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Welcome to libarchive!

The libarchive project develops a portable, efficient C library that can read and write streaming archives in a variety of formats. It also includes implementations of the common tar, cpio, and zcat command-line tools that use the libarchive library.

Questions? Issues?

Contents of the Distribution

This distribution bundle includes the following major components:

  • libarchive: a library for reading and writing streaming archives
  • tar: the 'bsdtar' program is a full-featured 'tar' implementation built on libarchive
  • cpio: the 'bsdcpio' program is a different interface to essentially the same functionality
  • cat: the 'bsdcat' program is a simple replacement tool for zcat, bzcat, xzcat, and such
  • unzip: the 'bsdunzip' program is a simple replacement tool for Info-ZIP's unzip
  • examples: Some small example programs that you may find useful.
  • examples/minitar: a compact sample demonstrating use of libarchive.
  • contrib: Various items sent to me by third parties; please contact the authors with any questions.

The top-level directory contains the following information files:

  • NEWS - highlights of recent changes
  • COPYING - what you can do with this
  • INSTALL - installation instructions
  • README - this file
  • CMakeLists.txt - input for "cmake" build tool, see INSTALL
  • configure - configuration script, see INSTALL for details. If your copy of the source lacks a configure script, you can try to construct it by running the script in build/autogen.sh (or use cmake).

The following files in the top-level directory are used by the 'configure' script:

  • Makefile.am, aclocal.m4, configure.ac - used to build this distribution, only needed by maintainers
  • Makefile.in, config.h.in - templates used by configure script

Documentation

In addition to the informational articles and documentation in the online libarchive Wiki, the distribution also includes a number of manual pages:

  • bsdtar.1 explains the use of the bsdtar program
  • bsdcpio.1 explains the use of the bsdcpio program
  • bsdcat.1 explains the use of the bsdcat program
  • libarchive.3 gives an overview of the library as a whole
  • archive_read.3, archive_write.3, archive_write_disk.3, and archive_read_disk.3 provide detailed calling sequences for the read and write APIs
  • archive_entry.3 details the "struct archive_entry" utility class
  • archive_internals.3 provides some insight into libarchive's internal structure and operation.
  • libarchive-formats.5 documents the file formats supported by the library
  • cpio.5, mtree.5, and tar.5 provide detailed information about these popular archive formats, including hard-to-find details about modern cpio and tar variants.

The manual pages above are provided in the 'doc' directory in a number of different formats.

You should also read the copious comments in archive.h and the source code for the sample programs for more details. Please let us know about any errors or omissions you find.

Supported Formats

Currently, the library automatically detects and reads the following formats:

  • Old V7 tar archives
  • POSIX ustar
  • GNU tar format (including GNU long filenames, long link names, and sparse files)
  • Solaris 9 extended tar format (including ACLs)
  • POSIX pax interchange format
  • POSIX octet-oriented cpio
  • SVR4 ASCII cpio
  • Binary cpio (big-endian or little-endian)
  • PWB binary cpio
  • ISO9660 CD-ROM images (with optional Rockridge or Joliet extensions)
  • ZIP archives (with uncompressed or "deflate" compressed entries, including support for encrypted Zip archives)
  • ZIPX archives (with support for bzip2, ppmd8, lzma and xz compressed entries)
  • GNU and BSD 'ar' archives
  • 'mtree' format
  • 7-Zip archives (including archives that use zstandard compression)
  • Microsoft CAB format
  • LHA and LZH archives
  • RAR and RAR 5.0 archives (with some limitations due to RAR's proprietary status)
  • XAR archives

The library also detects and handles any of the following before evaluating the archive:

  • uuencoded files
  • files with RPM wrapper
  • gzip compression
  • bzip2 compression
  • compress/LZW compression
  • lzma, lzip, and xz compression
  • lz4 compression
  • lzop compression
  • zstandard compression

The library can create archives in any of the following formats:

  • POSIX ustar
  • POSIX pax interchange format
  • "restricted" pax format, which will create ustar archives except for entries that require pax extensions (for long filenames, ACLs, etc).
  • Old GNU tar format
  • Old V7 tar format
  • POSIX octet-oriented cpio
  • SVR4 "newc" cpio
  • Binary cpio (little-endian)
  • PWB binary cpio
  • shar archives
  • ZIP archives (with uncompressed or "deflate" compressed entries)
  • GNU and BSD 'ar' archives
  • 'mtree' format
  • ISO9660 format
  • 7-Zip archives
  • XAR archives

When creating archives, the result can be filtered with any of the following:

  • uuencode
  • gzip compression
  • bzip2 compression
  • compress/LZW compression
  • lzma, lzip, and xz compression
  • lz4 compression
  • lzop compression
  • zstandard compression

Notes about the Library Design

The following notes address many of the most common questions we are asked about libarchive:

  • This is a heavily stream-oriented system. That means that it is optimized to read or write the archive in a single pass from beginning to end. For example, this allows libarchive to process archives too large to store on disk by processing them on-the-fly as they are read from or written to a network or tape drive. This also makes libarchive useful for tools that need to produce archives on-the-fly (such as webservers that provide archived contents of a users account).

  • In-place modification and random access to the contents of an archive are not directly supported. For some formats, this is not an issue: For example, tar.gz archives are not designed for random access. In some other cases, libarchive can re-open an archive and scan it from the beginning quickly enough to provide the needed abilities even without true random access. Of course, some applications do require true random access; those applications should consider alternatives to libarchive.

  • The library is designed to be extended with new compression and archive formats. The only requirement is that the format be readable or writable as a stream and that each archive entry be independent. There are articles on the libarchive Wiki explaining how to extend libarchive.

  • On read, compression and format are always detected automatically.

  • The same API is used for all formats; it should be very easy for software using libarchive to transparently handle any of libarchive's archiving formats.

  • Libarchive's automatic support for decompression can be used without archiving by explicitly selecting the "raw" and "empty" formats.

  • I've attempted to minimize static link pollution. If you don't explicitly invoke a particular feature (such as support for a particular compression or format), it won't get pulled in to statically-linked programs. In particular, if you don't explicitly enable a particular compression or decompression support, you won't need to link against the corresponding compression or decompression libraries. This also reduces the size of statically-linked binaries in environments where that matters.

  • The library is generally thread safe depending on the platform: it does not define any global variables of its own. However, some platforms do not provide fully thread-safe versions of key C library functions. On those platforms, libarchive will use the non-thread-safe functions. Patches to improve this are of great interest to us.

  • The function archive_write_disk_header() is not thread safe on POSIX machines and could lead to security issue resulting in world writeable directories. Thus it must be mutexed by the calling code. This is due to calling umask(oldumask = umask(0)), which sets the umask for the whole process to 0 for a short time frame. In case other thread calls the same function in parallel, it might get interrupted by it and cause the executable to use umask=0 for the remaining execution. This will then lead to implicitly created directories to have 777 permissions without sticky bit.

  • In particular, libarchive's modules to read or write a directory tree do use chdir() to optimize the directory traversals. This can cause problems for programs that expect to do disk access from multiple threads. Of course, those modules are completely optional and you can use the rest of libarchive without them.

  • The library is not thread aware, however. It does no locking or thread management of any kind. If you create a libarchive object and need to access it from multiple threads, you will need to provide your own locking.

  • On read, the library accepts whatever blocks you hand it. Your read callback is free to pass the library a byte at a time or mmap the entire archive and give it to the library at once. On write, the library always produces correctly-blocked output.

  • The object-style approach allows you to have multiple archive streams open at once. bsdtar uses this in its "@archive" extension.

  • The archive itself is read/written using callback functions. You can read an archive directly from an in-memory buffer or write it to a socket, if you wish. There are some utility functions to provide easy-to-use "open file," etc, capabilities.

  • The read/write APIs are designed to allow individual entries to be read or written to any data source: You can create a block of data in memory and add it to a tar archive without first writing a temporary file. You can also read an entry from an archive and write the data directly to a socket. If you want to read/write entries to disk, there are convenience functions to make this especially easy.

  • Note: The "pax interchange format" is a POSIX standard extended tar format that should be used when the older ustar format is not appropriate. It has many advantages over other tar formats (including the legacy GNU tar format) and is widely supported by current tar implementations.