The .xz file format specification version 1.0.0 is now
officially released. The format has been technically the same since 2008-11-19, but now that it is frozen, people can start using it without a fear that the format will break.
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Lasse Collin
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The .xz File Format
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-------------------
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===================
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Version 1.0.0 (2009-01-14)
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0. Preface
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0.1. Notices and Acknowledgements
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0.2. Changes
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0.2. Getting the Latest Version
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0.3. Version History
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1. Conventions
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1.1. Byte and Its Representation
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1.2. Multibyte Integers
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@ -61,29 +65,34 @@ The .xz File Format
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this format replace the old .lzma format used by LZMA SDK and
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LZMA Utils.
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IMPORTANT: The version described in this document is a
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draft, NOT a final, official version. Changes
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are possible.
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0.1. Notices and Acknowledgements
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This file format was designed by Lasse Collin
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<lasse.collin@tukaani.org> and Igor Pavlov.
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Special thanks for helping with this document goes to Ville
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Koskinen. Thanks for helping with this document goes to
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Special thanks for helping with this document goes to
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Ville Koskinen. Thanks for helping with this document goes to
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Mark Adler, H. Peter Anvin, Mikko Pouru, and Lars Wirzenius.
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This document has been put into the public domain.
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0.2. Changes
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0.2. Getting the Latest Version
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Last modified: 2008-12-05 12:45+0200
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The latest official version of this document can be downloaded
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from <http://tukaani.org/xz/xz-file-format.txt>.
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(A changelog will be kept once the first official version
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is made.)
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Specific versions of this document have a filename
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xz-file-format-X.Y.Z.txt where X.Y.Z is the version number.
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For example, the version 1.0.0 of this document is available
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at <http://tukaani.org/xz/xz-file-format-1.0.0.txt>.
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0.3. Version History
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Version Date Description
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1.0.0 2008-01-14 The first official version
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1. Conventions
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@ -171,7 +180,7 @@ The .xz File Format
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variable-length integers. The functions return the number of
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bytes occupied by the integer (1-9), or zero on error.
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#include <sys/types.h>
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#include <stddef.h>
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#include <inttypes.h>
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size_t
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@ -429,9 +438,9 @@ The .xz File Format
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Stream Padding. This can be convenient in certain situations
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[GNU-tar].
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The possibility of Padding MUST be taken into account when
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designing an application that parses Streams backwards, and
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the application supports concatenated Streams.
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The possibility of Stream Padding MUST be taken into account
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when designing an application that parses Streams backwards,
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and the application supports concatenated Streams.
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3. Block
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@ -599,19 +608,19 @@ The .xz File Format
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The Check, when used, is calculated from the original
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uncompressed data. If the calculated Check does not match the
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stored one, the decoder MUST indicate an error. If the selected
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type of Check is not supported by the decoder, it MUST indicate
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a warning or error.
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type of Check is not supported by the decoder, it SHOULD
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indicate a warning or error.
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4. Index
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+-----------------+=========================+
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| Index Indicator | Number of Index Records |
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+-----------------+=========================+
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+-----------------+===================+
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| Index Indicator | Number of Records |
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+-----------------+===================+
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+=================+=========+-+-+-+-+
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---> | List of Records | Padding | CRC32 |
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+=================+=========+-+-+-+-+
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+=================+===============+-+-+-+-+
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---> | List of Records | Index Padding | CRC32 |
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+=================+===============+-+-+-+-+
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Index serves several purporses. Using it, one can
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- verify that all Blocks in a Stream have been processed;
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Unpadded Size and Uncompressed Size of the respective Blocks.
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Implementation hint: It is possible to verify the Index with
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constant memory usage by calculating for example SHA256 of both
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the real size values and the List of Records, then comparing
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the check values. Implementing this using non-cryptographic
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check like CRC32 SHOULD be avoided unless small code size is
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important.
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constant memory usage by calculating for example SHA-256 of
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both the real size values and the List of Records, then
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comparing the hash values. Implementing this using
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non-cryptographic hash like CRC32 SHOULD be avoided unless
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small code size is important.
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If the decoder supports random-access reading, it MUST verify
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that Unpadded Size and Uncompressed Size of every completely
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@ -898,7 +907,7 @@ The .xz File Format
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Setting the start offset may be useful if an executable has
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multiple sections, and there are many cross-section calls.
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Taking advantage of this feature usually requires usage of
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the Subblock filter.
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the Subblock filter, whose design is not complete yet.
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5.3.3. Delta
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@ -985,6 +994,7 @@ The .xz File Format
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5.4.1. Reserved Custom Filter ID Ranges
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Range Description
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0x0000_0300 - 0x0000_04FF Reserved to ease .7z compatibility
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0x0002_0000 - 0x0007_FFFF Reserved to ease .7z compatibility
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0x0200_0000 - 0x07FF_FFFF Reserved to ease .7z compatibility
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@ -1001,7 +1011,7 @@ The .xz File Format
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the CRC32 and CRC64 values, and prints the calculated values
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as big endian hexadecimal strings to standard output.
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#include <sys/types.h>
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#include <stddef.h>
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#include <inttypes.h>
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#include <stdio.h>
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@ -1067,7 +1077,8 @@ The .xz File Format
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uint8_t buf[8192];
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while (1) {
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const size_t buf_size = fread(buf, 1, 8192, stdin);
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const size_t buf_size
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= fread(buf, 1, sizeof(buf), stdin);
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if (buf_size == 0)
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break;
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@ -1092,6 +1103,9 @@ The .xz File Format
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LZMA Utils - LZMA adapted to POSIX-like systems
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http://tukaani.org/lzma/
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XZ Utils - The next generation of LZMA Utils
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http://tukaani.org/xz/
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[RFC-1952]
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GZIP file format specification version 4.3
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http://www.ietf.org/rfc/rfc1952.txt
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@ -1102,12 +1116,12 @@ The .xz File Format
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http://www.ietf.org/rfc/rfc2119.txt
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[GNU-tar]
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GNU tar 1.20 manual
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GNU tar 1.21 manual
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http://www.gnu.org/software/tar/manual/html_node/Blocking-Factor.html
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- Node 9.4.2 "Blocking Factor", paragraph that begins
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"gzip will complain about trailing garbage"
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- Note that this URL points to the latest version of the
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manual, and may some day not contain the note which is in
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1.20. For the exact version of the manual, download GNU
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tar 1.20: ftp://ftp.gnu.org/pub/gnu/tar/tar-1.20.tar.gz
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1.21. For the exact version of the manual, download GNU
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tar 1.21: ftp://ftp.gnu.org/pub/gnu/tar/tar-1.21.tar.gz
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