src/pkg/io/io.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Package io provides basic interfaces to I/O primitives.
 // Its primary job is to wrap existing implementations of such primitives,
 // such as those in package os, into shared public interfaces that
 // abstract the functionality, plus some other related primitives.
 package io

 import "os"

 // Error represents an unexpected I/O behavior.
 type Error struct {
 	ErrorString string
 }

 func (err *Error) String() string { return err.ErrorString }

 // ErrShortWrite means that a write accepted fewer bytes than requested
 // but failed to return an explicit error.
 var ErrShortWrite os.Error = &Error{"short write"}

 // ErrShortBuffer means that a read required a longer buffer than was provided.
 var ErrShortBuffer os.Error = &Error{"short buffer"}

 // ErrUnexpectedEOF means that os.EOF was encountered in the
 // middle of reading a fixed-size block or data structure.
 var ErrUnexpectedEOF os.Error = &Error{"unexpected EOF"}

 // Reader is the interface that wraps the basic Read method.
 //
 // Read reads up to len(p) bytes into p.  It returns the number of bytes
 // read (0 <= n <= len(p)) and any error encountered.  Even if Read
 // returns n < len(p), it may use all of p as scratch space during the call.
 // If some data is available but not len(p) bytes, Read conventionally
 // returns what is available instead of waiting for more.
 //
 // When Read encounters an error or end-of-file condition after
 // successfully reading n > 0 bytes, it returns the number of
 // bytes read.  It may return the (non-nil) error from the same call
 // or return the error (and n == 0) from a subsequent call.
 // An instance of this general case is that a Reader returning
 // a non-zero number of bytes at the end of the input stream may
 // return either err == os.EOF or err == nil.  The next Read should
 // return 0, os.EOF regardless.
 //
 // Callers should always process the n > 0 bytes returned before
 // considering the error err.  Doing so correctly handles I/O errors
 // that happen after reading some bytes and also both of the
 // allowed EOF behaviors.
 type Reader interface {
 	Read(p []byte) (n int, err os.Error)
 }

 // Writer is the interface that wraps the basic Write method.
 //
 // Write writes len(p) bytes from p to the underlying data stream.
 // It returns the number of bytes written from p (0 <= n <= len(p))
 // and any error encountered that caused the write to stop early.
 // Write must return a non-nil error if it returns n < len(p).
 type Writer interface {
 	Write(p []byte) (n int, err os.Error)
 }

 // Closer is the interface that wraps the basic Close method.
 type Closer interface {
 	Close() os.Error
 }

 // Seeker is the interface that wraps the basic Seek method.
 //
 // Seek sets the offset for the next Read or Write to offset,
 // interpreted according to whence: 0 means relative to the origin of
 // the file, 1 means relative to the current offset, and 2 means
 // relative to the end.  Seek returns the new offset and an Error, if
 // any.
 type Seeker interface {
 	Seek(offset int64, whence int) (ret int64, err os.Error)
 }

 // ReadWriter is the interface that groups the basic Read and Write methods.
 type ReadWriter interface {
 	Reader
 	Writer
 }

 // ReadCloser is the interface that groups the basic Read and Close methods.
 type ReadCloser interface {
 	Reader
 	Closer
 }

 // WriteCloser is the interface that groups the basic Write and Close methods.
 type WriteCloser interface {
 	Writer
 	Closer
 }

 // ReadWriteCloser is the interface that groups the basic Read, Write and Close methods.
 type ReadWriteCloser interface {
 	Reader
 	Writer
 	Closer
 }

 // ReadSeeker is the interface that groups the basic Read and Seek methods.
 type ReadSeeker interface {
 	Reader
 	Seeker
 }

 // WriteSeeker is the interface that groups the basic Write and Seek methods.
 type WriteSeeker interface {
 	Writer
 	Seeker
 }

 // ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods.
 type ReadWriteSeeker interface {
 	Reader
 	Writer
 	Seeker
 }

 // ReaderFrom is the interface that wraps the ReadFrom method.
 type ReaderFrom interface {
 	ReadFrom(r Reader) (n int64, err os.Error)
 }

 // WriterTo is the interface that wraps the WriteTo method.
 type WriterTo interface {
 	WriteTo(w Writer) (n int64, err os.Error)
 }

 // ReaderAt is the interface that wraps the basic ReadAt method.
 //
 // ReadAt reads len(p) bytes into p starting at offset off in the
 // underlying input source.  It returns the number of bytes
 // read (0 <= n <= len(p)) and any error encountered.
 //
 // When ReadAt returns n < len(p), it returns a non-nil error
 // explaining why more bytes were not returned.  In this respect,
 // ReadAt is stricter than Read.
 //
 // Even if ReadAt returns n < len(p), it may use all of p as scratch
 // space during the call.  If some data is available but not len(p) bytes,
 // ReadAt blocks until either all the data is available or an error occurs.
 // In this respect ReadAt is different from Read.
 //
 // If the n = len(p) bytes returned by ReadAt are at the end of the
 // input source, ReadAt may return either err == os.EOF or err == nil.
 //
 // If ReadAt is reading from an input source with a seek offset,
 // ReadAt should not affect nor be affected by the underlying
 // seek offset.
 type ReaderAt interface {
 	ReadAt(p []byte, off int64) (n int, err os.Error)
 }

 // WriterAt is the interface that wraps the basic WriteAt method.
 //
 // WriteAt writes len(p) bytes from p to the underlying data stream
 // at offset off.  It returns the number of bytes written from p (0 <= n <= len(p))
 // and any error encountered that caused the write to stop early.
 // WriteAt must return a non-nil error if it returns n < len(p).
 type WriterAt interface {
 	WriteAt(p []byte, off int64) (n int, err os.Error)
 }

 // ByteReader is the interface that wraps the ReadByte method.
 //
 // ReadByte reads and returns the next byte from the input.
 // If no byte is available, err will be set.
 type ByteReader interface {
 	ReadByte() (c byte, err os.Error)
 }

 // ByteScanner is the interface that adds the UnreadByte method to the
 // basic ReadByte method.
 //
 // UnreadByte causes the next call to ReadByte to return the same byte
 // as the previous call to ReadByte.
 // It may be an error to call UnreadByte twice without an intervening
 // call to ReadByte.
 type ByteScanner interface {
 	ByteReader
 	UnreadByte() os.Error
 }

 // RuneReader is the interface that wraps the ReadRune method.
 //
 // ReadRune reads a single UTF-8 encoded Unicode character
 // and returns the rune and its size in bytes. If no character is
 // available, err will be set.
 type RuneReader interface {
 	ReadRune() (rune int, size int, err os.Error)
 }

 // RuneScanner is the interface that adds the UnreadRune method to the
 // basic ReadRune method.
 //
 // UnreadRune causes the next call to ReadRune to return the same rune
 // as the previous call to ReadRune.
 // It may be an error to call UnreadRune twice without an intervening
 // call to ReadRune.
 type RuneScanner interface {
 	RuneReader
 	UnreadRune() os.Error
 }

 // stringWriter is the interface that wraps the WriteString method.
 type stringWriter interface {
 	WriteString(s string) (n int, err os.Error)
 }

 // WriteString writes the contents of the string s to w, which accepts an array of bytes.
 func WriteString(w Writer, s string) (n int, err os.Error) {
 	if sw, ok := w.(stringWriter); ok {
 		return sw.WriteString(s)
 	}
 	return w.Write([]byte(s))
 }

 // ReadAtLeast reads from r into buf until it has read at least min bytes.
 // It returns the number of bytes copied and an error if fewer bytes were read.
 // The error is os.EOF only if no bytes were read.
 // If an EOF happens after reading fewer than min bytes,
 // ReadAtLeast returns ErrUnexpectedEOF.
 // If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer.
 func ReadAtLeast(r Reader, buf []byte, min int) (n int, err os.Error) {
 	if len(buf) < min {
 		return 0, ErrShortBuffer
 	}
 	for n < min && err == nil {
 		var nn int
 		nn, err = r.Read(buf[n:])
 		n += nn
 	}
 	if err == os.EOF {
 		if n >= min {
 			err = nil
 		} else if n > 0 {
 			err = ErrUnexpectedEOF
 		}
 	}
 	return
 }

 // ReadFull reads exactly len(buf) bytes from r into buf.
 // It returns the number of bytes copied and an error if fewer bytes were read.
 // The error is os.EOF only if no bytes were read.
 // If an EOF happens after reading some but not all the bytes,
 // ReadFull returns ErrUnexpectedEOF.
 func ReadFull(r Reader, buf []byte) (n int, err os.Error) {
 	return ReadAtLeast(r, buf, len(buf))
 }

 // Copyn copies n bytes (or until an error) from src to dst.
 // It returns the number of bytes copied and the earliest
 // error encountered while copying.  Because Read can
 // return the full amount requested as well as an error
 // (including os.EOF), so can Copyn.
 //
 // If dst implements the ReaderFrom interface,
 // the copy is implemented by calling dst.ReadFrom(src).
 func Copyn(dst Writer, src Reader, n int64) (written int64, err os.Error) {
 	// If the writer has a ReadFrom method, use it to do the copy.
 	// Avoids a buffer allocation and a copy.
 	if rt, ok := dst.(ReaderFrom); ok {
 		written, err = rt.ReadFrom(LimitReader(src, n))
 		if written < n && err == nil {
 			// rt stopped early; must have been EOF.
 			err = os.EOF
 		}
 		return
 	}
 	buf := make([]byte, 32*1024)
 	for written < n {
 		l := len(buf)
 		if d := n - written; d < int64(l) {
 			l = int(d)
 		}
 		nr, er := src.Read(buf[0:l])
 		if nr > 0 {
 			nw, ew := dst.Write(buf[0:nr])
 			if nw > 0 {
 				written += int64(nw)
 			}
 			if ew != nil {
 				err = ew
 				break
 			}
 			if nr != nw {
 				err = ErrShortWrite
 				break
 			}
 		}
 		if er != nil {
 			err = er
 			break
 		}
 	}
 	return written, err
 }

 // Copy copies from src to dst until either EOF is reached
 // on src or an error occurs.  It returns the number of bytes
 // copied and the first error encountered while copying, if any.
 //
 // A successful Copy returns err == nil, not err == os.EOF.
 // Because Copy is defined to read from src until EOF, it does
 // not treat an EOF from Read as an error to be reported.
 //
 // If dst implements the ReaderFrom interface,
 // the copy is implemented by calling dst.ReadFrom(src).
 // Otherwise, if src implements the WriterTo interface,
 // the copy is implemented by calling src.WriteTo(dst).
 func Copy(dst Writer, src Reader) (written int64, err os.Error) {
 	// If the writer has a ReadFrom method, use it to do the copy.
 	// Avoids an allocation and a copy.
 	if rt, ok := dst.(ReaderFrom); ok {
 		return rt.ReadFrom(src)
 	}
 	// Similarly, if the reader has a WriteTo method, use it to do the copy.
 	if wt, ok := src.(WriterTo); ok {
 		return wt.WriteTo(dst)
 	}
 	buf := make([]byte, 32*1024)
 	for {
 		nr, er := src.Read(buf)
 		if nr > 0 {
 			nw, ew := dst.Write(buf[0:nr])
 			if nw > 0 {
 				written += int64(nw)
 			}
 			if ew != nil {
 				err = ew
 				break
 			}
 			if nr != nw {
 				err = ErrShortWrite
 				break
 			}
 		}
 		if er == os.EOF {
 			break
 		}
 		if er != nil {
 			err = er
 			break
 		}
 	}
 	return written, err
 }

 // LimitReader returns a Reader that reads from r
 // but stops with os.EOF after n bytes.
 // The underlying implementation is a *LimitedReader.
 func LimitReader(r Reader, n int64) Reader { return &LimitedReader{r, n} }

 // A LimitedReader reads from R but limits the amount of
 // data returned to just N bytes. Each call to Read
 // updates N to reflect the new amount remaining.
 type LimitedReader struct {
 	R Reader // underlying reader
 	N int64  // max bytes remaining
 }

 func (l *LimitedReader) Read(p []byte) (n int, err os.Error) {
 	if l.N <= 0 {
 		return 0, os.EOF
 	}
 	if int64(len(p)) > l.N {
 		p = p[0:l.N]
 	}
 	n, err = l.R.Read(p)
 	l.N -= int64(n)
 	return
 }

 // NewSectionReader returns a SectionReader that reads from r
 // starting at offset off and stops with os.EOF after n bytes.
 func NewSectionReader(r ReaderAt, off int64, n int64) *SectionReader {
 	return &SectionReader{r, off, off, off + n}
 }

 // SectionReader implements Read, Seek, and ReadAt on a section
 // of an underlying ReaderAt.
 type SectionReader struct {
 	r     ReaderAt
 	base  int64
 	off   int64
 	limit int64
 }

 func (s *SectionReader) Read(p []byte) (n int, err os.Error) {
 	if s.off >= s.limit {
 		return 0, os.EOF
 	}
 	if max := s.limit - s.off; int64(len(p)) > max {
 		p = p[0:max]
 	}
 	n, err = s.r.ReadAt(p, s.off)
 	s.off += int64(n)
 	return
 }

 func (s *SectionReader) Seek(offset int64, whence int) (ret int64, err os.Error) {
 	switch whence {
 	default:
 		return 0, os.EINVAL
 	case 0:
 		offset += s.base
 	case 1:
 		offset += s.off
 	case 2:
 		offset += s.limit
 	}
 	if offset < s.base || offset > s.limit {
 		return 0, os.EINVAL
 	}
 	s.off = offset
 	return offset - s.base, nil
 }

 func (s *SectionReader) ReadAt(p []byte, off int64) (n int, err os.Error) {
 	if off < 0 || off >= s.limit-s.base {
 		return 0, os.EOF
 	}
 	off += s.base
 	if max := s.limit - off; int64(len(p)) > max {
 		p = p[0:max]
 	}
 	return s.r.ReadAt(p, off)
 }

 // Size returns the size of the section in bytes.
 func (s *SectionReader) Size() int64 { return s.limit - s.base }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Package io provides basic interfaces to I/O primitives.
	// Its primary job is to wrap existing implementations of such primitives,
	// such as those in package os, into shared public interfaces that
	// abstract the functionality, plus some other related primitives.
	package io

	import "os"

	// Error represents an unexpected I/O behavior.
	type Error struct {
	ErrorString string
	}

	func (err *Error) String() string { return err.ErrorString }

	// ErrShortWrite means that a write accepted fewer bytes than requested
	// but failed to return an explicit error.
	var ErrShortWrite os.Error = &Error{"short write"}

	// ErrShortBuffer means that a read required a longer buffer than was provided.
	var ErrShortBuffer os.Error = &Error{"short buffer"}

	// ErrUnexpectedEOF means that os.EOF was encountered in the
	// middle of reading a fixed-size block or data structure.
	var ErrUnexpectedEOF os.Error = &Error{"unexpected EOF"}

	// Reader is the interface that wraps the basic Read method.
	//
	// Read reads up to len(p) bytes into p. It returns the number of bytes
	// read (0 <= n <= len(p)) and any error encountered. Even if Read
	// returns n < len(p), it may use all of p as scratch space during the call.
	// If some data is available but not len(p) bytes, Read conventionally
	// returns what is available instead of waiting for more.
	//
	// When Read encounters an error or end-of-file condition after
	// successfully reading n > 0 bytes, it returns the number of
	// bytes read. It may return the (non-nil) error from the same call
	// or return the error (and n == 0) from a subsequent call.
	// An instance of this general case is that a Reader returning
	// a non-zero number of bytes at the end of the input stream may
	// return either err == os.EOF or err == nil. The next Read should
	// return 0, os.EOF regardless.
	//
	// Callers should always process the n > 0 bytes returned before
	// considering the error err. Doing so correctly handles I/O errors
	// that happen after reading some bytes and also both of the
	// allowed EOF behaviors.
	type Reader interface {
	Read(p []byte) (n int, err os.Error)
	}

	// Writer is the interface that wraps the basic Write method.
	//
	// Write writes len(p) bytes from p to the underlying data stream.
	// It returns the number of bytes written from p (0 <= n <= len(p))
	// and any error encountered that caused the write to stop early.
	// Write must return a non-nil error if it returns n < len(p).
	type Writer interface {
	Write(p []byte) (n int, err os.Error)
	}

	// Closer is the interface that wraps the basic Close method.
	type Closer interface {
	Close() os.Error
	}

	// Seeker is the interface that wraps the basic Seek method.
	//
	// Seek sets the offset for the next Read or Write to offset,
	// interpreted according to whence: 0 means relative to the origin of
	// the file, 1 means relative to the current offset, and 2 means
	// relative to the end. Seek returns the new offset and an Error, if
	// any.
	type Seeker interface {
	Seek(offset int64, whence int) (ret int64, err os.Error)
	}

	// ReadWriter is the interface that groups the basic Read and Write methods.
	type ReadWriter interface {
	Reader
	Writer
	}

	// ReadCloser is the interface that groups the basic Read and Close methods.
	type ReadCloser interface {
	Reader
	Closer
	}

	// WriteCloser is the interface that groups the basic Write and Close methods.
	type WriteCloser interface {
	Writer
	Closer
	}

	// ReadWriteCloser is the interface that groups the basic Read, Write and Close methods.
	type ReadWriteCloser interface {
	Reader
	Writer
	Closer
	}

	// ReadSeeker is the interface that groups the basic Read and Seek methods.
	type ReadSeeker interface {
	Reader
	Seeker
	}

	// WriteSeeker is the interface that groups the basic Write and Seek methods.
	type WriteSeeker interface {
	Writer
	Seeker
	}

	// ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods.
	type ReadWriteSeeker interface {
	Reader
	Writer
	Seeker
	}

	// ReaderFrom is the interface that wraps the ReadFrom method.
	type ReaderFrom interface {
	ReadFrom(r Reader) (n int64, err os.Error)
	}

	// WriterTo is the interface that wraps the WriteTo method.
	type WriterTo interface {
	WriteTo(w Writer) (n int64, err os.Error)
	}

	// ReaderAt is the interface that wraps the basic ReadAt method.
	//
	// ReadAt reads len(p) bytes into p starting at offset off in the
	// underlying input source. It returns the number of bytes
	// read (0 <= n <= len(p)) and any error encountered.
	//
	// When ReadAt returns n < len(p), it returns a non-nil error
	// explaining why more bytes were not returned. In this respect,
	// ReadAt is stricter than Read.
	//
	// Even if ReadAt returns n < len(p), it may use all of p as scratch
	// space during the call. If some data is available but not len(p) bytes,
	// ReadAt blocks until either all the data is available or an error occurs.
	// In this respect ReadAt is different from Read.
	//
	// If the n = len(p) bytes returned by ReadAt are at the end of the
	// input source, ReadAt may return either err == os.EOF or err == nil.
	//
	// If ReadAt is reading from an input source with a seek offset,
	// ReadAt should not affect nor be affected by the underlying
	// seek offset.
	type ReaderAt interface {
	ReadAt(p []byte, off int64) (n int, err os.Error)
	}

	// WriterAt is the interface that wraps the basic WriteAt method.
	//
	// WriteAt writes len(p) bytes from p to the underlying data stream
	// at offset off. It returns the number of bytes written from p (0 <= n <= len(p))
	// and any error encountered that caused the write to stop early.
	// WriteAt must return a non-nil error if it returns n < len(p).
	type WriterAt interface {
	WriteAt(p []byte, off int64) (n int, err os.Error)
	}

	// ByteReader is the interface that wraps the ReadByte method.
	//
	// ReadByte reads and returns the next byte from the input.
	// If no byte is available, err will be set.
	type ByteReader interface {
	ReadByte() (c byte, err os.Error)
	}

	// ByteScanner is the interface that adds the UnreadByte method to the
	// basic ReadByte method.
	//
	// UnreadByte causes the next call to ReadByte to return the same byte
	// as the previous call to ReadByte.
	// It may be an error to call UnreadByte twice without an intervening
	// call to ReadByte.
	type ByteScanner interface {
	ByteReader
	UnreadByte() os.Error
	}

	// RuneReader is the interface that wraps the ReadRune method.
	//
	// ReadRune reads a single UTF-8 encoded Unicode character
	// and returns the rune and its size in bytes. If no character is
	// available, err will be set.
	type RuneReader interface {
	ReadRune() (rune int, size int, err os.Error)
	}

	// RuneScanner is the interface that adds the UnreadRune method to the
	// basic ReadRune method.
	//
	// UnreadRune causes the next call to ReadRune to return the same rune
	// as the previous call to ReadRune.
	// It may be an error to call UnreadRune twice without an intervening
	// call to ReadRune.
	type RuneScanner interface {
	RuneReader
	UnreadRune() os.Error
	}

	// stringWriter is the interface that wraps the WriteString method.
	type stringWriter interface {
	WriteString(s string) (n int, err os.Error)
	}

	// WriteString writes the contents of the string s to w, which accepts an array of bytes.
	func WriteString(w Writer, s string) (n int, err os.Error) {
	if sw, ok := w.(stringWriter); ok {
	return sw.WriteString(s)
	}
	return w.Write([]byte(s))
	}

	// ReadAtLeast reads from r into buf until it has read at least min bytes.
	// It returns the number of bytes copied and an error if fewer bytes were read.
	// The error is os.EOF only if no bytes were read.
	// If an EOF happens after reading fewer than min bytes,
	// ReadAtLeast returns ErrUnexpectedEOF.
	// If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer.
	func ReadAtLeast(r Reader, buf []byte, min int) (n int, err os.Error) {
	if len(buf) < min {
	return 0, ErrShortBuffer
	}
	for n < min && err == nil {
	var nn int
	nn, err = r.Read(buf[n:])
	n += nn
	}
	if err == os.EOF {
	if n >= min {
	err = nil
	} else if n > 0 {
	err = ErrUnexpectedEOF
	}
	}
	return
	}

	// ReadFull reads exactly len(buf) bytes from r into buf.
	// It returns the number of bytes copied and an error if fewer bytes were read.
	// The error is os.EOF only if no bytes were read.
	// If an EOF happens after reading some but not all the bytes,
	// ReadFull returns ErrUnexpectedEOF.
	func ReadFull(r Reader, buf []byte) (n int, err os.Error) {
	return ReadAtLeast(r, buf, len(buf))
	}

	// Copyn copies n bytes (or until an error) from src to dst.
	// It returns the number of bytes copied and the earliest
	// error encountered while copying. Because Read can
	// return the full amount requested as well as an error
	// (including os.EOF), so can Copyn.
	//
	// If dst implements the ReaderFrom interface,
	// the copy is implemented by calling dst.ReadFrom(src).
	func Copyn(dst Writer, src Reader, n int64) (written int64, err os.Error) {
	// If the writer has a ReadFrom method, use it to do the copy.
	// Avoids a buffer allocation and a copy.
	if rt, ok := dst.(ReaderFrom); ok {
	written, err = rt.ReadFrom(LimitReader(src, n))
	if written < n && err == nil {
	// rt stopped early; must have been EOF.
	err = os.EOF
	}
	return
	}
	buf := make([]byte, 32*1024)
	for written < n {
	l := len(buf)
	if d := n - written; d < int64(l) {
	l = int(d)
	}
	nr, er := src.Read(buf[0:l])
	if nr > 0 {
	nw, ew := dst.Write(buf[0:nr])
	if nw > 0 {
	written += int64(nw)
	}
	if ew != nil {
	err = ew
	break
	}
	if nr != nw {
	err = ErrShortWrite
	break
	}
	}
	if er != nil {
	err = er
	break
	}
	}
	return written, err
	}

	// Copy copies from src to dst until either EOF is reached
	// on src or an error occurs. It returns the number of bytes
	// copied and the first error encountered while copying, if any.
	//
	// A successful Copy returns err == nil, not err == os.EOF.
	// Because Copy is defined to read from src until EOF, it does
	// not treat an EOF from Read as an error to be reported.
	//
	// If dst implements the ReaderFrom interface,
	// the copy is implemented by calling dst.ReadFrom(src).
	// Otherwise, if src implements the WriterTo interface,
	// the copy is implemented by calling src.WriteTo(dst).
	func Copy(dst Writer, src Reader) (written int64, err os.Error) {
	// If the writer has a ReadFrom method, use it to do the copy.
	// Avoids an allocation and a copy.
	if rt, ok := dst.(ReaderFrom); ok {
	return rt.ReadFrom(src)
	}
	// Similarly, if the reader has a WriteTo method, use it to do the copy.
	if wt, ok := src.(WriterTo); ok {
	return wt.WriteTo(dst)
	}
	buf := make([]byte, 32*1024)
	for {
	nr, er := src.Read(buf)
	if nr > 0 {
	nw, ew := dst.Write(buf[0:nr])
	if nw > 0 {
	written += int64(nw)
	}
	if ew != nil {
	err = ew
	break
	}
	if nr != nw {
	err = ErrShortWrite
	break
	}
	}
	if er == os.EOF {
	break
	}
	if er != nil {
	err = er
	break
	}
	}
	return written, err
	}

	// LimitReader returns a Reader that reads from r
	// but stops with os.EOF after n bytes.
	// The underlying implementation is a *LimitedReader.
	func LimitReader(r Reader, n int64) Reader { return &LimitedReader{r, n} }

	// A LimitedReader reads from R but limits the amount of
	// data returned to just N bytes. Each call to Read
	// updates N to reflect the new amount remaining.
	type LimitedReader struct {
	R Reader // underlying reader
	N int64 // max bytes remaining
	}

	func (l *LimitedReader) Read(p []byte) (n int, err os.Error) {
	if l.N <= 0 {
	return 0, os.EOF
	}
	if int64(len(p)) > l.N {
	p = p[0:l.N]
	}
	n, err = l.R.Read(p)
	l.N -= int64(n)
	return
	}

	// NewSectionReader returns a SectionReader that reads from r
	// starting at offset off and stops with os.EOF after n bytes.
	func NewSectionReader(r ReaderAt, off int64, n int64) *SectionReader {
	return &SectionReader{r, off, off, off + n}
	}

	// SectionReader implements Read, Seek, and ReadAt on a section
	// of an underlying ReaderAt.
	type SectionReader struct {
	r ReaderAt
	base int64
	off int64
	limit int64
	}

	func (s *SectionReader) Read(p []byte) (n int, err os.Error) {
	if s.off >= s.limit {
	return 0, os.EOF
	}
	if max := s.limit - s.off; int64(len(p)) > max {
	p = p[0:max]
	}
	n, err = s.r.ReadAt(p, s.off)
	s.off += int64(n)
	return
	}

	func (s *SectionReader) Seek(offset int64, whence int) (ret int64, err os.Error) {
	switch whence {
	default:
	return 0, os.EINVAL
	case 0:
	offset += s.base
	case 1:
	offset += s.off
	case 2:
	offset += s.limit
	}
	if offset < s.base \|\| offset > s.limit {
	return 0, os.EINVAL
	}
	s.off = offset
	return offset - s.base, nil
	}

	func (s *SectionReader) ReadAt(p []byte, off int64) (n int, err os.Error) {
	if off < 0 \|\| off >= s.limit-s.base {
	return 0, os.EOF
	}
	off += s.base
	if max := s.limit - off; int64(len(p)) > max {
	p = p[0:max]
	}
	return s.r.ReadAt(p, off)
	}

	// Size returns the size of the section in bytes.
	func (s *SectionReader) Size() int64 { return s.limit - s.base }