Tribblix: manual page: libarchive-formats.5

LIBARCHIVE-FORMATS(5) File Formats and Configurations LIBARCHIVE-FORMATS(5)

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:
+o Device major and minor numbers are limited to 21 bits.
Nodes with larger numbers will not be added to the archive.
+o Path names in the archive are limited to 255 bytes.
(Shorter if there is no / character in exactly the right
place.)
+o Symbolic links and hard links are stored in the archive
with the name of the referenced file. This name is limited
to 100 bytes.
+o Extended attributes, file flags, and other extended
security information cannot be stored.
+o 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:
+o Only regular files, directories, and symbolic links can be
archived. Block and character device nodes, FIFOs, and
sockets cannot be archived.
+o Path names in the archive are limited to 100 bytes.
+o Symbolic links and hard links are stored in the archive
with the name of the referenced file. This name is limited
to 100 bytes.
+o User and group information are stored as numeric IDs; there
is no provision for storing user or group names.
+o Extended attributes, file flags, and other extended
security information cannot be stored.
+o 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