAt can execute parallel ReadAt calls on the
   222  // same input source.
   223  //
   224  // Implementations must not retain p.
   225  type ReaderAt interface {
   226  	ReadAt(p []byte, off int64) (n int, err error)
   227  }
   228  
   229  // WriterAt is the interface that wraps the basic WriteAt method.
   230  //
   231  // WriteAt writes len(p) bytes from p to the underlying data stream
   232  // at offset off. It returns the number of bytes written from p (0 <= n <= len(p))
   233  // and any error encountered that caused the write to stop early.
   234  // WriteAt must return a non-nil error if it returns n < len(p).
   235  //
   236  // If WriteAt is writing to a destination with a seek offset,
   237  // WriteAt should not affect nor be affected by the underlying
   238  // seek offset.
   239  //
   240  // Clients of WriteAt can execute parallel WriteAt calls on the same
   241  // destination if the ranges do not overlap.
   242  //
   243  // Implementations must not retain p.
   244  type WriterAt interface {
   245  	WriteAt(p []byte, off int64) (n int, err error)
   246  }
   247  
   248  // ByteReader is the interface that wraps the ReadByte method.
   249  //
   250  // ReadByte reads and returns the next byte from the input or
   251  // any error encountered. If ReadByte returns an error, no input
   252  // byte was consumed, and the returned byte value is undefined.
   253  //
   254  // ReadByte provides an efficient interface for byte-at-time
   255  // processing. A Reader that does not implement  ByteReader
   256  // can be wrapped using bufio.NewReader to add this method.
   257  type ByteReader interface {
   258  	ReadByte() (byte, error)
   259  }
   260  
   261  // ByteScanner is the interface that adds the UnreadByte method to the
   262  // basic ReadByte method.
   263  //
   264  // UnreadByte causes the next call to ReadByte to return the same byte
   265  // as the previous call to ReadByte.
   266  // It may be an error to call UnreadByte twice without an intervening
   267  // call to ReadByte.
   268  type ByteScanner interface {
   269  	ByteReader
   270  	UnreadByte() error
   271  }
   272  
   273  // ByteWriter is the interface that wraps the WriteByte method.
   274  type ByteWriter interface {
   275  	WriteByte(c byte) error
   276  }
   277  
   278  // RuneReader is the interface that wraps the ReadRune method.
   279  //
   280  // ReadRune reads a single UTF-8 encoded Unicode character
   281  // and returns the rune and its size in bytes. If no character is
   282  // available, err will be set.
   283  type RuneReader interface {
   284  	ReadRune() (r rune, size int, err error)
   285  }
   286  
   287  // RuneScanner is the interface that adds the UnreadRune method to the
   288  // basic ReadRune method.
   289  //
   290  // UnreadRune causes the next call to ReadRune to return the same rune
   291  // as the previous call to ReadRune.
   292  // It may be an error to call UnreadRune twice without an intervening
   293  // call to ReadRune.
   294  type RuneScanner interface {
   295  	RuneReader
   296  	UnreadRune() error
   297  }
   298  
   299  // StringWriter is the interface that wraps the WriteString method.
   300  type StringWriter interface {
   301  	WriteString(s string) (n int, err error)
   302  }
   303  
   304  // WriteString writes the contents of the string s to w, which accepts a slice of bytes.
   305  // If w implements StringWriter, its WriteString method is invoked directly.
   306  // Otherwise, w.Write is called exactly once.
   307  func WriteString(w Writer, s string) (n int, err error) {
   308  	if sw, ok := w.(StringWriter); ok {
   309  		return sw.WriteString(s)
   310  	}
   311  	return w.Write([]byte(s))
   312  }
   313  
   314  // ReadAtLeast reads from r into buf until it has read at least min bytes.
   315  // It returns the number of bytes copied and an error if fewer bytes were read.
   316  // The error is EOF only if no bytes were read.
   317  // If an EOF happens after reading fewer than min bytes,
   318  // ReadAtLeast returns ErrUnexpectedEOF.
   319  // If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer.
   320  // On return, n >= min if and only if err == nil.
   321  // If r returns an error having read at least min bytes, the error is dropped.
   322  func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
   323  	if len(buf) < min {
   324  		return 0, ErrShortBuffer
   325  	}
   326  	for n < min && err == nil {
   327  		var nn int
   328  		nn, err = r.Read(buf[n:])
   329  		n += nn
   330  	}
   331  	if n >= min {
   332  		err = nil
   333  	} else if n > 0 && err == EOF {
   334  		err = ErrUnexpectedEOF
   335  	}
   336  	return
   337  }
   338  
   339  // ReadFull reads exactly len(buf) bytes from r into buf.
   340  // It returns the number of bytes copied and an error if fewer bytes were read.
   341  // The error is EOF only if no bytes were read.
   342  // If an EOF happens after reading some but not all the bytes,
   343  // ReadFull returns ErrUnexpectedEOF.
   344  // On return, n == len(buf) if and only if err == nil.
   345  // If r returns an error having read at least len(buf) bytes, the error is dropped.
   346  func ReadFull(r Reader, buf []byte) (n int, err error) {
   347  	return ReadAtLeast(r, buf, len(buf))
   348  }
   349  
   350  // CopyN copies n bytes (or until an error) from src to dst.
   351  // It returns the number of bytes copied and the earliest
   352  // error encountered while copying.
   353  // On return, written == n if and only if err == nil.
   354  //
   355  // If dst implements the ReaderFrom interface,
   356  // the copy is implemented using it.
   357  func CopyN(dst Writer, src Reader, n int64) (written int64, err error) {
   358  	written, err = Copy(dst, LimitReader(src, n))
   359  	if written == n {
   360  		return n, nil
   361  	}
   362  	if written < n && err == nil {
   363  		// src stopped early; must have been EOF.
   364  		err = EOF
   365  	}
   366  	return
   367  }
   368  
   369  // Copy copies from src to dst until either EOF is reached
   370  // on src or an error occurs. It returns the number of bytes
   371  // copied and the first error encountered while copying, if any.
   372  //
   373  // A successful Copy returns err == nil, not err == EOF.
   374  // Because Copy is defined to read from src until EOF, it does
   375  // not treat an EOF from Read as an error to be reported.
   376  //
   377  // If src implements the WriterTo interface,
   378  // the copy is implemented by calling src.WriteTo(dst).
   379  // Otherwise, if dst implements the ReaderFrom interface,
   380  // the copy is implemented by calling dst.ReadFrom(src).
   381  func Copy(dst Writer, src Reader) (written int64, err error) {
   382  	return copyBuffer(dst, src, nil)
   383  }
   384  
   385  // CopyBuffer is identical to Copy except that it stages through the
   386  // provided buffer (if one is required) rather than allocating a
   387  // temporary one. If buf is nil, one is allocated; otherwise if it has
   388  // zero length, CopyBuffer panics.
   389  //
   390  // If either src implements WriterTo or dst implements ReaderFrom,
   391  // buf will not be used to perform the copy.
   392  func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
   393  	if buf != nil && len(buf) == 0 {
   394  		panic("empty buffer in CopyBuffer")
   395  	}
   396  	return copyBuffer(dst, src, buf)
   397  }
   398  
   399  // copyBuffer is the actual implementation of Copy and CopyBuffer.
   400  // if buf is nil, one is allocated.
   401  func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
   402  	// If the reader has a WriteTo method, use it to do the copy.
   403  	// Avoids an allocation and a copy.
   404  	if wt, ok := src.(WriterTo); ok {
   405  		return wt.WriteTo(dst)
   406  	}
   407  	// Similarly, if the writer has a ReadFrom method, use it to do the copy.
   408  	if rt, ok := dst.(ReaderFrom); ok {
   409  		return rt.ReadFrom(src)
   410  	}
   411  	if buf == nil {
   412  		size := 32 * 1024
   413  		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
   414  			if l.N < 1 {
   415  				size = 1
   416  			} else {
   417  				size = int(l.N)
   418  			}
   419  		}
   420  		buf = make([]byte, size)
   421  	}
   422  	for {
   423  		nr, er := src.Read(buf)
   424  		if nr > 0 {
   425  			nw, ew := dst.Write(buf[0:nr])
   426  			if nw < 0 || nr < nw {
   427  				nw = 0
   428  				if ew == nil {
   429  					ew = errInvalidWrite
   430  				}
   431  			}
   432  			written += int64(nw)
   433  			if ew != nil {
   434  				err = ew
   435  				break
   436  			}
   437  			if nr != nw {
   438  				err = ErrShortWrite
   439  				break
   440  			}
   441  		}
   442  		if er != nil {
   443  			if er != EOF {
   444  				err = er
   445  			}
   446  			break
   447  		}
   448  	}
   449  	return written, err
   450  }
   451  
   452  // LimitReader returns a Reader that reads from r
   453  // but stops with EOF after n bytes.
   454  // The underlying implementation is a *LimitedReader.
   455  func LimitReader(r Reader, n int64) Reader { return &LimitedReader{r, n} }
   456  
   457  // A LimitedReader reads from R but limits the amount of
   458  // data returned to just N bytes. Each call to Read
   459  // updates N to reflect the new amount remaining.
   460  // Read returns EOF when N <= 0 or when the underlying R returns EOF.
   461  type LimitedReader struct {
   462  	R Reader // underlying reader
   463  	N int64  // max bytes remaining
   464  }
   465  
   466  func (l *LimitedReader) Read(p []byte) (n int, err error) {
   467  	if l.N <= 0 {
   468  		return 0, EOF
   469  	}
   470  	if int64(len(p)) > l.N {
   471  		p = p[0:l.N]
   472  	}
   473  	n, err = l.R.Read(p)
   474  	l.N -= int64(n)
   475  	return
   476  }
   477  
   478  // NewSectionReader returns a SectionReader that reads from r
   479  // starting at offset off and stops with EOF after n bytes.
   480  func NewSectionReader(r ReaderAt, off int64, n int64) *SectionReader {
   481  	return &SectionReader{r, off, off, off + n}
   482  }
   483  
   484  // SectionReader implements Read, Seek, and ReadAt on a section
   485  // of an underlying ReaderAt.
   486  type SectionReader struct {
   487  	r     ReaderAt
   488  	base  int64
   489  	off   int64
   490  	limit int64
   491  }
   492  
   493  func (s *SectionReader) Read(p []byte) (n int, err error) {
   494  	if s.off >= s.limit {
   495  		return 0, EOF
   496  	}
   497  	if max := s.limit - s.off; int64(len(p)) > max {
   498  		p = p[0:max]
   499  	}
   500  	n, err = s.r.ReadAt(p, s.off)
   501  	s.off += int64(n)
   502  	return
   503  }
   504  
   505  var errWhence = errors.New("Seek: invalid whence")
   506  var errOffset = errors.New("Seek: invalid offset")
   507  
   508  func (s *SectionReader) Seek(offset int64, whence int) (int64, error) {
   509  	switch whence {
   510  	default:
   511  		return 0, errWhence
   512  	case SeekStart:
   513  		offset += s.base
   514  	case SeekCurrent:
   515  		offset += s.off
   516  	case SeekEnd:
   517  		offset += s.limit
   518  	}
   519  	if offset < s.base {
   520  		return 0, errOffset
   521  	}
   522  	s.off = offset
   523  	return offset - s.base, nil
   524  }
   525  
   526  func (s *SectionReader) ReadAt(p []byte, off int64) (n int, err error) {
   527  	if off < 0 || off >= s.limit-s.base {
   528  		return 0, EOF
   529  	}
   530  	off += s.base
   531  	if max := s.limit - off; int64(len(p)) > max {
   532  		p = p[0:max]
   533  		n, err = s.r.ReadAt(p, off)
   534  		if err == nil {
   535  			err = EOF
   536  		}
   537  		return n, err
   538  	}
   539  	return s.r.ReadAt(p, off)
   540  }
   541  
   542  // Size returns the size of the section in bytes.
   543  func (s *SectionReader) Size() int64 { return s.limit - s.base }
   544  
   545  // TeeReader returns a Reader that writes to w what it reads from r.
   546  // All reads from r performed through it are matched with
   547  // corresponding writes to w. There is no internal buffering -
   548  // the write must complete before the read completes.
   549  // Any error encountered while writing is reported as a read error.
   550  func TeeReader(r Reader, w Writer) Reader {
   551  	return &teeReader{r, w}
   552  }
   553  
   554  type teeReader struct {
   555  	r Reader
   556  	w Writer
   557  }
   558  
   559  func (t *teeReader) Read(p []byte) (n int, err error) {
   560  	n, err = t.r.Read(p)
   561  	if n > 0 {
   562  		if n, err := t.w.Write(p[:n]); err != nil {
   563  			return n, err
   564  		}
   565  	}
   566  	return
   567  }
   568  
   569  // Discard is an Writer on which all Write calls succeed
   570  // without doing anything.
   571  var Discard Writer = discard{}
   572  
   573  type discard struct{}
   574  
   575  // discard implements ReaderFrom as an optimization so Copy to
   576  // io.Discard can avoid doing unnecessary work.
   577  var _ ReaderFrom = discard{}
   578  
   579  func (discard) Write(p []byte) (int, error) {
   580  	return len(p), nil
   581  }
   582  
   583  func (discard) WriteString(s string) (int, error) {
   584  	return len(s), nil
   585  }
   586  
   587  var blackHolePool = sync.Pool{
   588  	New: func() interface{} {
   589  		b := make([]byte, 8192)
   590  		return &b
   591  	},
   592  }
   593  
   594  func (discard) ReadFrom(r Reader) (n int64, err error) {
   595  	bufp := blackHolePool.Get().(*[]byte)
   596  	readSize := 0
   597  	for {
   598  		readSize, err = r.Read(*bufp)
   599  		n += int64(readSize)
   600  		if err != nil {
   601  			blackHolePool.Put(bufp)
   602  			if err == EOF {
   603  				return n, nil
   604  			}
   605  			return
   606  		}
   607  	}
   608  }
   609  
   610  // NopCloser returns a ReadCloser with a no-op Close method wrapping
   611  // the provided Reader r.
   612  func NopCloser(r Reader) ReadCloser {
   613  	return nopCloser{r}
   614  }
   615  
   616  type nopCloser struct {
   617  	Reader
   618  }
   619  
   620  func (nopCloser) Close() error { return nil }
   621  
   622  // ReadAll reads from r until an error or EOF and returns the data it read.
   623  // A successful call returns err == nil, not err == EOF. Because ReadAll is
   624  // defined to read from src until EOF, it does not treat an EOF from Read
   625  // as an error to be reported.
   626  func ReadAll(r Reader) ([]byte, error) {
   627  	b := make([]byte, 0, 512)
   628  	for {
   629  		if len(b) == cap(b) {
   630  			// Add more capacity (let append pick how much).
   631  			b = append(b, 0)[:len(b)]
   632  		}
   633  		n, err := r.Read(b[len(b):cap(b)])
   634  		b = b[:len(b)+n]
   635  		if err != nil {
   636  			if err == EOF {
   637  				err = nil
   638  			}
   639  			return b, err
   640  		}
   641  	}
   642  }
   643  

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