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// 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.
// 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) ([]byte, os.Error) {
buf := bytes.NewBuffer(make([]byte, 0, capacity))
_, err := buf.ReadFrom(r)
return buf.Bytes(), err
// ReadAll reads from r until an error or EOF and returns the data it read.
func ReadAll(r io.Reader) ([]byte, os.Error) {
return readAll(r, bytes.MinRead)
// ReadFile reads the file named by filename and returns the contents.
func ReadFile(filename string) ([]byte, os.Error) {
f, err := os.Open(filename, os.O_RDONLY, 0)
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.
fi, err := f.Stat()
var n int64
if err == nil && fi.Size < 2e9 { // Don't preallocate a huge buffer, just in case.
n = fi.Size
// As initial capacity for readAll, use n + 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.
return readAll(f, n+bytes.MinRead)
// 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 uint32) os.Error {
f, err := os.Open(filename, os.O_WRONLY|os.O_CREAT|os.O_TRUNC, perm)
if err != nil {
return err
n, err := f.Write(data)
if err == nil && n < len(data) {
err = io.ErrShortWrite
return err
// A dirList implements sort.Interface.
type fileInfoList []*os.FileInfo
func (f fileInfoList) Len() int { return len(f) }
func (f fileInfoList) Less(i, j int) bool { return f[i].Name < f[j].Name }
func (f fileInfoList) Swap(i, j int) { f[i], f[j] = f[j], f[i] }
// ReadDir reads the directory named by dirname and returns
// a list of sorted directory entries.
func ReadDir(dirname string) ([]*os.FileInfo, os.Error) {
f, err := os.Open(dirname, os.O_RDONLY, 0)
if err != nil {
return nil, err
list, err := f.Readdir(-1)
if err != nil {
return nil, err
fi := make(fileInfoList, len(list))
for i := range list {
fi[i] = &list[i]
return fi, nil
type nopCloser struct {
func (nopCloser) Close() os.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}