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libarchive-formats(3)  FreeBSD 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

     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.

     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
             attributes are of unlimited length and are stored as UTF-8 Uni-
             code strings.  Keywords defined in the standard are in all lower-
             case; vendors are allowed to define custom keys by preceding them
             with the vendor name in all uppercase.  When writing pax ar-
             chives, 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 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
             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:
             +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 Sixth Edition Unix (circa 1976).  This
     makes the tar format one of the oldest and most widely-supported archive
     formats.  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.  There have also been many custom variations.

   Cpio Formats
     The libarchive library can read a number of common cpio variants and can
     write ``odc'' format archives.  A cpio archive stores each entry as a
     fixed-size header followed by a variable-length filename and variable-
     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 is an old format that was widely used because of its simplicity and
     its support for very long filenames.  Unfortunately, it 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.

   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.

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

FreeBSD 6.2                     April 27, 2004                     FreeBSD 6.2


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