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     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 limita-
     tions 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.

   Tar Formats
     The libarchive(3) library can read	most tar archives.  However, it	only
     writes POSIX-standard ``ustar'' and ``pax interchange'' formats.

     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

     gnutar  The libarchive(3) library can read	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, but
	     cannot write GNU sparse file entries.

     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.  The	pres-
	     ence of this additional entry 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	Uni-
	     code strings.  Keywords defined in	the standard are in all	lower-
	     case; vendors are allowed to define custom	keys by	preceding them
	     with the vendor name in all uppercase.  When writing pax ar-
	     chives, 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 key-
	     words that	it does	not understand.

     restricted	pax
	     The libarchive library can	also write pax archives	in which it
	     attempts to suppress the extended attributes entry	whenever pos-
	     sible.  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-com-
	     pliant pax	interchange format archives.  Programs that correctly
	     read ustar	format (see below) will	also be	able to	read this for-
	     mat; 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
	     +o	 Extended attributes, file flags, and other extended security
		 information cannot be stored.
	     +o	 Archive entries are limited to	2 gigabytes in size.
	     Note that the pax interchange format has none of these restric-

     The libarchive library can	also read a variety of commonly-used exten-
     sions to the basic	tar format.  In	particular, it supports	base-256 val-
     ues in certain numeric fields.  This essentially removes the limitations
     on	file size, modification	time, and device numbers.

     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 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 a variety of cpio
     formats, 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 can	read 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-stan-
	     dard format, which	is officially known as the ``cpio interchange
	     format'' or the ``octet-oriented cpio archive format'' and	some-
	     times 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    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 limita-
     tions 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 dissim-
     ilar user numbering.

   Shar	Formats
     A ``shell archive'' is a shell script that, when executed on a POSIX-com-
     pliant 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.	How-
	     ever, 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 regard-
	     less 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 com-
	     mands 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.  It also has	partial	support	for Rockridge extensions.  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

   Zip format
     Libarchive	can extract from most zip format archives, including jar ar-
     chives, archives that use Zip64 extensions	and many self-extracting zip
     archives.	It currently only supports uncompressed	entries	and entries
     compressed	with the ``deflate'' algorithm.	 Older zip compression algo-
     rithms are	not supported.	Libarchive reads Zip archives as they are
     being streamed, which allows it to	read archives of arbitrary size.  It
     currently does not	use the	central	directory; this	limits libarchive's
     ability to	support	some self-extracting archives and ones that have been
     modified in certain ways.

   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	stan-
     dardised.	There are two common variants: the GNU format derived from
     SVR4, and the BSD format, which first appeared in 4.4BSD.	Libarchive
     provides read and write support for both variants.

     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	key-
     words supported by	both the NetBSD	and FreeBSD versions of	mtree(1),
     although many of the keywords cannot currently be stored in an
     archive_entry object.  When reading, libarchive supports an extension
     that allows it to obtain the contents of the files	described by the
     mtree(5) description from files on	disk.  When writing, libarchive	sup-
     ports use of the archive_write_set_options(3) interface to	specify	which
     keywords should be	included in the	output.	 This includes the ability to
     compute hash entries such as sha512 or md5	from file data being written
     to	the mtree writer.

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

FreeBSD	11.1			April 17, 2009			  FreeBSD 11.1


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