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

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