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. 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.
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.
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.
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.
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.
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.
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.
Libarchive can read and write 7-Zip format archives. TODO: Need more information
Libarchive can read Microsoft Cabinet ( “CAB”) format archives.
TODO: Need more information.
TODO: Information about libarchive's LHA support
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.
Libarchive can read and write “web archives”. TODO: Need more
information
Libarchive can read and write the XAR format used by many Apple tools. TODO:
Need more information
ar(1),
cpio(1),
mkisofs(1),
shar(1),
tar(1),
zip(1),
zlib(3),
cpio(5),
mtree(5),
tar(5)