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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.
package tar
// TODO(dsymonds):
// - pax extensions
import (
"bytes"
"errors"
"io"
"io/ioutil"
"os"
"strconv"
"time"
)
var (
HeaderError = errors.New("invalid tar header")
)
// A Reader provides sequential access to the contents of a tar archive.
// A tar archive consists of a sequence of files.
// The Next method advances to the next file in the archive (including the first),
// and then it can be treated as an io.Reader to access the file's data.
//
// Example:
// tr := tar.NewReader(r)
// for {
// hdr, err := tr.Next()
// if err == io.EOF {
// // end of tar archive
// break
// }
// if err != nil {
// // handle error
// }
// io.Copy(data, tr)
// }
type Reader struct {
r io.Reader
err error
nb int64 // number of unread bytes for current file entry
pad int64 // amount of padding (ignored) after current file entry
}
// NewReader creates a new Reader reading from r.
func NewReader(r io.Reader) *Reader { return &Reader{r: r} }
// Next advances to the next entry in the tar archive.
func (tr *Reader) Next() (*Header, error) {
var hdr *Header
if tr.err == nil {
tr.skipUnread()
}
if tr.err == nil {
hdr = tr.readHeader()
}
return hdr, tr.err
}
// Parse bytes as a NUL-terminated C-style string.
// If a NUL byte is not found then the whole slice is returned as a string.
func cString(b []byte) string {
n := 0
for n < len(b) && b[n] != 0 {
n++
}
return string(b[0:n])
}
func (tr *Reader) octal(b []byte) int64 {
// Removing leading spaces.
for len(b) > 0 && b[0] == ' ' {
b = b[1:]
}
// Removing trailing NULs and spaces.
for len(b) > 0 && (b[len(b)-1] == ' ' || b[len(b)-1] == '\x00') {
b = b[0 : len(b)-1]
}
x, err := strconv.ParseUint(cString(b), 8, 64)
if err != nil {
tr.err = err
}
return int64(x)
}
// Skip any unread bytes in the existing file entry, as well as any alignment padding.
func (tr *Reader) skipUnread() {
nr := tr.nb + tr.pad // number of bytes to skip
tr.nb, tr.pad = 0, 0
if sr, ok := tr.r.(io.Seeker); ok {
if _, err := sr.Seek(nr, os.SEEK_CUR); err == nil {
return
}
}
_, tr.err = io.CopyN(ioutil.Discard, tr.r, nr)
}
func (tr *Reader) verifyChecksum(header []byte) bool {
if tr.err != nil {
return false
}
given := tr.octal(header[148:156])
unsigned, signed := checksum(header)
return given == unsigned || given == signed
}
func (tr *Reader) readHeader() *Header {
header := make([]byte, blockSize)
if _, tr.err = io.ReadFull(tr.r, header); tr.err != nil {
return nil
}
// Two blocks of zero bytes marks the end of the archive.
if bytes.Equal(header, zeroBlock[0:blockSize]) {
if _, tr.err = io.ReadFull(tr.r, header); tr.err != nil {
return nil
}
if bytes.Equal(header, zeroBlock[0:blockSize]) {
tr.err = io.EOF
} else {
tr.err = HeaderError // zero block and then non-zero block
}
return nil
}
if !tr.verifyChecksum(header) {
tr.err = HeaderError
return nil
}
// Unpack
hdr := new(Header)
s := slicer(header)
hdr.Name = cString(s.next(100))
hdr.Mode = tr.octal(s.next(8))
hdr.Uid = int(tr.octal(s.next(8)))
hdr.Gid = int(tr.octal(s.next(8)))
hdr.Size = tr.octal(s.next(12))
hdr.ModTime = time.Unix(tr.octal(s.next(12)), 0)
s.next(8) // chksum
hdr.Typeflag = s.next(1)[0]
hdr.Linkname = cString(s.next(100))
// The remainder of the header depends on the value of magic.
// The original (v7) version of tar had no explicit magic field,
// so its magic bytes, like the rest of the block, are NULs.
magic := string(s.next(8)) // contains version field as well.
var format string
switch magic {
case "ustar\x0000": // POSIX tar (1003.1-1988)
if string(header[508:512]) == "tar\x00" {
format = "star"
} else {
format = "posix"
}
case "ustar \x00": // old GNU tar
format = "gnu"
}
switch format {
case "posix", "gnu", "star":
hdr.Uname = cString(s.next(32))
hdr.Gname = cString(s.next(32))
devmajor := s.next(8)
devminor := s.next(8)
if hdr.Typeflag == TypeChar || hdr.Typeflag == TypeBlock {
hdr.Devmajor = tr.octal(devmajor)
hdr.Devminor = tr.octal(devminor)
}
var prefix string
switch format {
case "posix", "gnu":
prefix = cString(s.next(155))
case "star":
prefix = cString(s.next(131))
hdr.AccessTime = time.Unix(tr.octal(s.next(12)), 0)
hdr.ChangeTime = time.Unix(tr.octal(s.next(12)), 0)
}
if len(prefix) > 0 {
hdr.Name = prefix + "/" + hdr.Name
}
}
if tr.err != nil {
tr.err = HeaderError
return nil
}
// Maximum value of hdr.Size is 64 GB (12 octal digits),
// so there's no risk of int64 overflowing.
tr.nb = int64(hdr.Size)
tr.pad = -tr.nb & (blockSize - 1) // blockSize is a power of two
return hdr
}
// Read reads from the current entry in the tar archive.
// It returns 0, io.EOF when it reaches the end of that entry,
// until Next is called to advance to the next entry.
func (tr *Reader) Read(b []byte) (n int, err error) {
if tr.nb == 0 {
// file consumed
return 0, io.EOF
}
if int64(len(b)) > tr.nb {
b = b[0:tr.nb]
}
n, err = tr.r.Read(b)
tr.nb -= int64(n)
if err == io.EOF && tr.nb > 0 {
err = io.ErrUnexpectedEOF
}
tr.err = err
return
}
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