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, though many programs do use
libarchive convenience functions to enable all supported formats.
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. It can read GNU
sparse file entries, including the new POSIX-based formats, but
cannot write GNU sparse file entries.
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 immediately
before each regular entry. The presence of these additional
entries 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 and
a few LIBARCHIVE keys. The libarchive library can read most of
the SCHILY keys and most of the GNU keys introduced by GNU tar.
It silently 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 8 gigabytes in size.
Note that the pax interchange format has none of these
The libarchive library also reads a variety of commonly-used extensions
to the basic tar format. These extensions are recognized automatically
whenever they appear.
The POSIX standards require fixed-length numeric fields to be
written with some character position reserved for terminators.
Libarchive allows these fields to be written without terminator
characters. This extends the allowable range; in particular,
ustar archives with this extension can support entries up to 64
gigabytes in size. Libarchive also recognizes base-256 values in
most numeric fields. This essentially removes all limitations on
file size, modification time, and device numbers.
Libarchive recognizes ACL and extended attribute records written
by Solaris tar. Currently, libarchive only has support for old-
style ACLs; the newer NFSv4 ACLs are recognized but discarded.
The first tar program appeared in Seventh Edition Unix in 1979. 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.
The libarchive library can read a number of common cpio variants and can
write “odc” and “newc” format archives. A cpio archive stores each entry
as a fixed-size header followed by a variable-length filename and
variable-length data. Unlike the tar format, the cpio format does only
minimal padding of the header or file data. There are several cpio
variants, 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 transparently reads 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, which is officially known as the “cpio
interchange format” or the “octet-oriented cpio archive format”
and sometimes unofficially referred to as the “old character
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 first appeared in PWB/UNIX 1.0, which was released within AT&T in
1977. PWB/UNIX 1.0 formed the basis of System III Unix, released outside
of AT&T in 1981. This makes cpio older than tar, although cpio was not
included in Version 7 AT&T Unix. As a result, the tar command became
much better known in universities and research groups that used Version
7. The combination of the find and cpio utilities provided very precise
control over file selection. Unfortunately, the format 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 with
dissimilar user numbering.
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. In many cases, this can remove the need to burn a physical
CDROM just in order to read the files contained in an ISO9660 image. It
also avoids security and complexity issues that come with virtual mounts
and loopback devices. Libarchive supports the most common Rockridge
extensions and has partial support for Joliet extensions. If both
extensions are present, the Joliet extensions will be used and the
Rockridge extensions will be ignored. In particular, this can create
problems with hardlinks and symlinks, which are supported by Rockridge
but not by Joliet.
Libarchive can read and write zip format archives that have uncompressed
entries and entries compressed with the “deflate” algorithm. Older zip
compression algorithms are not supported. It can extract jar archives,
archives that use Zip64 extensions and many self-extracting zip archives.
Libarchive reads Zip archives as they are being streamed, which allows it
to read archives of arbitrary size. It currently does not use the
central directory; this limits libarchive’s ability to support some self-
extracting archives and ones that have been modified in certain ways.
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. The two differ
primarily in their handling of filenames longer than 15 characters: the
GNU/SVR4 variant writes a filename table at the beginning of the archive;
the BSD format stores each long filename in an extension area adjacent to
the entry. Libarchive can read both extensions, including archives that
may include both types of long filenames. Programs using libarchive can
write GNU/SVR4 format if they provide a filename table to be written into
the archive before any of the entries. Any entries whose names are not
in the filename table will be written using BSD-style long filenames.
This can cause problems for programs such as GNU ld that do not support
the BSD-style long filenames.
Libarchive can read and write files in mtree(5) format. This format is
not a true archive format, but rather a textual description of a file
hierarchy in which each line specifies the name of a file and provides
specific metadata about that file. Libarchive can read all of the
keywords supported by both the NetBSD and FreeBSD versions of mtree(1),
although many of the keywords cannot currently be stored in an
archive_entry object. When writing, libarchive supports use of the
archive_write_set_options(3) interface to specify which keywords should
be included in the output. If libarchive was compiled with access to
suitable cryptographic libraries (such as the OpenSSL libraries), it can
compute hash entries such as sha512 or md5 from file data being written
to the mtree writer.
When reading an mtree file, libarchive will locate the corresponding
files on disk using the contents keyword if present or the regular
filename. If it can locate and open the file on disk, it will use that
to fill in any metadata that is missing from the mtree file and will read
the file contents and return those to the program using libarchive. If
it cannot locate and open the file on disk, libarchive will return an
error for any attempt to read the entry body.
ar(1), cpio(1), mkisofs(1), shar(1), tar(1), zip(1), zlib(3), cpio(5),