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Memory-efficient, streaming implementation of the tar file format

tar #

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This package provides stream-based readers and writers for tar files.

When working with large tar files, this library consumes considerably less memory than package:archive, although it is slightly slower due to the async overhead.

Reading #

To read entries from a tar file, use a TarReader with a Stream emitting bytes (as List<int>):

import 'dart:convert';
import 'dart:io';
import 'package:tar/tar.dart';

Future<void> main() async {
  final reader = TarReader(File('file.tar').openRead());

  while (await reader.moveNext()) {
    final entry = reader.current;
    // Use reader.header to see the header of the current tar entry
    // And reader.contents to read the content of the current entry as a stream
    print(await entry.contents.transform(utf8.decoder).first);
  // Note that the reader will automatically close if moveNext() returns false or
  // throws. If you want to close a tar stream before that happens, use
  // reader.cancel();

To read .tar.gz files, transform the stream with gzip.decoder before passing it to the TarReader.

To easily go through all entries in a tar file, use TarReader.forEach:

Future<void> main() async {
  final inputStream = File('file.tar').openRead();

  await TarReader.forEach(inputStream, (entry) {
    print(await entry.contents.transform(utf8.decoder).first);

Warning: Since the reader is backed by a single stream, concurrent calls to read are not allowed! Similarly, if you're reading from an entry's contents, make sure to fully drain the stream before calling read() again. Not subscribing to contents before calling moveNext() is acceptable too. In this case, the reader will implicitly drain the stream. The reader detects concurrency misuses and will throw an error when they occur, there's no risk of reading faulty data.

Writing #

When writing archives, package:tar expects a Stream of tar entries to include in the archive. This stream can then be converted into a stream of byte-array chunks forming the encoded tar archive.

To write a tar stream into a StreamSink<List<int>>, such as an IOSink returned by File.openWrite, use tarWritingSink:

import 'dart:convert';
import 'dart:io';
import 'package:tar/tar.dart';

Future<void> main() async {
  final output = File('test.tar').openWrite();
  final tarEntries = Stream<TarEntry>.value(
        name: 'hello.txt',
        mode: int.parse('644', radix: 8),
      utf8.encode('Hello world'),

  await tarEntries.pipe(tarWritingSink(output));

For more complex stream transformations, tarWriter can be used as a stream transformer converting a stream of tar entries into archive bytes.

Together with the gzip.encoder transformer from dart:io, this can be used to write a .tar.gz file:

import 'dart:io';
import 'package:tar/tar.dart';

Future<void> write(Stream<TarEntry> entries) {
  return entries
      .transform(tarWriter) // convert entries into a .tar stream
      .transform(gzip.encoder) // convert the .tar stream into a .tar.gz stream

A more complex example for writing files can be found in example/archive_self.dart.

Encoding options #

By default, tar files are written in the pax format defined by the POSIX.1-2001 specification (--format=posix in GNU tar). When all entries have file names shorter than 100 chars and a size smaller than 8 GB, this is equivalent to the ustar format. This library won't write PAX headers when there is no reason to do so. If you prefer writing GNU-style long filenames instead, you can use the format option:

Future<void> write(Stream<TarEntry> entries) {
  return entries
          format: OutputFormat.gnuLongName,

To change the output format on the tarWriter transformer, use tarWriterWith.

Synchronous writing #

As the content of tar entries is defined as an asynchronous stream, the tar encoder is asynchronous too. The more specific SynchronousTarEntry class stores tar content as a list of bytes, meaning that it can be written synchronously.

To synchronously write tar files, use tarConverter (or tarConverterWith for options):

List<int> createTarArchive(Iterable<SynchronousTarEntry> entries) {
  late List<int> result;
  final sink = ByteConversionSink.withCallback((data) => result = data);

  final output = tarConverter.startChunkedConversion(sink);

  return result;

Features #

  • Supports v7, ustar, pax, gnu and star archives
  • Supports extended pax headers for long file or link names
  • Supports long file and link names generated by GNU-tar
  • Hardened against denial-of-service attacks with invalid tar files
  • Supports being compiled to JavaScript, tested on Node.js

Security considerations #

Internally, this package contains checks to guard against some invalid tar files. In particular,

  • The reader doesn't allocate memory based on values in a tar file (so there's a guard against DoS attacks with tar files containing huge headers).
  • When encountering malformed tar files, the reader will throw a TarException. Any other exception thrown indicates a bug in package:tar or how it's used. The reader should never crash.
  • Reading a tar file can be cancelled mid-stream without leaking resources.

However, the tar reader does not throw exceptions for well-formed archives with suspicious contents, such as

  • File names beginning with ../, / or names pointing out of the archive by other means.
  • Link references to files outside of the archive.
  • Paths not using forward slashes.
  • Gzip + tar bombs.
  • Invalid permission bits in entries.
  • ...

When reading or extracting untrusted tar files, it is your responsibility to detect and handle these cases. For instance, this naive extraction function is susceptible to invalid tar files containing paths outside of the target directory:

Future<void> extractTarGz(File tarGz, Directory target) async {
  final input = tarGz.openRead().transform(gzip.decoder);

  await TarReader.forEach(input, (entry) async {
    final destination =
        // DON'T DO THIS! If `entry.name` contained `../`, this may escape the
        // target directory.
        path.joinAll([target.path, ...path.posix.split(entry.name)]);

    final f = File(destination);
    await f.create(recursive: true);
    await entry.contents.pipe(f.openWrite());

For an idea on how to guard against this, see the extraction logic used by the pub client.

Big thanks to Garett Tok Ern Liang for writing the initial Dart tar reader that this library is based on.

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Memory-efficient, streaming implementation of the tar file format

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