306 lines
17 KiB
Plaintext
306 lines
17 KiB
Plaintext
LIBARCHIVE-FORMATS(5) BSD File Formats Manual LIBARCHIVE-FORMATS(5)
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NAME
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libarchive-formats — archive formats supported by the libarchive library
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DESCRIPTION
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The libarchive(3) library reads and writes a variety of streaming archive
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formats. Generally speaking, all of these archive formats consist of a
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series of “entries”. Each entry stores a single file system object, such
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as a file, directory, or symbolic link.
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The following provides a brief description of each format supported by
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libarchive, with some information about recognized extensions or limita‐
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tions of the current library support. Note that just because a format is
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supported by libarchive does not imply that a program that uses
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libarchive will support that format. Applications that use libarchive
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specify which formats they wish to support, though many programs do use
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libarchive convenience functions to enable all supported formats.
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Tar Formats
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The libarchive(3) library can read most tar archives. It can write
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POSIX-standard “ustar” and “pax interchange” formats and a subset of the
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legacy GNU tar format.
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All tar formats store each entry in one or more 512-byte records. The
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first record is used for file metadata, including filename, timestamp,
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and mode information, and the file data is stored in subsequent records.
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Later variants have extended this by either appropriating undefined areas
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of the header record, extending the header to multiple records, or by
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storing special entries that modify the interpretation of subsequent
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entries.
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gnutar The libarchive(3) library can read most GNU-format tar archives.
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It currently supports the most popular GNU extensions, including
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modern long filename and linkname support, as well as atime and
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ctime data. The libarchive library does not support multi-volume
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archives, nor the old GNU long filename format. It can read GNU
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sparse file entries, including the new POSIX-based formats.
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The libarchive(3) library can write GNU tar format, including
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long filename and linkname support, as well as atime and ctime
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data.
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pax The libarchive(3) library can read and write POSIX-compliant pax
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interchange format archives. Pax interchange format archives are
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an extension of the older ustar format that adds a separate entry
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with additional attributes stored as key/value pairs immediately
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before each regular entry. The presence of these additional
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entries is the only difference between pax interchange format and
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the older ustar format. The extended attributes are of unlimited
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length and are stored as UTF-8 Unicode strings. Keywords defined
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in the standard are in all lowercase; vendors are allowed to
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define custom keys by preceding them with the vendor name in all
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uppercase. When writing pax archives, libarchive uses many of
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the SCHILY keys defined by Joerg Schilling's “star” archiver and
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a few LIBARCHIVE keys. The libarchive library can read most of
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the SCHILY keys and most of the GNU keys introduced by GNU tar.
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It silently ignores any keywords that it does not understand.
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The pax interchange format converts filenames to Unicode and
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stores them using the UTF-8 encoding. Prior to libarchive 3.0,
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libarchive erroneously assumed that the system wide-character
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routines natively supported Unicode. This caused it to mis-han‐
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dle non-ASCII filenames on systems that did not satisfy this
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assumption.
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restricted pax
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The libarchive library can also write pax archives in which it
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attempts to suppress the extended attributes entry whenever pos‐
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sible. The result will be identical to a ustar archive unless
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the extended attributes entry is required to store a long file
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name, long linkname, extended ACL, file flags, or if any of the
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standard ustar data (user name, group name, UID, GID, etc) cannot
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be fully represented in the ustar header. In all cases, the
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result can be dearchived by any program that can read POSIX-com‐
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pliant pax interchange format archives. Programs that correctly
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read ustar format (see below) will also be able to read this for‐
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mat; any extended attributes will be extracted as separate files
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stored in PaxHeader directories.
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ustar The libarchive library can both read and write this format. This
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format has the following limitations:
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· Device major and minor numbers are limited to 21 bits. Nodes
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with larger numbers will not be added to the archive.
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· Path names in the archive are limited to 255 bytes. (Shorter
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if there is no / character in exactly the right place.)
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· Symbolic links and hard links are stored in the archive with
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the name of the referenced file. This name is limited to 100
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bytes.
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· Extended attributes, file flags, and other extended security
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information cannot be stored.
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· Archive entries are limited to 8 gigabytes in size.
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Note that the pax interchange format has none of these restric‐
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tions. The ustar format is old and widely supported. It is rec‐
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ommended when compatibility is the primary concern.
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The libarchive library also reads a variety of commonly-used extensions
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to the basic tar format. These extensions are recognized automatically
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whenever they appear.
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Numeric extensions.
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The POSIX standards require fixed-length numeric fields to be
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written with some character position reserved for terminators.
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Libarchive allows these fields to be written without terminator
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characters. This extends the allowable range; in particular,
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ustar archives with this extension can support entries up to 64
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gigabytes in size. Libarchive also recognizes base-256 values in
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most numeric fields. This essentially removes all limitations on
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file size, modification time, and device numbers.
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Solaris extensions
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Libarchive recognizes ACL and extended attribute records written
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by Solaris tar. Currently, libarchive only has support for old-
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style ACLs; the newer NFSv4 ACLs are recognized but discarded.
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The first tar program appeared in Seventh Edition Unix in 1979. The
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first official standard for the tar file format was the “ustar” (Unix
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Standard Tar) format defined by POSIX in 1988. POSIX.1-2001 extended the
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ustar format to create the “pax interchange” format.
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Cpio Formats
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The libarchive library can read a number of common cpio variants and can
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write “odc” and “newc” format archives. A cpio archive stores each entry
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as a fixed-size header followed by a variable-length filename and vari‐
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able-length data. Unlike the tar format, the cpio format does only mini‐
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mal padding of the header or file data. There are several cpio variants,
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which differ primarily in how they store the initial header: some store
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the values as octal or hexadecimal numbers in ASCII, others as binary
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values of varying byte order and length.
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binary The libarchive library transparently reads both big-endian and
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little-endian variants of the original binary cpio format. This
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format used 32-bit binary values for file size and mtime, and
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16-bit binary values for the other fields.
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odc The libarchive library can both read and write this POSIX-stan‐
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dard format, which is officially known as the “cpio interchange
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format” or the “octet-oriented cpio archive format” and sometimes
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unofficially referred to as the “old character format”. This
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format stores the header contents as octal values in ASCII. It
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is standard, portable, and immune from byte-order confusion.
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File sizes and mtime are limited to 33 bits (8GB file size),
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other fields are limited to 18 bits.
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SVR4 The libarchive library can read both CRC and non-CRC variants of
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this format. The SVR4 format uses eight-digit hexadecimal values
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for all header fields. This limits file size to 4GB, and also
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limits the mtime and other fields to 32 bits. The SVR4 format
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can optionally include a CRC of the file contents, although
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libarchive does not currently verify this CRC.
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Cpio first appeared in PWB/UNIX 1.0, which was released within AT&T in
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1977. PWB/UNIX 1.0 formed the basis of System III Unix, released outside
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of AT&T in 1981. This makes cpio older than tar, although cpio was not
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included in Version 7 AT&T Unix. As a result, the tar command became
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much better known in universities and research groups that used Version
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7. The combination of the find and cpio utilities provided very precise
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control over file selection. Unfortunately, the format has many limita‐
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tions that make it unsuitable for widespread use. Only the POSIX format
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permits files over 4GB, and its 18-bit limit for most other fields makes
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it unsuitable for modern systems. In addition, cpio formats only store
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numeric UID/GID values (not usernames and group names), which can make it
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very difficult to correctly transfer archives across systems with dissim‐
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ilar user numbering.
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Shar Formats
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A “shell archive” is a shell script that, when executed on a POSIX-com‐
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pliant system, will recreate a collection of file system objects. The
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libarchive library can write two different kinds of shar archives:
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shar The traditional shar format uses a limited set of POSIX commands,
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including echo(1), mkdir(1), and sed(1). It is suitable for
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portably archiving small collections of plain text files. How‐
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ever, it is not generally well-suited for large archives (many
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implementations of sh(1) have limits on the size of a script) nor
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should it be used with non-text files.
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shardump
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This format is similar to shar but encodes files using
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uuencode(1) so that the result will be a plain text file regard‐
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less of the file contents. It also includes additional shell
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commands that attempt to reproduce as many file attributes as
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possible, including owner, mode, and flags. The additional com‐
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mands used to restore file attributes make shardump archives less
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portable than plain shar archives.
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ISO9660 format
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Libarchive can read and extract from files containing ISO9660-compliant
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CDROM images. In many cases, this can remove the need to burn a physical
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CDROM just in order to read the files contained in an ISO9660 image. It
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also avoids security and complexity issues that come with virtual mounts
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and loopback devices. Libarchive supports the most common Rockridge
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extensions and has partial support for Joliet extensions. If both exten‐
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sions are present, the Joliet extensions will be used and the Rockridge
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extensions will be ignored. In particular, this can create problems with
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hardlinks and symlinks, which are supported by Rockridge but not by
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Joliet.
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Libarchive reads ISO9660 images using a streaming strategy. This allows
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it to read compressed images directly (decompressing on the fly) and
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allows it to read images directly from network sockets, pipes, and other
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non-seekable data sources. This strategy works well for optimized
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ISO9660 images created by many popular programs. Such programs collect
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all directory information at the beginning of the ISO9660 image so it can
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be read from a physical disk with a minimum of seeking. However, not all
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ISO9660 images can be read in this fashion.
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Libarchive can also write ISO9660 images. Such images are fully opti‐
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mized with the directory information preceding all file data. This is
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done by storing all file data to a temporary file while collecting direc‐
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tory information in memory. When the image is finished, libarchive
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writes out the directory structure followed by the file data. The loca‐
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tion used for the temporary file can be changed by the usual environment
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variables.
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Zip format
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Libarchive can read and write zip format archives that have uncompressed
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entries and entries compressed with the “deflate” algorithm. Other zip
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compression algorithms are not supported. It can extract jar archives,
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archives that use Zip64 extensions and self-extracting zip archives.
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Libarchive can use either of two different strategies for reading Zip ar‐
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chives: a streaming strategy which is fast and can handle extremely large
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archives, and a seeking strategy which can correctly process self-
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extracting Zip archives and archives with deleted members or other in-
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place modifications.
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The streaming reader processes Zip archives as they are read. It can
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read archives of arbitrary size from tape or network sockets, and can
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decode Zip archives that have been separately compressed or encoded.
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However, self-extracting Zip archives and archives with certain types of
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modifications cannot be correctly handled. Such archives require that
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the reader first process the Central Directory, which is ordinarily
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located at the end of a Zip archive and is thus inaccessible to the
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streaming reader. If the program using libarchive has enabled seek sup‐
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port, then libarchive will use this to processes the central directory
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first.
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In particular, the seeking reader must be used to correctly handle self-
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extracting archives. Such archives consist of a program followed by a
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regular Zip archive. The streaming reader cannot parse the initial pro‐
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gram portion, but the seeking reader starts by reading the Central Direc‐
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tory from the end of the archive. Similarly, Zip archives that have been
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modified in-place can have deleted entries or other garbage data that can
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only be accurately detected by first reading the Central Directory.
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Archive (library) file format
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The Unix archive format (commonly created by the ar(1) archiver) is a
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general-purpose format which is used almost exclusively for object files
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to be read by the link editor ld(1). The ar format has never been stan‐
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dardised. There are two common variants: the GNU format derived from
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SVR4, and the BSD format, which first appeared in 4.4BSD. The two differ
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primarily in their handling of filenames longer than 15 characters: the
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GNU/SVR4 variant writes a filename table at the beginning of the archive;
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the BSD format stores each long filename in an extension area adjacent to
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the entry. Libarchive can read both extensions, including archives that
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may include both types of long filenames. Programs using libarchive can
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write GNU/SVR4 format if they provide a filename table to be written into
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the archive before any of the entries. Any entries whose names are not
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in the filename table will be written using BSD-style long filenames.
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This can cause problems for programs such as GNU ld that do not support
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the BSD-style long filenames.
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mtree
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Libarchive can read and write files in mtree(5) format. This format is
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not a true archive format, but rather a textual description of a file
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hierarchy in which each line specifies the name of a file and provides
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specific metadata about that file. Libarchive can read all of the key‐
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words supported by both the NetBSD and FreeBSD versions of mtree(8),
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although many of the keywords cannot currently be stored in an
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archive_entry object. When writing, libarchive supports use of the
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archive_write_set_options(3) interface to specify which keywords should
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be included in the output. If libarchive was compiled with access to
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suitable cryptographic libraries (such as the OpenSSL libraries), it can
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compute hash entries such as sha512 or md5 from file data being written
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to the mtree writer.
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When reading an mtree file, libarchive will locate the corresponding
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files on disk using the contents keyword if present or the regular file‐
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name. If it can locate and open the file on disk, it will use that to
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fill in any metadata that is missing from the mtree file and will read
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the file contents and return those to the program using libarchive. If
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it cannot locate and open the file on disk, libarchive will return an
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error for any attempt to read the entry body.
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LHA
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XXX Information about libarchive's LHA support XXX
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CAB
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XXX Information about libarchive's CAB support XXX
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XAR
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XXX Information about libarchive's XAR support XXX
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RAR
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Libarchive has limited support for reading RAR format archives. Cur‐
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rently, libarchive can read RARv3 format archives which have been either
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created uncompressed, or compressed using any of the compression methods
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supported by the RARv3 format. Libarchive can also read self-extracting
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RAR archives.
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SEE ALSO
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ar(1), cpio(1), mkisofs(1), shar(1), tar(1), zip(1), zlib(3), cpio(5),
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mtree(5), tar(5)
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BSD March 18, 2012 BSD
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