Libarchive is a programming library that can create and read several
different streaming archive formats, including most popular tar
variants and several cpio formats. It can also write shar archives and
read ISO9660 CDROM images. The bsdtar program is an implementation of
tar(1) that is built on top of libarchive. It started as a test
harness, but has grown and is now the standard system tar for FreeBSD 5
The libarchive library offers a number of features that make it both
very flexible and very powerful.
- Automatic format detection: libarchive can automatically determine
both the compression and the archive format, regardless of the
data source. Most tar implementations do not automatically detect
the compression format, few implementation that can correctly do
this when reading from stdin or a socket. (The tar program
included with Gunnar Ritter's heirloom collection also does full
automatic format detection.)
- Writes POSIX formats: libarchive writes POSIX-standard formats,
including "ustar," "pax interchange format," and the POSIX "cpio"
- Supports pax interchange format: Pax interchange format (which,
despite the name, is really an extended tar format) eliminates
almost all limitations of historic tar formats and provides a
standard method for incorporating vendor-specific extensions.
libarchive exploits this extension mechanism to support ACLs and
file flags, for example. (Joerg Schilling's star archiver is
another open-source tar program that supports pax interchange
- Reads popular formats: libarchive can read GNU tar, ustar, pax
interchange format, cpio, and older tar variants. The internal
architecture is easily extensible. The only requirement for
support is that it be possible to read the format without seeking
in the file. (For example, a format that includes a compressed
size field before the data cannot be correctly written without
- High-Level API: the libarchive API makes it fairly simple to build
an archive from a list of filenames or to extract the entries
from an archive. However, the API also provides extreme
flexibility with regards to data sources. For example, there are
generic hooks that allow you to write an archive to a socket or
read data from an archive entry into a memory buffer.
- Extensible. The internal design uses generic interfaces for
compression, archive format detection and decoding, and archive data
I/O. It should be very easy to add new formats, new compression
methods, or new ways of reading/writing archives.
Tim Kientzle <firstname.lastname@example.org>