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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.  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

     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

     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-han‐
	     dle non-ASCII filenames on systems that did not satisfy this

     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:
	     ·	 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
	     ·	 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 restric‐
	     tions.  The ustar format is old and widely supported.  It is rec‐
	     ommended when compatibility is the primary concern.

     v7      The libarchive library can read and write the legacy v7 tar for‐
	     mat.  This format has the following limitations:
	     ·	 Only regular files, directories, and symbolic links can be
		 archived.  Block and character device nodes, FIFOs, and sock‐
		 ets 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
	     ·	 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 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.

	     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
     extensions 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

     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 opti‐
     mized with the directory information preceding all file data.  This is
     done by storing all file data to a temporary file while collecting direc‐
     tory information in memory.  When the image is finished, libarchive
     writes out the directory structure followed by the file data.  The loca‐
     tion used for the temporary file can be changed by the usual environment

   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 ar‐
     chives: 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 sup‐
     port, 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 pro‐
     gram portion, but the seeking reader starts by reading the Central Direc‐
     tory 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 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 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 key‐
     words 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 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
     error for any attempt to read the entry body.

     Libarchive can read and write 7-Zip format archives.  TODO: Need more

     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.  Cur‐
     rently, 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 informa‐

     Libarchive can read and write the XAR format used by many Apple tools.
     TODO: Need more information

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

BSD			       December 27, 2016			   BSD