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 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/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