LIBARCHIVE-FORMATS(5)	    BSD File Formats Manual	 LIBARCHIVE-FORMATS(5)

NAME

     libarchive-formats — archive formats supported by the libarchive library

DESCRIPTION

     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

     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 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

	     assumption.

     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

		 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 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

		 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 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/newc

	     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.

     shardump

	     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

     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 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

     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 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

     first.

     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.

   mtree

     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.

   7-Zip

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

     information

   CAB

     Libarchive can read Microsoft Cabinet ( “CAB”) format archives.  TODO:

     Need more information.

   LHA

     TODO: Information about libarchive's LHA support

   RAR

     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.

   Warc

     Libarchive can read and write “web archives”.  TODO: Need more informa‐

     tion

   XAR

     Libarchive can read and write the XAR format used by many Apple tools.

     TODO: Need more information

SEE ALSO

     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