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

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