1LIBARCHIVE-FORMATS(5)       BSD File Formats Manual      LIBARCHIVE-FORMATS(5)
2

NAME

4     libarchive-formats — archive formats supported by the libarchive library
5

DESCRIPTION

7     The libarchive(3) library reads and writes a variety of streaming archive
8     formats.  Generally speaking, all of these archive formats consist of a
9     series of “entries”.  Each entry stores a single file system object, such
10     as a file, directory, or symbolic link.
11
12     The following provides a brief description of each format supported by
13     libarchive, with some information about recognized extensions or limita‐
14     tions of the current library support.  Note that just because a format is
15     supported by libarchive does not imply that a program that uses
16     libarchive will support that format.  Applications that use libarchive
17     specify which formats they wish to support, though many programs do use
18     libarchive convenience functions to enable all supported formats.
19
20   Tar Formats
21     The libarchive(3) library can read most tar archives.  It can write
22     POSIX-standard “ustar” and “pax interchange” formats as well as v7 tar
23     format and a subset of the legacy GNU tar format.
24
25     All tar formats store each entry in one or more 512-byte records.  The
26     first record is used for file metadata, including filename, timestamp,
27     and mode information, and the file data is stored in subsequent records.
28     Later variants have extended this by either appropriating undefined areas
29     of the header record, extending the header to multiple records, or by
30     storing special entries that modify the interpretation of subsequent
31     entries.
32
33     gnutar  The libarchive(3) library can read most GNU-format tar archives.
34             It currently supports the most popular GNU extensions, including
35             modern long filename and linkname support, as well as atime and
36             ctime data.  The libarchive library does not support multi-volume
37             archives, nor the old GNU long filename format.  It can read GNU
38             sparse file entries, including the new POSIX-based formats.
39
40             The libarchive(3) library can write GNU tar format, including
41             long filename and linkname support, as well as atime and ctime
42             data.
43
44     pax     The libarchive(3) library can read and write POSIX-compliant pax
45             interchange format archives.  Pax interchange format archives are
46             an extension of the older ustar format that adds a separate entry
47             with additional attributes stored as key/value pairs immediately
48             before each regular entry.  The presence of these additional
49             entries is the only difference between pax interchange format and
50             the older ustar format.  The extended attributes are of unlimited
51             length and are stored as UTF-8 Unicode strings.  Keywords defined
52             in the standard are in all lowercase; vendors are allowed to
53             define custom keys by preceding them with the vendor name in all
54             uppercase.  When writing pax archives, libarchive uses many of
55             the SCHILY keys defined by Joerg Schilling's “star” archiver and
56             a few LIBARCHIVE keys.  The libarchive library can read most of
57             the SCHILY keys and most of the GNU keys introduced by GNU tar.
58             It silently ignores any keywords that it does not understand.
59
60             The pax interchange format converts filenames to Unicode and
61             stores them using the UTF-8 encoding.  Prior to libarchive 3.0,
62             libarchive erroneously assumed that the system wide-character
63             routines natively supported Unicode.  This caused it to mis-han‐
64             dle non-ASCII filenames on systems that did not satisfy this
65             assumption.
66
67     restricted pax
68             The libarchive library can also write pax archives in which it
69             attempts to suppress the extended attributes entry whenever pos‐
70             sible.  The result will be identical to a ustar archive unless
71             the extended attributes entry is required to store a long file
72             name, long linkname, extended ACL, file flags, or if any of the
73             standard ustar data (user name, group name, UID, GID, etc) cannot
74             be fully represented in the ustar header.  In all cases, the
75             result can be dearchived by any program that can read POSIX-com‐
76             pliant pax interchange format archives.  Programs that correctly
77             read ustar format (see below) will also be able to read this for‐
78             mat; any extended attributes will be extracted as separate files
79             stored in PaxHeader directories.
80
81     ustar   The libarchive library can both read and write this format.  This
82             format has the following limitations:
83             ·   Device major and minor numbers are limited to 21 bits.  Nodes
84                 with larger numbers will not be added to the archive.
85             ·   Path names in the archive are limited to 255 bytes.  (Shorter
86                 if there is no / character in exactly the right place.)
87             ·   Symbolic links and hard links are stored in the archive with
88                 the name of the referenced file.  This name is limited to 100
89                 bytes.
90             ·   Extended attributes, file flags, and other extended security
91                 information cannot be stored.
92             ·   Archive entries are limited to 8 gigabytes in size.
93             Note that the pax interchange format has none of these restric‐
94             tions.  The ustar format is old and widely supported.  It is rec‐
95             ommended when compatibility is the primary concern.
96
97     v7      The libarchive library can read and write the legacy v7 tar for‐
98             mat.  This format has the following limitations:
99             ·   Only regular files, directories, and symbolic links can be
100                 archived.  Block and character device nodes, FIFOs, and sock‐
101                 ets cannot be archived.
102             ·   Path names in the archive are limited to 100 bytes.
103             ·   Symbolic links and hard links are stored in the archive with
104                 the name of the referenced file.  This name is limited to 100
105                 bytes.
106             ·   User and group information are stored as numeric IDs; there
107                 is no provision for storing user or group names.
108             ·   Extended attributes, file flags, and other extended security
109                 information cannot be stored.
110             ·   Archive entries are limited to 8 gigabytes in size.
111             Generally, users should prefer the ustar format for portability
112             as the v7 tar format is both less useful and less portable.
113
114     The libarchive library also reads a variety of commonly-used extensions
115     to the basic tar format.  These extensions are recognized automatically
116     whenever they appear.
117
118     Numeric extensions.
119             The POSIX standards require fixed-length numeric fields to be
120             written with some character position reserved for terminators.
121             Libarchive allows these fields to be written without terminator
122             characters.  This extends the allowable range; in particular,
123             ustar archives with this extension can support entries up to 64
124             gigabytes in size.  Libarchive also recognizes base-256 values in
125             most numeric fields.  This essentially removes all limitations on
126             file size, modification time, and device numbers.
127
128     Solaris extensions
129             Libarchive recognizes ACL and extended attribute records written
130             by Solaris tar.
131
132     The first tar program appeared in Seventh Edition Unix in 1979.  The
133     first official standard for the tar file format was the “ustar” (Unix
134     Standard Tar) format defined by POSIX in 1988.  POSIX.1-2001 extended the
135     ustar format to create the “pax interchange” format.
136
137   Cpio Formats
138     The libarchive library can read a number of common cpio variants and can
139     write “odc” and “newc” format archives.  A cpio archive stores each entry
140     as a fixed-size header followed by a variable-length filename and vari‐
141     able-length data.  Unlike the tar format, the cpio format does only mini‐
142     mal padding of the header or file data.  There are several cpio variants,
143     which differ primarily in how they store the initial header: some store
144     the values as octal or hexadecimal numbers in ASCII, others as binary
145     values of varying byte order and length.
146
147     binary  The libarchive library transparently reads both big-endian and
148             little-endian variants of the original binary cpio format.  This
149             format used 32-bit binary values for file size and mtime, and
150             16-bit binary values for the other fields.
151
152     odc     The libarchive library can both read and write this POSIX-stan‐
153             dard format, which is officially known as the “cpio interchange
154             format” or the “octet-oriented cpio archive format” and sometimes
155             unofficially referred to as the “old character format”.  This
156             format stores the header contents as octal values in ASCII.  It
157             is standard, portable, and immune from byte-order confusion.
158             File sizes and mtime are limited to 33 bits (8GB file size),
159             other fields are limited to 18 bits.
160
161     SVR4/newc
162             The libarchive library can read both CRC and non-CRC variants of
163             this format.  The SVR4 format uses eight-digit hexadecimal values
164             for all header fields.  This limits file size to 4GB, and also
165             limits the mtime and other fields to 32 bits.  The SVR4 format
166             can optionally include a CRC of the file contents, although
167             libarchive does not currently verify this CRC.
168
169     Cpio first appeared in PWB/UNIX 1.0, which was released within AT&T in
170     1977.  PWB/UNIX 1.0 formed the basis of System III Unix, released outside
171     of AT&T in 1981.  This makes cpio older than tar, although cpio was not
172     included in Version 7 AT&T Unix.  As a result, the tar command became
173     much better known in universities and research groups that used Version
174     7.  The combination of the find and cpio utilities provided very precise
175     control over file selection.  Unfortunately, the format has many limita‐
176     tions that make it unsuitable for widespread use.  Only the POSIX format
177     permits files over 4GB, and its 18-bit limit for most other fields makes
178     it unsuitable for modern systems.  In addition, cpio formats only store
179     numeric UID/GID values (not usernames and group names), which can make it
180     very difficult to correctly transfer archives across systems with dissim‐
181     ilar user numbering.
182
183   Shar Formats
184     A “shell archive” is a shell script that, when executed on a POSIX-com‐
185     pliant system, will recreate a collection of file system objects.  The
186     libarchive library can write two different kinds of shar archives:
187
188     shar    The traditional shar format uses a limited set of POSIX commands,
189             including echo(1), mkdir(1), and sed(1).  It is suitable for
190             portably archiving small collections of plain text files.  How‐
191             ever, it is not generally well-suited for large archives (many
192             implementations of sh(1) have limits on the size of a script) nor
193             should it be used with non-text files.
194
195     shardump
196             This format is similar to shar but encodes files using
197             uuencode(1) so that the result will be a plain text file regard‐
198             less of the file contents.  It also includes additional shell
199             commands that attempt to reproduce as many file attributes as
200             possible, including owner, mode, and flags.  The additional com‐
201             mands used to restore file attributes make shardump archives less
202             portable than plain shar archives.
203
204   ISO9660 format
205     Libarchive can read and extract from files containing ISO9660-compliant
206     CDROM images.  In many cases, this can remove the need to burn a physical
207     CDROM just in order to read the files contained in an ISO9660 image.  It
208     also avoids security and complexity issues that come with virtual mounts
209     and loopback devices.  Libarchive supports the most common Rockridge
210     extensions and has partial support for Joliet extensions.  If both exten‐
211     sions are present, the Joliet extensions will be used and the Rockridge
212     extensions will be ignored.  In particular, this can create problems with
213     hardlinks and symlinks, which are supported by Rockridge but not by
214     Joliet.
215
216     Libarchive reads ISO9660 images using a streaming strategy.  This allows
217     it to read compressed images directly (decompressing on the fly) and
218     allows it to read images directly from network sockets, pipes, and other
219     non-seekable data sources.  This strategy works well for optimized
220     ISO9660 images created by many popular programs.  Such programs collect
221     all directory information at the beginning of the ISO9660 image so it can
222     be read from a physical disk with a minimum of seeking.  However, not all
223     ISO9660 images can be read in this fashion.
224
225     Libarchive can also write ISO9660 images.  Such images are fully opti‐
226     mized with the directory information preceding all file data.  This is
227     done by storing all file data to a temporary file while collecting direc‐
228     tory information in memory.  When the image is finished, libarchive
229     writes out the directory structure followed by the file data.  The loca‐
230     tion used for the temporary file can be changed by the usual environment
231     variables.
232
233   Zip format
234     Libarchive can read and write zip format archives that have uncompressed
235     entries and entries compressed with the “deflate” algorithm.  Other zip
236     compression algorithms are not supported.  It can extract jar archives,
237     archives that use Zip64 extensions and self-extracting zip archives.
238     Libarchive can use either of two different strategies for reading Zip ar‐
239     chives: a streaming strategy which is fast and can handle extremely large
240     archives, and a seeking strategy which can correctly process self-
241     extracting Zip archives and archives with deleted members or other in-
242     place modifications.
243
244     The streaming reader processes Zip archives as they are read.  It can
245     read archives of arbitrary size from tape or network sockets, and can
246     decode Zip archives that have been separately compressed or encoded.
247     However, self-extracting Zip archives and archives with certain types of
248     modifications cannot be correctly handled.  Such archives require that
249     the reader first process the Central Directory, which is ordinarily
250     located at the end of a Zip archive and is thus inaccessible to the
251     streaming reader.  If the program using libarchive has enabled seek sup‐
252     port, then libarchive will use this to processes the central directory
253     first.
254
255     In particular, the seeking reader must be used to correctly handle self-
256     extracting archives.  Such archives consist of a program followed by a
257     regular Zip archive.  The streaming reader cannot parse the initial pro‐
258     gram portion, but the seeking reader starts by reading the Central Direc‐
259     tory from the end of the archive.  Similarly, Zip archives that have been
260     modified in-place can have deleted entries or other garbage data that can
261     only be accurately detected by first reading the Central Directory.
262
263   Archive (library) file format
264     The Unix archive format (commonly created by the ar(1) archiver) is a
265     general-purpose format which is used almost exclusively for object files
266     to be read by the link editor ld(1).  The ar format has never been stan‐
267     dardised.  There are two common variants: the GNU format derived from
268     SVR4, and the BSD format, which first appeared in 4.4BSD.  The two differ
269     primarily in their handling of filenames longer than 15 characters: the
270     GNU/SVR4 variant writes a filename table at the beginning of the archive;
271     the BSD format stores each long filename in an extension area adjacent to
272     the entry.  Libarchive can read both extensions, including archives that
273     may include both types of long filenames.  Programs using libarchive can
274     write GNU/SVR4 format if they provide an entry called // containing a
275     filename table to be written into the archive before any of the entries.
276     Any entries whose names are not in the filename table will be written
277     using BSD-style long filenames.  This can cause problems for programs
278     such as GNU ld that do not support the BSD-style long filenames.
279
280   mtree
281     Libarchive can read and write files in mtree(5) format.  This format is
282     not a true archive format, but rather a textual description of a file
283     hierarchy in which each line specifies the name of a file and provides
284     specific metadata about that file.  Libarchive can read all of the key‐
285     words supported by both the NetBSD and FreeBSD versions of mtree(8),
286     although many of the keywords cannot currently be stored in an
287     archive_entry object.  When writing, libarchive supports use of the
288     archive_write_set_options(3) interface to specify which keywords should
289     be included in the output.  If libarchive was compiled with access to
290     suitable cryptographic libraries (such as the OpenSSL libraries), it can
291     compute hash entries such as sha512 or md5 from file data being written
292     to the mtree writer.
293
294     When reading an mtree file, libarchive will locate the corresponding
295     files on disk using the contents keyword if present or the regular file‐
296     name.  If it can locate and open the file on disk, it will use that to
297     fill in any metadata that is missing from the mtree file and will read
298     the file contents and return those to the program using libarchive.  If
299     it cannot locate and open the file on disk, libarchive will return an
300     error for any attempt to read the entry body.
301
302   7-Zip
303     Libarchive can read and write 7-Zip format archives.  TODO: Need more
304     information
305
306   CAB
307     Libarchive can read Microsoft Cabinet ( “CAB”) format archives.  TODO:
308     Need more information.
309
310   LHA
311     TODO: Information about libarchive's LHA support
312
313   RAR
314     Libarchive has limited support for reading RAR format archives.  Cur‐
315     rently, libarchive can read RARv3 format archives which have been either
316     created uncompressed, or compressed using any of the compression methods
317     supported by the RARv3 format.  Libarchive can also read self-extracting
318     RAR archives.
319
320   Warc
321     Libarchive can read and write “web archives”.  TODO: Need more informa‐
322     tion
323
324   XAR
325     Libarchive can read and write the XAR format used by many Apple tools.
326     TODO: Need more information
327

SEE ALSO

329     ar(1), cpio(1), mkisofs(1), shar(1), tar(1), zip(1), zlib(3), cpio(5),
330     mtree(5), tar(5)
331
332BSD                            December 27, 2016                           BSD
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