libarchive-formats(5)       BSD File Formats Manual      libarchive-formats(5)

     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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