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Source file src/io/ioutil/ioutil.go

Documentation: io/ioutil

  // 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 ioutil implements some I/O utility functions.
  package ioutil
  import (
  // readAll reads from r until an error or EOF and returns the data it read
  // from the internal buffer allocated with a specified capacity.
  func readAll(r io.Reader, capacity int64) (b []byte, err error) {
  	var buf bytes.Buffer
  	// If the buffer overflows, we will get bytes.ErrTooLarge.
  	// Return that as an error. Any other panic remains.
  	defer func() {
  		e := recover()
  		if e == nil {
  		if panicErr, ok := e.(error); ok && panicErr == bytes.ErrTooLarge {
  			err = panicErr
  		} else {
  	if int64(int(capacity)) == capacity {
  	_, err = buf.ReadFrom(r)
  	return buf.Bytes(), err
  // ReadAll reads from r until an error or EOF and returns the data it read.
  // A successful call returns err == nil, not err == EOF. Because ReadAll is
  // defined to read from src until EOF, it does not treat an EOF from Read
  // as an error to be reported.
  func ReadAll(r io.Reader) ([]byte, error) {
  	return readAll(r, bytes.MinRead)
  // ReadFile reads the file named by filename and returns the contents.
  // A successful call returns err == nil, not err == EOF. Because ReadFile
  // reads the whole file, it does not treat an EOF from Read as an error
  // to be reported.
  func ReadFile(filename string) ([]byte, error) {
  	f, err := os.Open(filename)
  	if err != nil {
  		return nil, err
  	defer f.Close()
  	// It's a good but not certain bet that FileInfo will tell us exactly how much to
  	// read, so let's try it but be prepared for the answer to be wrong.
  	var n int64 = bytes.MinRead
  	if fi, err := f.Stat(); err == nil {
  		// As initial capacity for readAll, use Size + a little extra in case Size
  		// is zero, and to avoid another allocation after Read has filled the
  		// buffer. The readAll call will read into its allocated internal buffer
  		// cheaply. If the size was wrong, we'll either waste some space off the end
  		// or reallocate as needed, but in the overwhelmingly common case we'll get
  		// it just right.
  		if size := fi.Size() + bytes.MinRead; size > n {
  			n = size
  	return readAll(f, n)
  // WriteFile writes data to a file named by filename.
  // If the file does not exist, WriteFile creates it with permissions perm;
  // otherwise WriteFile truncates it before writing.
  func WriteFile(filename string, data []byte, perm os.FileMode) error {
  	f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
  	if err != nil {
  		return err
  	n, err := f.Write(data)
  	if err == nil && n < len(data) {
  		err = io.ErrShortWrite
  	if err1 := f.Close(); err == nil {
  		err = err1
  	return err
  // ReadDir reads the directory named by dirname and returns
  // a list of directory entries sorted by filename.
  func ReadDir(dirname string) ([]os.FileInfo, error) {
  	f, err := os.Open(dirname)
  	if err != nil {
  		return nil, err
  	list, err := f.Readdir(-1)
  	if err != nil {
  		return nil, err
  	sort.Slice(list, func(i, j int) bool { return list[i].Name() < list[j].Name() })
  	return list, nil
  type nopCloser struct {
  func (nopCloser) Close() error { return nil }
  // NopCloser returns a ReadCloser with a no-op Close method wrapping
  // the provided Reader r.
  func NopCloser(r io.Reader) io.ReadCloser {
  	return nopCloser{r}
  type devNull int
  // devNull implements ReaderFrom as an optimization so io.Copy to
  // ioutil.Discard can avoid doing unnecessary work.
  var _ io.ReaderFrom = devNull(0)
  func (devNull) Write(p []byte) (int, error) {
  	return len(p), nil
  func (devNull) WriteString(s string) (int, error) {
  	return len(s), nil
  var blackHolePool = sync.Pool{
  	New: func() interface{} {
  		b := make([]byte, 8192)
  		return &b
  func (devNull) ReadFrom(r io.Reader) (n int64, err error) {
  	bufp := blackHolePool.Get().(*[]byte)
  	readSize := 0
  	for {
  		readSize, err = r.Read(*bufp)
  		n += int64(readSize)
  		if err != nil {
  			if err == io.EOF {
  				return n, nil
  // Discard is an io.Writer on which all Write calls succeed
  // without doing anything.
  var Discard io.Writer = devNull(0)

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