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+.TH libarchive-formats 3 "April 27, 2004" ""
+.SH NAME
+.ad l
+\fB\%libarchive-formats\fP
+\- archive formats supported by the libarchive library
+.SH DESCRIPTION
+.ad l
+The
+\fBlibarchive\fP(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.
+.PP
+The following provides a brief description of each format supported
+by libarchive, with some information about recognized extensions or
+limitations 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.
+.SS Tar Formats
+The
+\fBlibarchive\fP(3)
+library can read most tar archives.
+However, it only writes POSIX-standard
+``ustar''
+and
+``pax interchange''
+formats.
+.PP
+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.
+.PP
+.RS 5
+.TP
+\fBgnutar\fP
+The
+\fBlibarchive\fP(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.
+.TP
+\fBpax\fP
+The
+\fBlibarchive\fP(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 presence 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 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.
+.TP
+\fBrestricted\fP pax
+The libarchive library can also write pax archives in which it
+attempts to suppress the extended attributes entry whenever
+possible.
+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-compliant pax interchange format archives.
+Programs that correctly read ustar format (see below) will also be
+able to read this format; any extended attributes will be extracted as
+separate files stored in
+\fIPaxHeader\fP
+directories.
+.TP
+\fBustar\fP
+The libarchive library can both read and write this format.
+This format has the following limitations:
+.RS 5
+.IP \(bu
+Device major and minor numbers are limited to 21 bits.
+Nodes with larger numbers will not be added to the archive.
+.IP \(bu
+Path names in the archive are limited to 255 bytes.
+(Shorter if there is no / character in exactly the right place.)
+.IP \(bu
+Symbolic links and hard links are stored in the archive with
+the name of the referenced file.
+This name is limited to 100 bytes.
+.IP \(bu
+Extended attributes, file flags, and other extended
+security information cannot be stored.
+.IP \(bu
+Archive entries are limited to 2 gigabytes in size.
+.RE
+Note that the pax interchange format has none of these restrictions.
+.RE
+.PP
+The libarchive library can also read a variety of commonly-used extensions to
+the basic tar format.
+In particular, it supports base-256 values in certain numeric fields.
+This essentially removes the limitations on file size, modification time,
+and device numbers.
+.PP
+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.
+.SS 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 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.
+.RS 5
+.TP
+\fBbinary\fP
+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.
+.TP
+\fBodc\fP
+The libarchive library can both read and write this
+POSIX-standard 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.
+.TP
+\fBSVR4\fP
+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.
+.RE
+.PP
+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
+\fB\%find\fP
+and
+\fB\%cpio\fP
+utilities provided very precise control over file selection.
+Unfortunately, the format has many limitations 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 dissimilar user numbering.
+.SS Shar Formats
+A
+``shell archive''
+is a shell script that, when executed on a POSIX-compliant
+system, will recreate a collection of file system objects.
+The libarchive library can write two different kinds of shar archives:
+.RS 5
+.TP
+\fBshar\fP
+The traditional shar format uses a limited set of POSIX
+commands, including
+\fBecho\fP(1),
+\fBmkdir\fP(1),
+and
+\fBsed\fP(1).
+It is suitable for portably archiving small collections of plain text files.
+However, it is not generally well-suited for large archives
+(many implementations of
+\fBsh\fP(1)
+have limits on the size of a script) nor should it be used with non-text files.
+.TP
+\fBshardump\fP
+This format is similar to shar but encodes files using
+\fBuuencode\fP(1)
+so that the result will be a plain text file regardless 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 commands used to restore file attributes make
+shardump archives less portable than plain shar archives.
+.RE
+.SS 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.
+.SS 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.
+.SS Archive (library) file format
+The Unix archive format (commonly created by the
+\fBar\fP(1)
+archiver) is a general-purpose format which is
+used almost exclusively for object files to be
+read by the link editor
+\fBld\fP(1).
+The ar format has never been standardised.
+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.
+.SS mtree
+Libarchive can read files in
+\fBmtree\fP(5)
+format. This format is not a true archive format, but rather a description
+of a file hierarchy. When requested, libarchive obtains the contents of
+the files described by the
+\fBmtree\fP(5)
+format from files on disk instead.
+.SH SEE ALSO
+.ad l
+\fBar\fP(1),
+\fBcpio\fP(1),
+\fBmkisofs\fP(1),
+\fBshar\fP(1),
+\fBtar\fP(1),
+\fBzip\fP(1),
+\fBzlib\fP(3),
+\fBcpio\fP(5),
+\fBmtree\fP(5),
+\fBtar\fP(5)