.TH LIBARCHIVE-FORMATS 5 "December 27, 2016" ""

.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, though many programs do use libarchive convenience

functions to enable all supported formats.

.SS Tar Formats

The

\fBlibarchive\fP(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.

.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.

.RS 5

.TP

\fBgnutar\fP

The

\fBlibarchive\fP(3)

library can read most 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.

.PP

The

\fBlibarchive\fP(3)

library can write GNU tar format, including long filename

and linkname support, as well as atime and ctime data.

.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 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 understand.

.PP

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.

.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 8 gigabytes in size.

.RE

Note that the pax interchange format has none of these restrictions.

The ustar format is old and widely supported.

It is recommended when compatibility is the primary concern.

.TP

\fBv7\fP

The libarchive library can read and write the legacy v7 tar format.

This format has the following limitations:

.RS 5

.IP \(bu

Only regular files, directories, and symbolic links can be archived.

Block and character device nodes, FIFOs, and sockets cannot be archived.

.IP \(bu

Path names in the archive are limited to 100 bytes.

.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

User and group information are stored as numeric IDs; there

is no provision for storing user or group names.

.IP \(bu

Extended attributes, file flags, and other extended

security information cannot be stored.

.IP \(bu

Archive entries are limited to 8 gigabytes in size.

.RE

Generally, users should prefer the ustar format for portability

as the v7 tar format is both less useful and less portable.

.RE

.PP

The libarchive library also reads a variety of commonly-used extensions to

the basic tar format.

These extensions are recognized automatically whenever they appear.

.RS 5

.TP

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.

.TP

Solaris extensions

Libarchive recognizes ACL and extended attribute records written

by Solaris tar.

.RE

.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 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.

.RS 5

.TP

\fBbinary\fP

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.

.TP

\fBodc\fP

The libarchive library can both read and write this

POSIX-standard format, which is officially known as the

``cpio interchange format''

or the

``octet-oriented cpio archive format''

and sometimes 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.

.TP

\fBSVR4/newc\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.

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.

.PP

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.

.PP

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.

.SS Zip format

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.

.PP

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.

.PP

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.

.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.

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

\fI//\fP

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.

.SS mtree

Libarchive can read and write files in

\fBmtree\fP(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

\fBmtree\fP(8),

although many of the keywords cannot currently be stored in an

Tn archive_entry

object.

When writing, libarchive supports use of the

\fBarchive_write_set_options\fP(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

\fBsha512\fP

or

\fBmd5\fP

from file data being written to the mtree writer.

.PP

When reading an mtree file, libarchive will locate the corresponding

files on disk using the

\fBcontents\fP

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.

.SS 7-Zip

Libarchive can read and write 7-Zip format archives.

TODO: Need more information

.SS CAB

Libarchive can read Microsoft Cabinet (

``CAB )''

format archives.

TODO: Need more information.

.SS LHA

TODO: Information about libarchive's LHA support

.SS RAR

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.

.SS Warc

Libarchive can read and write

``web archives''.

TODO: Need more information

.SS XAR

Libarchive can read and write the XAR format used by many Apple tools.

TODO: Need more information

.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)