Ubuntu Manpage: libarchive-formats — archive formats supported by the libarchive library

Provided by: libarchive-dev_3.7.7-0ubuntu2.3_amd64

NAME

       libarchive-formats — archive formats supported by the libarchive library

DESCRIPTION

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.

Tar Formats
The libarchive(3) library can read most tar archives. It can write POSIX-standard “ustar” and “pax
interchange” formats as well as v7 tar format and a subset of the legacy GNU tar format.

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 entries.

gnutar The libarchive(3) library can read most 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.

The libarchive(3) library can write GNU tar format, including long filename and linkname support,
as well as atime and ctime data.

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 pax interchange format converts filenames to Unicode and stores them using the UTF-8
encoding. Prior to libarchive 3.0, libarchive erroneously assumed that the system wide-character
routines natively supported Unicode. This caused it to mis-handle non-ASCII filenames on systems
that did not satisfy this assumption.

restricted pax
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 bytes.
• 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 restrictions. The ustar format is old and
widely supported. It is recommended when compatibility is the primary concern.

v7 The libarchive library can read and write the legacy v7 tar format. This format has the
following limitations:
• Only regular files, directories, and symbolic links can be archived. Block and character
device nodes, FIFOs, and sockets cannot be archived.
• Path names in the archive are limited to 100 bytes.
• Symbolic links and hard links are stored in the archive with the name of the referenced file.
This name is limited to 100 bytes.
• User and group information are stored as numeric IDs; there is no provision for storing user
or group names.
• Extended attributes, file flags, and other extended security information cannot be stored.
• Archive entries are limited to 8 gigabytes in size.
Generally, users should prefer the ustar format for portability as the v7 tar format is both less
useful and less portable.

The libarchive library also reads a variety of commonly-used extensions to the basic tar format. These
extensions are recognized automatically whenever they appear.

Numeric extensions.
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.

Solaris extensions
Libarchive recognizes ACL and extended attribute records written by Solaris tar.

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.

Cpio Formats
The libarchive library can read and write a number of common cpio variants. 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 the
two binary cpio formats; the original one from PWB/UNIX, and the later, more widely used,
variant. This format used 32-bit binary values for file size and mtime, and 16-bit binary values
for the other fields. The formats support only the file types present in UNIX at the time of
their creation. File sizes are limited to 24 bits in the PWB format, because of the limits of
the file system, and to 31 bits in the newer binary format, where signed 32 bit longs were used.

odc This is the POSIX standardized 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/newc
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.

Shar Formats
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.

shardump
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.

ISO9660 format
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 reads ISO9660 images using a streaming strategy. This allows it to read compressed images
directly (decompressing on the fly) and allows it to read images directly from network sockets, pipes,
and other non-seekable data sources. This strategy works well for optimized ISO9660 images created by
many popular programs. Such programs collect all directory information at the beginning of the ISO9660
image so it can be read from a physical disk with a minimum of seeking. However, not all ISO9660 images
can be read in this fashion.

Libarchive can also write ISO9660 images. Such images are fully optimized with the directory information
preceding all file data. This is done by storing all file data to a temporary file while collecting
directory information in memory. When the image is finished, libarchive writes out the directory
structure followed by the file data. The location used for the temporary file can be changed by the
usual environment variables.

Zip format
Libarchive can read and write zip format archives that have uncompressed entries and entries compressed
with the “deflate” algorithm. Other zip compression algorithms are not supported. It can extract jar
archives, archives that use Zip64 extensions and self-extracting zip archives. Libarchive can use either
of two different strategies for reading Zip archives: a streaming strategy which is fast and can handle
extremely large archives, and a seeking strategy which can correctly process self-extracting Zip archives
and archives with deleted members or other in-place modifications.

The streaming reader processes Zip archives as they are read. It can read archives of arbitrary size
from tape or network sockets, and can decode Zip archives that have been separately compressed or
encoded. However, self-extracting Zip archives and archives with certain types of modifications cannot
be correctly handled. Such archives require that the reader first process the Central Directory, which
is ordinarily located at the end of a Zip archive and is thus inaccessible to the streaming reader. If
the program using libarchive has enabled seek support, then libarchive will use this to processes the
central directory first.

In particular, the seeking reader must be used to correctly handle self-extracting archives. Such
archives consist of a program followed by a regular Zip archive. The streaming reader cannot parse the
initial program portion, but the seeking reader starts by reading the Central Directory from the end of
the archive. Similarly, Zip archives that have been modified in-place can have deleted entries or other
garbage data that can only be accurately detected by first reading the Central Directory.

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 an entry called // containing 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.

mtree
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(8), 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.

7-Zip
Libarchive can read and write 7-Zip format archives. TODO: Need more information

CAB
Libarchive can read Microsoft Cabinet ( “CAB”) format archives. TODO: Need more information.

LHA
TODO: Information about libarchive's LHA support

RAR
Libarchive has limited support for reading RAR format archives. Currently, libarchive can read RARv3
format archives which have been either created uncompressed, or compressed using any of the compression
methods supported by the RARv3 format. Libarchive can also read self-extracting RAR archives.

Warc
Libarchive can read and write “web archives”. TODO: Need more information

XAR
Libarchive can read and write the XAR format used by many Apple tools. TODO: Need more information

NAME

DESCRIPTION

SEE ALSO