Provided by: libarchive-dev_3.6.0-1ubuntu1_amd64 bug


     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.

   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

     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

             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.

             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.

             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

     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.

     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

     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


     ar(1), cpio(1), mkisofs(1), shar(1), tar(1), zip(1), zlib(3), cpio(5), mtree(5), tar(5)