diff options
Diffstat (limited to 'libarchive/libarchive-2.8.0/doc/text/libarchive-formats.5.txt')
| -rw-r--r-- | libarchive/libarchive-2.8.0/doc/text/libarchive-formats.5.txt | 241 |
1 files changed, 241 insertions, 0 deletions
diff --git a/libarchive/libarchive-2.8.0/doc/text/libarchive-formats.5.txt b/libarchive/libarchive-2.8.0/doc/text/libarchive-formats.5.txt new file mode 100644 index 0000000..9e6523d --- /dev/null +++ b/libarchive/libarchive-2.8.0/doc/text/libarchive-formats.5.txt @@ -0,0 +1,241 @@ +libarchive-formats(5) FreeBSD File Formats Manual libarchive-formats(5) + +NAME + libarchive-formats -- archive formats supported by the libarchive library + +DESCRIPTION + 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 + entries. + + 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 + entries 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 + 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 under- + stand. + + 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 + bytes. + 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- + tions. + + 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, + ustar archives with this extension can support entries up to 64 + gigabytes 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 + variable-length data. Unlike the tar format, the cpio format does only + minimal padding of the header or file data. There are several cpio vari- + ants, 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 some- + times 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. + + shardump + 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 + extensions 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 + Joliet. + + 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- + extracting 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. + + mtree + 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 key- + words supported by both the NetBSD and FreeBSD versions of mtree(1), + although many of the keywords cannot currently be stored in an + archive_entry object. 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 + error for any attempt to read the entry body. + +SEE ALSO + ar(1), cpio(1), mkisofs(1), shar(1), tar(1), zip(1), zlib(3), cpio(5), + mtree(5), tar(5) + +FreeBSD 8.0 December 27, 2009 FreeBSD 8.0 |