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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, though	many programs do use
     libarchive	convenience functions to enable	all supported formats.

   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 en-

     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 immediately
	     before each regular entry.	 The presence of these additional en-
	     tries 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 de-
	     fine 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.

     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 re-
	     sult can be dearchived by any program that	can read POSIX-compli-
	     ant 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	8 gigabytes in size.
	     Note that the pax interchange format has none of these restric-

     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, us-
	     tar archives with this extension can support entries up to	64 gi-
	     gabytes 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.  Currently, libarchive only has support for old-
	     style ACLs; the newer NFSv4 ACLs are recognized but discarded.

     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 vari-
     able-length data.	Unlike the tar format, the cpio	format does only mini-
     mal 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-stan-
	     dard 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    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.  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 ex-
     tensions and has partial support for Joliet extensions.  If both exten-
     sions 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

   Zip format
     Libarchive	can read and write zip format archives that have uncompressed
     entries and entries compressed with the "deflate" algorithm.  Older zip
     compression algorithms are	not supported.	It can extract jar archives,
     archives that use Zip64 extensions	and many self-extracting zip archives.
     Libarchive	reads Zip archives as they are being streamed, which allows it
     to	read archives of arbitrary size.  It currently does not	use the	cen-
     tral directory; this limits libarchive's ability to support some self-ex-
     tracting 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.	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 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 hi-
     erarchy in	which each line	specifies the name of a	file and provides spe-
     cific metadata about that file.  Libarchive can read all of the keywords
     supported by both the NetBSD and FreeBSD versions of mtree(1), although
     many of the keywords cannot currently be stored in	an archive_entry ob-
     ject.  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	file-
     name.  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 er-
     ror for any attempt to read the entry body.

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

BSD			       December	27, 2009			   BSD


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