|LIBARCHIVE-FORMATS(5)||File Formats Manual||LIBARCHIVE-FORMATS(5)|
archive formats supported by the libarchive
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 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, though many programs do use libarchive convenience functions to enable all supported 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.
The libarchive(3) library can write GNU tar format, including long filename and linkname support, as well as atime and ctime data.
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-handle non-ASCII filenames on systems that did not satisfy this assumption.
The libarchive library also reads a variety of commonly-used extensions to the basic tar format. These extensions are recognized automatically whenever they appear.
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.
The libarchive library can read and write a number of common cpio variants. A cpio archive stores each entry as a fixed-size header followed by a variable-length filename and variable-length data. Unlike the tar format, the cpio format does only minimal 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.
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
cpio 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.
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:
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 extensions 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 optimized with the directory information preceding all file data. This is done by storing all file data to a temporary file while collecting directory information in memory. When the image is finished, libarchive writes out the directory structure followed by the file data. The location used for the temporary file can be changed by the usual environment variables.
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 archives: 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 support, 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 program portion, but the seeking reader starts by reading the Central Directory 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.
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 standardised. 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.
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 keywords 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
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
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
keyword if present or the regular filename. 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.
Libarchive can read and write 7-Zip format archives. TODO: Need more information
Libarchive can read Microsoft Cabinet ( “CAB”) format archives. TODO: Need more information.
TODO: Information about libarchive's LHA support
Libarchive has limited support for reading RAR format archives. Currently, 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.
Libarchive can read and write “web archives”. TODO: Need more information
Libarchive can read and write the XAR format used by many Apple tools. TODO: Need more information
|December 27, 2016||macOS 14.1|