Provided by: libarchive1_2.2.4-1ubuntu1_i386
libarchive-formats - archive formats supported by the libarchive library
The libarchive(3) library reads and writes a variety of streaming archive
formats. Generally speaking, all of these archive formats consist of a
series of “entries”. Each entry stores a single file system object, such
as a file, directory, or symbolic link.
The following provides a brief description of each format supported by
libarchive, with some information about recognized extensions or
limitations of the current library support. Note that just because a
format is supported by libarchive does not imply that a program that uses
libarchive will support that format. Applications that use libarchive
specify which formats they wish to support.
The libarchive(3) library can read most tar archives. However, it only
writes POSIX-standard “ustar” and “pax interchange” formats.
All tar formats store each entry in one or more 512-byte records. The
first record is used for file metadata, including filename, timestamp,
and mode information, and the file data is stored in subsequent records.
Later variants have extended this by either appropriating undefined areas
of the header record, extending the header to multiple records, or by
storing special entries that modify the interpretation of subsequent
gnutar The libarchive(3) library can read GNU-format tar archives. It
currently supports the most popular GNU extensions, including
modern long filename and linkname support, as well as atime and
ctime data. The libarchive library does not support multi-volume
archives, nor the old GNU long filename format.
pax The libarchive(3) library can read and write POSIX-compliant pax
interchange format archives. Pax interchange format archives are
an extension of the older ustar format that adds a separate entry
with additional attributes stored as key/value pairs. The
presence of this additional entry is the only difference between
pax interchange format and the older ustar format. The extended
attributes are of unlimited length and are stored as UTF-8
Unicode strings. Keywords defined in the standard are in all
lowercase; vendors are allowed to define custom keys by preceding
them with the vendor name in all uppercase. When writing pax
archives, libarchive uses many of the SCHILY keys defined by
Joerg Schilling’s “star” archiver. The libarchive library can
read most of the SCHILY keys. It ignores any keywords that it
does not understand.
The libarchive library can also write pax archives in which it
attempts to suppress the extended attributes entry whenever
possible. The result will be identical to a ustar archive unless
the extended attributes entry is required to store a long file
name, long linkname, extended ACL, file flags, or if any of the
standard ustar data (user name, group name, UID, GID, etc) cannot
be fully represented in the ustar header. In all cases, the
result can be dearchived by any program that can read POSIX-
compliant pax interchange format archives. Programs that
correctly read ustar format (see below) will also be able to read
this format; any extended attributes will be extracted as
separate files stored in PaxHeader directories.
ustar The libarchive library can both read and write this format. This
format has the following limitations:
· Device major and minor numbers are limited to 21 bits. Nodes
with larger numbers will not be added to the archive.
· Path names in the archive are limited to 255 bytes. (Shorter
if there is no / character in exactly the right place.)
· Symbolic links and hard links are stored in the archive with
the name of the referenced file. This name is limited to 100
· Extended attributes, file flags, and other extended security
information cannot be stored.
· Archive entries are limited to 2 gigabytes in size.
Note that the pax interchange format has none of these
The libarchive library can also read a variety of commonly-used
extensions to the basic tar format. In particular, it supports base-256
values in certain numeric fields. This essentially removes the
limitations on file size, modification time, and device numbers.
The first tar program appeared in Sixth Edition Unix (circa 1976). This
makes the tar format one of the oldest and most widely-supported archive
formats. The first official standard for the tar file format was the
“ustar” (Unix Standard Tar) format defined by POSIX in 1988.
POSIX.1-2001 extended the ustar format to create the “pax interchange”
format. There have also been many custom variations.
The libarchive library can read a number of common cpio variants and can
write “odc” format archives. A cpio archive stores each entry as a
fixed-size header followed by a variable-length filename and variable-
length data. Unlike tar, cpio does only minimal padding of the header or
file data. There are a variety of cpio formats, which differ primarily
in how they store the initial header: some store the values as octal or
hexadecimal numbers in ASCII, others as binary values of varying byte
order and length.
binary The libarchive library can read both big-endian and little-endian
variants of the original binary cpio format. This format used
32-bit binary values for file size and mtime, and 16-bit binary
values for the other fields.
odc The libarchive library can both read and write this POSIX-
standard format. This format stores the header contents as octal
values in ASCII. It is standard, portable, and immune from byte-
order confusion. File sizes and mtime are limited to 33 bits
(8GB file size), other fields are limited to 18 bits.
SVR4 The libarchive library can read both CRC and non-CRC variants of
this format. The SVR4 format uses eight-digit hexadecimal values
for all header fields. This limits file size to 4GB, and also
limits the mtime and other fields to 32 bits. The SVR4 format
can optionally include a CRC of the file contents, although
libarchive does not currently verify this CRC.
Cpio is an old format that was widely used because of its simplicity and
its support for very long filenames. Unfortunately, it has many
limitations that make it unsuitable for widespread use. Only the POSIX
format permits files over 4GB, and its 18-bit limit for most other fields
makes it unsuitable for modern systems. In addition, cpio formats only
store numeric UID/GID values (not usernames and group names), which can
make it very difficult to correctly transfer archives across systems.
A “shell archive” is a shell script that, when executed on a POSIX-
compliant system, will recreate a collection of file system objects. The
libarchive library can write two different kinds of shar archives:
shar The traditional shar format uses a limited set of POSIX commands,
including echo(1), mkdir(1), and sed(1). It is suitable for
portably archiving small collections of plain text files.
However, it is not generally well-suited for large archives (many
implementations of sh(1) have limits on the size of a script) nor
should it be used with non-text files.
This format is similar to shar but encodes files using
uuencode(1) so that the result will be a plain text file
regardless of the file contents. It also includes additional
shell commands that attempt to reproduce as many file attributes
as possible, including owner, mode, and flags. The additional
commands used to restore file attributes make shardump archives
less portable than plain shar archives.
Libarchive can read and extract from files containing ISO9660-compliant
CDROM images. It also has partial support for Rockridge extensions. In
many cases, this can remove the need to burn a physical CDROM. It also
avoids security and complexity issues that come with virtual mounts and
Libarchive can extract from most zip format archives. It currently only
supports uncompressed entries and entries compressed with the “deflate”
algorithm. Older zip compression algorithms are not supported.
Archive (library) file format
The Unix archive format (commonly created by the ar(1) archiver) is a
general-purpose format which is used almost exclusively for object files
to be read by the link editor ld(1). The ar format has never been
standardised. There are two common variants: the GNU format derived from
SVR4, and the BSD format, which first appeared in 4.4BSD. Libarchive
provides read and write support for both variants.
ar(1), cpio(1), mkisofs(1), shar(1), tar(1), zip(1), zlib(3), tar(5)