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The .xz File Format
===================
Version 1.2.1 (2024-04-08)
0. Preface
0.1. Notices and Acknowledgements
0.2. Getting the Latest Version
0.3. Version History
1. Conventions
1.1. Byte and Its Representation
1.2. Multibyte Integers
2. Overall Structure of .xz File
2.1. Stream
2.1.1. Stream Header
2.1.1.1. Header Magic Bytes
2.1.1.2. Stream Flags
2.1.1.3. CRC32
2.1.2. Stream Footer
2.1.2.1. CRC32
2.1.2.2. Backward Size
2.1.2.3. Stream Flags
2.1.2.4. Footer Magic Bytes
2.2. Stream Padding
3. Block
3.1. Block Header
3.1.1. Block Header Size
3.1.2. Block Flags
3.1.3. Compressed Size
3.1.4. Uncompressed Size
3.1.5. List of Filter Flags
3.1.6. Header Padding
3.1.7. CRC32
3.2. Compressed Data
3.3. Block Padding
3.4. Check
4. Index
4.1. Index Indicator
4.2. Number of Records
4.3. List of Records
4.3.1. Unpadded Size
4.3.2. Uncompressed Size
4.4. Index Padding
4.5. CRC32
5. Filter Chains
5.1. Alignment
5.2. Security
5.3. Filters
5.3.1. LZMA2
5.3.2. Branch/Call/Jump Filters for Executables
5.3.3. Delta
5.3.3.1. Format of the Encoded Output
5.4. Custom Filter IDs
5.4.1. Reserved Custom Filter ID Ranges
6. Cyclic Redundancy Checks
7. References
0. Preface
This document describes the .xz file format (filename suffix
".xz", MIME type "application/x-xz"). It is intended that this
this format replace the old .lzma format used by LZMA SDK and
LZMA Utils.
0.1. Notices and Acknowledgements
This file format was designed by Lasse Collin
<lasse.collin@tukaani.org> and Igor Pavlov.
Special thanks for helping with this document goes to
Ville Koskinen. Thanks for helping with this document goes to
Mark Adler, H. Peter Anvin, Mikko Pouru, and Lars Wirzenius.
This document has been put into the public domain.
0.2. Getting the Latest Version
The latest official version of this document can be downloaded
from <https://tukaani.org/xz/xz-file-format.txt>.
Specific versions of this document have a filename
xz-file-format-X.Y.Z.txt where X.Y.Z is the version number.
For example, the version 1.0.0 of this document is available
at <https://tukaani.org/xz/xz-file-format-1.0.0.txt>.
0.3. Version History
Version Date Description
1.2.1 2024-04-08 The URLs of this specification and
XZ Utils were changed back to the
original ones in Sections 0.2 and 7.
1.2.0 2024-01-19 Added RISC-V filter and updated URLs in
Sections 0.2 and 7. The URL of this
specification was changed.
1.1.0 2022-12-11 Added ARM64 filter and clarified 32-bit
ARM endianness in Section 5.3.2,
language improvements in Section 5.4
1.0.4 2009-08-27 Language improvements in Sections 1.2,
2.1.1.2, 3.1.1, 3.1.2, and 5.3.1
1.0.3 2009-06-05 Spelling fixes in Sections 5.1 and 5.4
1.0.2 2009-06-04 Typo fixes in Sections 4 and 5.3.1
1.0.1 2009-06-01 Typo fix in Section 0.3 and minor
clarifications to Sections 2, 2.2,
3.3, 4.4, and 5.3.2
1.0.0 2009-01-14 The first official version
1. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC-2119].
Indicating a warning means displaying a message, returning
appropriate exit status, or doing something else to let the
user know that something worth warning occurred. The operation
SHOULD still finish if a warning is indicated.
Indicating an error means displaying a message, returning
appropriate exit status, or doing something else to let the
user know that something prevented successfully finishing the
operation. The operation MUST be aborted once an error has
been indicated.
1.1. Byte and Its Representation
In this document, byte is always 8 bits.
A "null byte" has all bits unset. That is, the value of a null
byte is 0x00.
To represent byte blocks, this document uses notation that
is similar to the notation used in [RFC-1952]:
+-------+
| Foo | One byte.
+-------+
+---+---+
| Foo | Two bytes; that is, some of the vertical bars
+---+---+ can be missing.
+=======+
| Foo | Zero or more bytes.
+=======+
In this document, a boxed byte or a byte sequence declared
using this notation is called "a field". The example field
above would be called "the Foo field" or plain "Foo".
If there are many fields, they may be split to multiple lines.
This is indicated with an arrow ("--->"):
+=====+
| Foo |
+=====+
+=====+
---> | Bar |
+=====+
The above is equivalent to this:
+=====+=====+
| Foo | Bar |
+=====+=====+
1.2. Multibyte Integers
Multibyte integers of static length, such as CRC values,
are stored in little endian byte order (least significant
byte first).
When smaller values are more likely than bigger values (for
example file sizes), multibyte integers are encoded in a
variable-length representation:
- Numbers in the range [0, 127] are copied as is, and take
one byte of space.
- Bigger numbers will occupy two or more bytes. All but the
last byte of the multibyte representation have the highest
(eighth) bit set.
For now, the value of the variable-length integers is limited
to 63 bits, which limits the encoded size of the integer to
nine bytes. These limits may be increased in the future if
needed.
The following C code illustrates encoding and decoding of
variable-length integers. The functions return the number of
bytes occupied by the integer (1-9), or zero on error.
#include <stddef.h>
#include <inttypes.h>
size_t
encode(uint8_t buf[static 9], uint64_t num)
{
if (num > UINT64_MAX / 2)
return 0;
size_t i = 0;
while (num >= 0x80) {
buf[i++] = (uint8_t)(num) | 0x80;
num >>= 7;
}
buf[i++] = (uint8_t)(num);
return i;
}
size_t
decode(const uint8_t buf[], size_t size_max, uint64_t *num)
{
if (size_max == 0)
return 0;
if (size_max > 9)
size_max = 9;
*num = buf[0] & 0x7F;
size_t i = 0;
while (buf[i++] & 0x80) {
if (i >= size_max || buf[i] == 0x00)
return 0;
*num |= (uint64_t)(buf[i] & 0x7F) << (i * 7);
}
return i;
}
2. Overall Structure of .xz File
A standalone .xz files consist of one or more Streams which may
have Stream Padding between or after them:
+========+================+========+================+
| Stream | Stream Padding | Stream | Stream Padding | ...
+========+================+========+================+
The sizes of Stream and Stream Padding are always multiples
of four bytes, thus the size of every valid .xz file MUST be
a multiple of four bytes.
While a typical file contains only one Stream and no Stream
Padding, a decoder handling standalone .xz files SHOULD support
files that have more than one Stream or Stream Padding.
In contrast to standalone .xz files, when the .xz file format
is used as an internal part of some other file format or
communication protocol, it usually is expected that the decoder
stops after the first Stream, and doesn't look for Stream
Padding or possibly other Streams.
2.1. Stream
+-+-+-+-+-+-+-+-+-+-+-+-+=======+=======+ +=======+
| Stream Header | Block | Block | ... | Block |
+-+-+-+-+-+-+-+-+-+-+-+-+=======+=======+ +=======+
+=======+-+-+-+-+-+-+-+-+-+-+-+-+
---> | Index | Stream Footer |
+=======+-+-+-+-+-+-+-+-+-+-+-+-+
All the above fields have a size that is a multiple of four. If
Stream is used as an internal part of another file format, it
is RECOMMENDED to make the Stream start at an offset that is
a multiple of four bytes.
Stream Header, Index, and Stream Footer are always present in
a Stream. The maximum size of the Index field is 16 GiB (2^34).
There are zero or more Blocks. The maximum number of Blocks is
limited only by the maximum size of the Index field.
Total size of a Stream MUST be less than 8 EiB (2^63 bytes).
The same limit applies to the total amount of uncompressed
data stored in a Stream.
If an implementation supports handling .xz files with multiple
concatenated Streams, it MAY apply the above limits to the file
as a whole instead of limiting per Stream basis.
2.1.1. Stream Header
+---+---+---+---+---+---+-------+------+--+--+--+--+
| Header Magic Bytes | Stream Flags | CRC32 |
+---+---+---+---+---+---+-------+------+--+--+--+--+
2.1.1.1. Header Magic Bytes
The first six (6) bytes of the Stream are so called Header
Magic Bytes. They can be used to identify the file type.
Using a C array and ASCII:
const uint8_t HEADER_MAGIC[6]
= { 0xFD, '7', 'z', 'X', 'Z', 0x00 };
In plain hexadecimal:
FD 37 7A 58 5A 00
Notes:
- The first byte (0xFD) was chosen so that the files cannot
be erroneously detected as being in .lzma format, in which
the first byte is in the range [0x00, 0xE0].
- The sixth byte (0x00) was chosen to prevent applications
from misdetecting the file as a text file.
If the Header Magic Bytes don't match, the decoder MUST
indicate an error.
2.1.1.2. Stream Flags
The first byte of Stream Flags is always a null byte. In the
future, this byte may be used to indicate a new Stream version
or other Stream properties.
The second byte of Stream Flags is a bit field:
Bit(s) Mask Description
0-3 0x0F Type of Check (see Section 3.4):
ID Size Check name
0x00 0 bytes None
0x01 4 bytes CRC32
0x02 4 bytes (Reserved)
0x03 4 bytes (Reserved)
0x04 8 bytes CRC64
0x05 8 bytes (Reserved)
0x06 8 bytes (Reserved)
0x07 16 bytes (Reserved)
0x08 16 bytes (Reserved)
0x09 16 bytes (Reserved)
0x0A 32 bytes SHA-256
0x0B 32 bytes (Reserved)
0x0C 32 bytes (Reserved)
0x0D 64 bytes (Reserved)
0x0E 64 bytes (Reserved)
0x0F 64 bytes (Reserved)
4-7 0xF0 Reserved for future use; MUST be zero for now.
Implementations SHOULD support at least the Check IDs 0x00
(None) and 0x01 (CRC32). Supporting other Check IDs is
OPTIONAL. If an unsupported Check is used, the decoder SHOULD
indicate a warning or error.
If any reserved bit is set, the decoder MUST indicate an error.
It is possible that there is a new field present which the
decoder is not aware of, and can thus parse the Stream Header
incorrectly.
2.1.1.3. CRC32
The CRC32 is calculated from the Stream Flags field. It is
stored as an unsigned 32-bit little endian integer. If the
calculated value does not match the stored one, the decoder
MUST indicate an error.
The idea is that Stream Flags would always be two bytes, even
if new features are needed. This way old decoders will be able
to verify the CRC32 calculated from Stream Flags, and thus
distinguish between corrupt files (CRC32 doesn't match) and
files that the decoder doesn't support (CRC32 matches but
Stream Flags has reserved bits set).
2.1.2. Stream Footer
+-+-+-+-+---+---+---+---+-------+------+----------+---------+
| CRC32 | Backward Size | Stream Flags | Footer Magic Bytes |
+-+-+-+-+---+---+---+---+-------+------+----------+---------+
2.1.2.1. CRC32
The CRC32 is calculated from the Backward Size and Stream Flags
fields. It is stored as an unsigned 32-bit little endian
integer. If the calculated value does not match the stored one,
the decoder MUST indicate an error.
The reason to have the CRC32 field before the Backward Size and
Stream Flags fields is to keep the four-byte fields aligned to
a multiple of four bytes.
2.1.2.2. Backward Size
Backward Size is stored as a 32-bit little endian integer,
which indicates the size of the Index field as multiple of
four bytes, minimum value being four bytes:
real_backward_size = (stored_backward_size + 1) * 4;
If the stored value does not match the real size of the Index
field, the decoder MUST indicate an error.
Using a fixed-size integer to store Backward Size makes
it slightly simpler to parse the Stream Footer when the
application needs to parse the Stream backwards.
2.1.2.3. Stream Flags
This is a copy of the Stream Flags field from the Stream
Header. The information stored to Stream Flags is needed
when parsing the Stream backwards. The decoder MUST compare
the Stream Flags fields in both Stream Header and Stream
Footer, and indicate an error if they are not identical.
2.1.2.4. Footer Magic Bytes
As the last step of the decoding process, the decoder MUST
verify the existence of Footer Magic Bytes. If they don't
match, an error MUST be indicated.
Using a C array and ASCII:
const uint8_t FOOTER_MAGIC[2] = { 'Y', 'Z' };
In hexadecimal:
59 5A
The primary reason to have Footer Magic Bytes is to make
it easier to detect incomplete files quickly, without
uncompressing. If the file does not end with Footer Magic Bytes
(excluding Stream Padding described in Section 2.2), it cannot
be undamaged, unless someone has intentionally appended garbage
after the end of the Stream.
2.2. Stream Padding
Only the decoders that support decoding of concatenated Streams
MUST support Stream Padding.
Stream Padding MUST contain only null bytes. To preserve the
four-byte alignment of consecutive Streams, the size of Stream
Padding MUST be a multiple of four bytes. Empty Stream Padding
is allowed. If these requirements are not met, the decoder MUST
indicate an error.
Note that non-empty Stream Padding is allowed at the end of the
file; there doesn't need to be a new Stream after non-empty
Stream Padding. This can be convenient in certain situations
[GNU-tar].
The possibility of Stream Padding MUST be taken into account
when designing an application that parses Streams backwards,
and the application supports concatenated Streams.
3. Block
+==============+=================+===============+=======+
| Block Header | Compressed Data | Block Padding | Check |
+==============+=================+===============+=======+
3.1. Block Header
+-------------------+-------------+=================+
| Block Header Size | Block Flags | Compressed Size |
+-------------------+-------------+=================+
+===================+======================+
---> | Uncompressed Size | List of Filter Flags |
+===================+======================+
+================+--+--+--+--+
---> | Header Padding | CRC32 |
+================+--+--+--+--+
3.1.1. Block Header Size
This field overlaps with the Index Indicator field (see
Section 4.1).
This field contains the size of the Block Header field,
including the Block Header Size field itself. Valid values are
in the range [0x01, 0xFF], which indicate the size of the Block
Header as multiples of four bytes, minimum size being eight
bytes:
real_header_size = (encoded_header_size + 1) * 4;
If a Block Header bigger than 1024 bytes is needed in the
future, a new field can be added between the Block Header and
Compressed Data fields. The presence of this new field would
be indicated in the Block Header field.
3.1.2. Block Flags
The Block Flags field is a bit field:
Bit(s) Mask Description
0-1 0x03 Number of filters (1-4)
2-5 0x3C Reserved for future use; MUST be zero for now.
6 0x40 The Compressed Size field is present.
7 0x80 The Uncompressed Size field is present.
If any reserved bit is set, the decoder MUST indicate an error.
It is possible that there is a new field present which the
decoder is not aware of, and can thus parse the Block Header
incorrectly.
3.1.3. Compressed Size
This field is present only if the appropriate bit is set in
the Block Flags field (see Section 3.1.2).
The Compressed Size field contains the size of the Compressed
Data field, which MUST be non-zero. Compressed Size is stored
using the encoding described in Section 1.2. If the Compressed
Size doesn't match the size of the Compressed Data field, the
decoder MUST indicate an error.
3.1.4. Uncompressed Size
This field is present only if the appropriate bit is set in
the Block Flags field (see Section 3.1.2).
The Uncompressed Size field contains the size of the Block
after uncompressing. Uncompressed Size is stored using the
encoding described in Section 1.2. If the Uncompressed Size
does not match the real uncompressed size, the decoder MUST
indicate an error.
Storing the Compressed Size and Uncompressed Size fields serves
several purposes:
- The decoder knows how much memory it needs to allocate
for a temporary buffer in multithreaded mode.
- Simple error detection: wrong size indicates a broken file.
- Seeking forwards to a specific location in streamed mode.
It should be noted that the only reliable way to determine
the real uncompressed size is to uncompress the Block,
because the Block Header and Index fields may contain
(intentionally or unintentionally) invalid information.
3.1.5. List of Filter Flags
+================+================+ +================+
| Filter 0 Flags | Filter 1 Flags | ... | Filter n Flags |
+================+================+ +================+
The number of Filter Flags fields is stored in the Block Flags
field (see Section 3.1.2).
The format of each Filter Flags field is as follows:
+===========+====================+===================+
| Filter ID | Size of Properties | Filter Properties |
+===========+====================+===================+
Both Filter ID and Size of Properties are stored using the
encoding described in Section 1.2. Size of Properties indicates
the size of the Filter Properties field as bytes. The list of
officially defined Filter IDs and the formats of their Filter
Properties are described in Section 5.3.
Filter IDs greater than or equal to 0x4000_0000_0000_0000
(2^62) are reserved for implementation-specific internal use.
These Filter IDs MUST never be used in List of Filter Flags.
3.1.6. Header Padding
This field contains as many null byte as it is needed to make
the Block Header have the size specified in Block Header Size.
If any of the bytes are not null bytes, the decoder MUST
indicate an error. It is possible that there is a new field
present which the decoder is not aware of, and can thus parse
the Block Header incorrectly.
3.1.7. CRC32
The CRC32 is calculated over everything in the Block Header
field except the CRC32 field itself. It is stored as an
unsigned 32-bit little endian integer. If the calculated
value does not match the stored one, the decoder MUST indicate
an error.
By verifying the CRC32 of the Block Header before parsing the
actual contents allows the decoder to distinguish between
corrupt and unsupported files.
3.2. Compressed Data
The format of Compressed Data depends on Block Flags and List
of Filter Flags. Excluding the descriptions of the simplest
filters in Section 5.3, the format of the filter-specific
encoded data is out of scope of this document.
3.3. Block Padding
Block Padding MUST contain 0-3 null bytes to make the size of
the Block a multiple of four bytes. This can be needed when
the size of Compressed Data is not a multiple of four. If any
of the bytes in Block Padding are not null bytes, the decoder
MUST indicate an error.
3.4. Check
The type and size of the Check field depends on which bits
are set in the Stream Flags field (see Section 2.1.1.2).
The Check, when used, is calculated from the original
uncompressed data. If the calculated Check does not match the
stored one, the decoder MUST indicate an error. If the selected
type of Check is not supported by the decoder, it SHOULD
indicate a warning or error.
4. Index
+-----------------+===================+
| Index Indicator | Number of Records |
+-----------------+===================+
+=================+===============+-+-+-+-+
---> | List of Records | Index Padding | CRC32 |
+=================+===============+-+-+-+-+
Index serves several purposes. Using it, one can
- verify that all Blocks in a Stream have been processed;
- find out the uncompressed size of a Stream; and
- quickly access the beginning of any Block (random access).
4.1. Index Indicator
This field overlaps with the Block Header Size field (see
Section 3.1.1). The value of Index Indicator is always 0x00.
4.2. Number of Records
This field indicates how many Records there are in the List
of Records field, and thus how many Blocks there are in the
Stream. The value is stored using the encoding described in
Section 1.2. If the decoder has decoded all the Blocks of the
Stream, and then notices that the Number of Records doesn't
match the real number of Blocks, the decoder MUST indicate an
error.
4.3. List of Records
List of Records consists of as many Records as indicated by the
Number of Records field:
+========+========+
| Record | Record | ...
+========+========+
Each Record contains information about one Block:
+===============+===================+
| Unpadded Size | Uncompressed Size |
+===============+===================+
If the decoder has decoded all the Blocks of the Stream, it
MUST verify that the contents of the Records match the real
Unpadded Size and Uncompressed Size of the respective Blocks.
Implementation hint: It is possible to verify the Index with
constant memory usage by calculating for example SHA-256 of
both the real size values and the List of Records, then
comparing the hash values. Implementing this using
non-cryptographic hash like CRC32 SHOULD be avoided unless
small code size is important.
If the decoder supports random-access reading, it MUST verify
that Unpadded Size and Uncompressed Size of every completely
decoded Block match the sizes stored in the Index. If only
partial Block is decoded, the decoder MUST verify that the
processed sizes don't exceed the sizes stored in the Index.
4.3.1. Unpadded Size
This field indicates the size of the Block excluding the Block
Padding field. That is, Unpadded Size is the size of the Block
Header, Compressed Data, and Check fields. Unpadded Size is
stored using the encoding described in Section 1.2. The value
MUST never be zero; with the current structure of Blocks, the
actual minimum value for Unpadded Size is five.
Implementation note: Because the size of the Block Padding
field is not included in Unpadded Size, calculating the total
size of a Stream or doing random-access reading requires
calculating the actual size of the Blocks by rounding Unpadded
Sizes up to the next multiple of four.
The reason to exclude Block Padding from Unpadded Size is to
ease making a raw copy of Compressed Data without Block
Padding. This can be useful, for example, if someone wants
to convert Streams to some other file format quickly.
4.3.2. Uncompressed Size
This field indicates the Uncompressed Size of the respective
Block as bytes. The value is stored using the encoding
described in Section 1.2.
4.4. Index Padding
This field MUST contain 0-3 null bytes to pad the Index to
a multiple of four bytes. If any of the bytes are not null
bytes, the decoder MUST indicate an error.
4.5. CRC32
The CRC32 is calculated over everything in the Index field
except the CRC32 field itself. The CRC32 is stored as an
unsigned 32-bit little endian integer. If the calculated
value does not match the stored one, the decoder MUST indicate
an error.
5. Filter Chains
The Block Flags field defines how many filters are used. When
more than one filter is used, the filters are chained; that is,
the output of one filter is the input of another filter. The
following figure illustrates the direction of data flow.
v Uncompressed Data ^
| Filter 0 |
Encoder | Filter 1 | Decoder
| Filter n |
v Compressed Data ^
5.1. Alignment
Alignment of uncompressed input data is usually the job of
the application producing the data. For example, to get the
best results, an archiver tool should make sure that all
PowerPC executable files in the archive stream start at
offsets that are multiples of four bytes.
Some filters, for example LZMA2, can be configured to take
advantage of specified alignment of input data. Note that
taking advantage of aligned input can be beneficial also when
a filter is not the first filter in the chain. For example,
if you compress PowerPC executables, you may want to use the
PowerPC filter and chain that with the LZMA2 filter. Because
not only the input but also the output alignment of the PowerPC
filter is four bytes, it is now beneficial to set LZMA2
settings so that the LZMA2 encoder can take advantage of its
four-byte-aligned input data.
The output of the last filter in the chain is stored to the
Compressed Data field, which is is guaranteed to be aligned
to a multiple of four bytes relative to the beginning of the
Stream. This can increase
- speed, if the filtered data is handled multiple bytes at
a time by the filter-specific encoder and decoder,
because accessing aligned data in computer memory is
usually faster; and
- compression ratio, if the output data is later compressed
with an external compression tool.
5.2. Security
If filters would be allowed to be chained freely, it would be
possible to create malicious files, that would be very slow to
decode. Such files could be used to create denial of service
attacks.
Slow files could occur when multiple filters are chained:
v Compressed input data
| Filter 1 decoder (last filter)
| Filter 0 decoder (non-last filter)
v Uncompressed output data
The decoder of the last filter in the chain produces a lot of
output from little input. Another filter in the chain takes the
output of the last filter, and produces very little output
while consuming a lot of input. As a result, a lot of data is
moved inside the filter chain, but the filter chain as a whole
gets very little work done.
To prevent this kind of slow files, there are restrictions on
how the filters can be chained. These restrictions MUST be
taken into account when designing new filters.
The maximum number of filters in the chain has been limited to
four, thus there can be at maximum of three non-last filters.
Of these three non-last filters, only two are allowed to change
the size of the data.
The non-last filters, that change the size of the data, MUST
have a limit how much the decoder can compress the data: the
decoder SHOULD produce at least n bytes of output when the
filter is given 2n bytes of input. This limit is not
absolute, but significant deviations MUST be avoided.
The above limitations guarantee that if the last filter in the
chain produces 4n bytes of output, the chain as a whole will
produce at least n bytes of output.
5.3. Filters
5.3.1. LZMA2
LZMA (Lempel-Ziv-Markov chain-Algorithm) is a general-purpose
compression algorithm with high compression ratio and fast
decompression. LZMA is based on LZ77 and range coding
algorithms.
LZMA2 is an extension on top of the original LZMA. LZMA2 uses
LZMA internally, but adds support for flushing the encoder,
uncompressed chunks, eases stateful decoder implementations,
and improves support for multithreading. Thus, the plain LZMA
will not be supported in this file format.
Filter ID: 0x21
Size of Filter Properties: 1 byte
Changes size of data: Yes
Allow as a non-last filter: No
Allow as the last filter: Yes
Preferred alignment:
Input data: Adjustable to 1/2/4/8/16 byte(s)
Output data: 1 byte
The format of the one-byte Filter Properties field is as
follows:
Bits Mask Description
0-5 0x3F Dictionary Size
6-7 0xC0 Reserved for future use; MUST be zero for now.
Dictionary Size is encoded with one-bit mantissa and five-bit
exponent. The smallest dictionary size is 4 KiB and the biggest
is 4 GiB.
Raw value Mantissa Exponent Dictionary size
0 2 11 4 KiB
1 3 11 6 KiB
2 2 12 8 KiB
3 3 12 12 KiB
4 2 13 16 KiB
5 3 13 24 KiB
6 2 14 32 KiB
... ... ... ...
35 3 27 768 MiB
36 2 28 1024 MiB
37 3 29 1536 MiB
38 2 30 2048 MiB
39 3 30 3072 MiB
40 2 31 4096 MiB - 1 B
Instead of having a table in the decoder, the dictionary size
can be decoded using the following C code:
const uint8_t bits = get_dictionary_flags() & 0x3F;
if (bits > 40)
return DICTIONARY_TOO_BIG; // Bigger than 4 GiB
uint32_t dictionary_size;
if (bits == 40) {
dictionary_size = UINT32_MAX;
} else {
dictionary_size = 2 | (bits & 1);
dictionary_size <<= bits / 2 + 11;
}
5.3.2. Branch/Call/Jump Filters for Executables
These filters convert relative branch, call, and jump
instructions to their absolute counterparts in executable
files. This conversion increases redundancy and thus
compression ratio.
Size of Filter Properties: 0 or 4 bytes
Changes size of data: No
Allow as a non-last filter: Yes
Allow as the last filter: No
Below is the list of filters in this category. The alignment
is the same for both input and output data.
Filter ID Alignment Description
0x04 1 byte x86 filter (BCJ)
0x05 4 bytes PowerPC (big endian) filter
0x06 16 bytes IA64 filter
0x07 4 bytes ARM filter [1]
0x08 2 bytes ARM Thumb filter [1]
0x09 4 bytes SPARC filter
0x0A 4 bytes ARM64 filter [2]
0x0B 2 bytes RISC-V filter
[1] These are for little endian instruction encoding.
This must not be confused with data endianness.
A processor configured for big endian data access
may still use little endian instruction encoding.
The filters don't care about the data endianness.
[2] 4096-byte alignment gives the best results
because the address in the ADRP instruction
is a multiple of 4096 bytes.
If the size of Filter Properties is four bytes, the Filter
Properties field contains the start offset used for address
conversions. It is stored as an unsigned 32-bit little endian
integer. The start offset MUST be a multiple of the alignment
of the filter as listed in the table above; if it isn't, the
decoder MUST indicate an error. If the size of Filter
Properties is zero, the start offset is zero.
Setting the start offset may be useful if an executable has
multiple sections, and there are many cross-section calls.
Taking advantage of this feature usually requires usage of
the Subblock filter, whose design is not complete yet.
5.3.3. Delta
The Delta filter may increase compression ratio when the value
of the next byte correlates with the value of an earlier byte
at specified distance.
Filter ID: 0x03
Size of Filter Properties: 1 byte
Changes size of data: No
Allow as a non-last filter: Yes
Allow as the last filter: No
Preferred alignment:
Input data: 1 byte
Output data: Same as the original input data
The Properties byte indicates the delta distance, which can be
1-256 bytes backwards from the current byte: 0x00 indicates
distance of 1 byte and 0xFF distance of 256 bytes.
5.3.3.1. Format of the Encoded Output
The code below illustrates both encoding and decoding with
the Delta filter.
// Distance is in the range [1, 256].
const unsigned int distance = get_properties_byte() + 1;
uint8_t pos = 0;
uint8_t delta[256];
memset(delta, 0, sizeof(delta));
while (1) {
const int byte = read_byte();
if (byte == EOF)
break;
uint8_t tmp = delta[(uint8_t)(distance + pos)];
if (is_encoder) {
tmp = (uint8_t)(byte) - tmp;
delta[pos] = (uint8_t)(byte);
} else {
tmp = (uint8_t)(byte) + tmp;
delta[pos] = tmp;
}
write_byte(tmp);
--pos;
}
5.4. Custom Filter IDs
If a developer wants to use custom Filter IDs, there are two
choices. The first choice is to contact Lasse Collin and ask
him to allocate a range of IDs for the developer.
The second choice is to generate a 40-bit random integer
which the developer can use as a personal Developer ID.
To minimize the risk of collisions, Developer ID has to be
a randomly generated integer, not manually selected "hex word".
The following command, which works on many free operating
systems, can be used to generate Developer ID:
dd if=/dev/urandom bs=5 count=1 | hexdump
The developer can then use the Developer ID to create unique
(well, hopefully unique) Filter IDs.
Bits Mask Description
0-15 0x0000_0000_0000_FFFF Filter ID
16-55 0x00FF_FFFF_FFFF_0000 Developer ID
56-62 0x3F00_0000_0000_0000 Static prefix: 0x3F
The resulting 63-bit integer will use 9 bytes of space when
stored using the encoding described in Section 1.2. To get
a shorter ID, see the beginning of this Section how to
request a custom ID range.
5.4.1. Reserved Custom Filter ID Ranges
Range Description
0x0000_0300 - 0x0000_04FF Reserved to ease .7z compatibility
0x0002_0000 - 0x0007_FFFF Reserved to ease .7z compatibility
0x0200_0000 - 0x07FF_FFFF Reserved to ease .7z compatibility
6. Cyclic Redundancy Checks
There are several incompatible variations to calculate CRC32
and CRC64. For simplicity and clarity, complete examples are
provided to calculate the checks as they are used in this file
format. Implementations MAY use different code as long as it
gives identical results.
The program below reads data from standard input, calculates
the CRC32 and CRC64 values, and prints the calculated values
as big endian hexadecimal strings to standard output.
#include <stddef.h>
#include <inttypes.h>
#include <stdio.h>
uint32_t crc32_table[256];
uint64_t crc64_table[256];
void
init(void)
{
static const uint32_t poly32 = UINT32_C(0xEDB88320);
static const uint64_t poly64
= UINT64_C(0xC96C5795D7870F42);
for (size_t i = 0; i < 256; ++i) {
uint32_t crc32 = i;
uint64_t crc64 = i;
for (size_t j = 0; j < 8; ++j) {
if (crc32 & 1)
crc32 = (crc32 >> 1) ^ poly32;
else
crc32 >>= 1;
if (crc64 & 1)
crc64 = (crc64 >> 1) ^ poly64;
else
crc64 >>= 1;
}
crc32_table[i] = crc32;
crc64_table[i] = crc64;
}
}
uint32_t
crc32(const uint8_t *buf, size_t size, uint32_t crc)
{
crc = ~crc;
for (size_t i = 0; i < size; ++i)
crc = crc32_table[buf[i] ^ (crc & 0xFF)]
^ (crc >> 8);
return ~crc;
}
uint64_t
crc64(const uint8_t *buf, size_t size, uint64_t crc)
{
crc = ~crc;
for (size_t i = 0; i < size; ++i)
crc = crc64_table[buf[i] ^ (crc & 0xFF)]
^ (crc >> 8);
return ~crc;
}
int
main()
{
init();
uint32_t value32 = 0;
uint64_t value64 = 0;
uint64_t total_size = 0;
uint8_t buf[8192];
while (1) {
const size_t buf_size
= fread(buf, 1, sizeof(buf), stdin);
if (buf_size == 0)
break;
total_size += buf_size;
value32 = crc32(buf, buf_size, value32);
value64 = crc64(buf, buf_size, value64);
}
printf("Bytes: %" PRIu64 "\n", total_size);
printf("CRC-32: 0x%08" PRIX32 "\n", value32);
printf("CRC-64: 0x%016" PRIX64 "\n", value64);
return 0;
}
7. References
LZMA SDK - The original LZMA implementation
https://7-zip.org/sdk.html
LZMA Utils - LZMA adapted to POSIX-like systems
https://tukaani.org/lzma/
XZ Utils - The next generation of LZMA Utils
https://tukaani.org/xz/
[RFC-1952]
GZIP file format specification version 4.3
https://www.ietf.org/rfc/rfc1952.txt
- Notation of byte boxes in section "2.1. Overall conventions"
[RFC-2119]
Key words for use in RFCs to Indicate Requirement Levels
https://www.ietf.org/rfc/rfc2119.txt
[GNU-tar]
GNU tar 1.35 manual
https://www.gnu.org/software/tar/manual/html_node/Blocking-Factor.html
- Node 9.4.2 "Blocking Factor", paragraph that begins
"gzip will complain about trailing garbage"
- Note that this URL points to the latest version of the
manual, and may some day not contain the note which is in
1.35. For the exact version of the manual, download GNU
tar 1.35: ftp://ftp.gnu.org/pub/gnu/tar/tar-1.35.tar.gz
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