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libarchive-formats(3)	 BSD Library Functions Manual	 libarchive-formats(3)

     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 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.  The	pres-
	     ence of this additional entry is the only difference between pax
	     interchange format	and the	older ustar format.  The extended at-
	     tributes 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.  The libarchive library can read
	     most of the SCHILY	keys.  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	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 vari-
     able-length data.	Unlike tar, cpio 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.  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.  It	also
     avoids security and complexity issues that	come with virtual mounts and
     loopback devices.

   Zip format
     Libarchive	can extract from most zip format archives.  It currently only
     supports uncompressed entries and entries compressed with the "deflate"
     algorithm.	 Older zip compression algorithms are not supported.

   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 files in mtree(5) format. This	format is not a	true
     archive format, but rather	a description of a file	hierarchy. When	re-
     quested, libarchive obtains the contents of the files described by	the
     mtree(5) format from files	on disk	instead.

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

BSD				April 27, 2004				   BSD


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