The libarchive library provides a flexible interface for reading and writing streaming archive files such as tar and cpio. the library is inherently stream-oriented; readers serially iterate through the archive, writers serially add things to the archive. in particular, note that there is no built-in support for random access nor for in-place modification.

libarchive can read at least five tar formats, four cpio formats, iso9660 cd/dvd images (including rockridge extensions), and zip files.

libarchive can write two tar formats (ustar and pax), one cpio format (odc/posix), and two types of shar files (with and without uuencoding).

the bsdtar debian package, which is the default tar(1) on freebsd, is built using libarchive.

there is also a manpage, tar(5), that provides an excellent description of the various tar file formats.