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@ -11,11 +11,12 @@ File: clzip.info, Node: Top, Next: Introduction, Up: (dir)
Clzip Manual
************
This manual is for Clzip (version 1.9, 13 April 2017).
This manual is for Clzip (version 1.10, 6 February 2018).
* Menu:
* Introduction:: Purpose and features of clzip
* Output:: Meaning of clzip's output
* Invoking clzip:: Command line interface
* Quality assurance:: Design, development and testing of lzip
* File format:: Detailed format of the compressed file
@ -28,13 +29,13 @@ This manual is for Clzip (version 1.9, 13 April 2017).
* Concept index:: Index of concepts
Copyright (C) 2010-2017 Antonio Diaz Diaz.
Copyright (C) 2010-2018 Antonio Diaz Diaz.
This manual is free documentation: you have unlimited permission to
copy, distribute and modify it.

File: clzip.info, Node: Introduction, Next: Invoking clzip, Prev: Top, Up: Top
File: clzip.info, Node: Introduction, Next: Output, Prev: Top, Up: Top
1 Introduction
**************
@ -55,7 +56,7 @@ archiving, taking into account both data integrity and decoder
availability:
* The lzip format provides very safe integrity checking and some data
recovery means. The lziprecover program can repair bit-flip errors
recovery means. The lziprecover program can repair bit flip errors
(one of the most common forms of data corruption) in lzip files,
and provides data recovery capabilities, including error-checked
merging of damaged copies of a file. *Note Data safety:
@ -129,7 +130,7 @@ entirely incomprehensible and therefore pointless.
Clzip will correctly decompress a file which is the concatenation of
two or more compressed files. The result is the concatenation of the
corresponding uncompressed files. Integrity testing of concatenated
corresponding decompressed files. Integrity testing of concatenated
compressed files is also supported.
Clzip can produce multimember files, and lziprecover can safely
@ -142,14 +143,58 @@ multivolume compressed tar archives.
automatically creating multimember output. The members so created are
large, about 2 PiB each.

File: clzip.info, Node: Output, Next: Invoking clzip, Prev: Introduction, Up: Top
2 Meaning of clzip's output
***************************
The output of clzip looks like this:
clzip -v foo
foo: 6.676:1, 14.98% ratio, 85.02% saved, 450560 in, 67493 out.
clzip -tvv foo.lz
foo.lz: 6.676:1, 14.98% ratio, 85.02% saved. ok
The meaning of each field is as follows:
'N:1'
The compression ratio (uncompressed_size / compressed_size), shown
as N to 1.
'ratio'
The inverse compression ratio
(compressed_size / uncompressed_size), shown as a percentage. A
decimal ratio is easily obtained by moving the decimal point two
places to the left; 14.98% = 0.1498.
'saved'
The space saved by compression (1 - ratio), shown as a percentage.
'in'
The size of the uncompressed data. When decompressing or testing,
it is shown as 'decompressed'. Note that clzip always prints the
uncompressed size before the compressed size when compressing,
decompressing, testing or listing.
'out'
The size of the compressed data. When decompressing or testing, it
is shown as 'compressed'.
When decompressing or testing at verbosity level 4 (-vvvv), the
dictionary size used to compress the file and the CRC32 of the
uncompressed data are also shown.
LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may
never have been compressed. Decompressed is used to refer to data which
have undergone the process of decompression.

File: clzip.info, Node: Invoking clzip, Next: Quality assurance, Prev: Introduction, Up: Top
File: clzip.info, Node: Invoking clzip, Next: Quality assurance, Prev: Output, Up: Top
2 Invoking clzip
3 Invoking clzip
****************
The format for running clzip is:
@ -160,7 +205,7 @@ The format for running clzip is:
other FILES and is read just once, the first time it appears in the
command line.
Clzip supports the following options:
clzip supports the following options:
'-h'
'--help'
@ -179,9 +224,10 @@ command line.
'-b BYTES'
'--member-size=BYTES'
Set the member size limit to BYTES. A small member size may
degrade compression ratio, so use it only when needed. Valid values
range from 100 kB to 2 PiB. Defaults to 2 PiB.
When compressing, set the member size limit to BYTES. A small
member size may degrade compression ratio, so use it only when
needed. Valid values range from 100 kB to 2 PiB. Defaults to
2 PiB.
'-c'
'--stdout'
@ -189,15 +235,15 @@ command line.
unchanged. If compressing several files, each file is compressed
independently. This option is needed when reading from a named
pipe (fifo) or from a device. Use it also to recover as much of
the uncompressed data as possible when decompressing a corrupt
the decompressed data as possible when decompressing a corrupt
file.
'-d'
'--decompress'
Decompress the specified file(s). If a file does not exist or
can't be opened, clzip continues decompressing the rest of the
files. If a file fails to decompress, clzip exits immediately
without decompressing the rest of the files.
Decompress the specified files. If a file does not exist or can't
be opened, clzip continues decompressing the rest of the files. If
a file fails to decompress, or is a terminal, clzip exits
immediately without decompressing the rest of the files.
'-f'
'--force'
@ -205,8 +251,8 @@ command line.
'-F'
'--recompress'
Force re-compression of files whose name already has the '.lz' or
'.tlz' suffix.
When compressing, force re-compression of files whose name already
has the '.lz' or '.tlz' suffix.
'-k'
'--keep'
@ -216,7 +262,7 @@ command line.
'-l'
'--list'
Print the uncompressed size, compressed size and percentage saved
of the specified file(s). Trailing data are ignored. The values
of the specified files. Trailing data are ignored. The values
produced are correct even for multimember files. If more than one
file is given, a final line containing the cumulative sizes is
printed. With '-v', the dictionary size, the number of members in
@ -230,19 +276,20 @@ command line.
'-m BYTES'
'--match-length=BYTES'
Set the match length limit in bytes. After a match this long is
found, the search is finished. Valid values range from 5 to 273.
Larger values usually give better compression ratios but longer
compression times.
When compressing, set the match length limit in bytes. After a
match this long is found, the search is finished. Valid values
range from 5 to 273. Larger values usually give better compression
ratios but longer compression times.
'-o FILE'
'--output=FILE'
When reading from standard input and '--stdout' has not been
specified, use 'FILE' as the virtual name of the uncompressed
file. This produces a file named 'FILE' when decompressing, a file
named 'FILE.lz' when compressing, and several files named
'FILE00001.lz', 'FILE00002.lz', etc, when compressing and
splitting the output in volumes.
file. This produces a file named 'FILE' when decompressing, or a
file named 'FILE.lz' when compressing. A second '.lz' extension is
not added if 'FILE' already ends in '.lz' or '.tlz'. When
compressing and splitting the output in volumes, several files
named 'FILE00001.lz', 'FILE00002.lz', etc, are created.
'-q'
'--quiet'
@ -250,13 +297,13 @@ command line.
'-s BYTES'
'--dictionary-size=BYTES'
Set the dictionary size limit in bytes. Clzip will use the smallest
possible dictionary size for each file without exceeding this
limit. Valid values range from 4 KiB to 512 MiB. Values 12 to 29
are interpreted as powers of two, meaning 2^12 to 2^29 bytes. Note
that dictionary sizes are quantized. If the specified size does
not match one of the valid sizes, it will be rounded upwards by
adding up to (BYTES / 8) to it.
When compressing, set the dictionary size limit in bytes. Clzip
will use the smallest possible dictionary size for each file
without exceeding this limit. Valid values range from 4 KiB to
512 MiB. Values 12 to 29 are interpreted as powers of two, meaning
2^12 to 2^29 bytes. Note that dictionary sizes are quantized. If
the specified size does not match one of the valid sizes, it will
be rounded upwards by adding up to (BYTES / 8) to it.
For maximum compression you should use a dictionary size limit as
large as possible, but keep in mind that the decompression memory
@ -265,38 +312,40 @@ command line.
'-S BYTES'
'--volume-size=BYTES'
Split the compressed output into several volume files with names
'original_name00001.lz', 'original_name00002.lz', etc, and set the
volume size limit to BYTES. Each volume is a complete, maybe
multimember, lzip file. A small volume size may degrade compression
ratio, so use it only when needed. Valid values range from 100 kB
to 4 EiB.
When compressing, split the compressed output into several volume
files with names 'original_name00001.lz', 'original_name00002.lz',
etc, and set the volume size limit to BYTES. Input files are kept
unchanged. Each volume is a complete, maybe multimember, lzip
file. A small volume size may degrade compression ratio, so use it
only when needed. Valid values range from 100 kB to 4 EiB.
'-t'
'--test'
Check integrity of the specified file(s), but don't decompress
them. This really performs a trial decompression and throws away
the result. Use it together with '-v' to see information about
the file(s). If a file fails the test, does not exist, can't be
opened, or is a terminal, clzip continues checking the rest of the
files.
Check integrity of the specified files, but don't decompress them.
This really performs a trial decompression and throws away the
result. Use it together with '-v' to see information about the
files. If a file fails the test, does not exist, can't be opened,
or is a terminal, clzip continues checking the rest of the files.
A final diagnostic is shown at verbosity level 1 or higher if any
file fails the test when testing multiple files.
'-v'
'--verbose'
Verbose mode.
When compressing, show the compression ratio for each file
processed. A second '-v' shows the progress of compression.
When compressing, show the compression ratio and size for each file
processed.
When decompressing or testing, further -v's (up to 4) increase the
verbosity level, showing status, compression ratio, dictionary
size, trailer contents (CRC, data size, member size), and up to 6
bytes of trailing data (if any) both in hexadecimal and as a
string of printable ASCII characters.
Two or more '-v' options show the progress of (de)compression.
'-0 .. -9'
Set the compression parameters (dictionary size and match length
limit) as shown in the table below. The default compression level
is '-6'. Note that '-9' can be much slower than '-0'. These
options have no effect when decompressing.
options have no effect when decompressing, testing or listing.
The bidimensional parameter space of LZMA can't be mapped to a
linear scale optimal for all files. If your files are large, very
@ -319,6 +368,13 @@ command line.
'--best'
Aliases for GNU gzip compatibility.
'--loose-trailing'
When decompressing, testing or listing, allow trailing data whose
first bytes are so similar to the magic bytes of a lzip header
that they can be confused with a corrupt header. Use this option
if a file triggers a "corrupt header" error and the cause is not
indeed a corrupt header.
Numbers given as arguments to options may be followed by a multiplier
and an optional 'B' for "byte".
@ -344,7 +400,7 @@ caused clzip to panic.

File: clzip.info, Node: Quality assurance, Next: File format, Prev: Invoking clzip, Up: Top
3 Design, development and testing of lzip
4 Design, development and testing of lzip
*****************************************
There are two ways of constructing a software design: One way is to make
@ -359,7 +415,7 @@ describes the lessons learned from previous compressors (gzip and
bzip2), and their application to the design of lzip.
3.1 Format design
4.1 Format design
=================
When gzip was designed in 1992, computers and operating systems were
@ -377,7 +433,7 @@ one of gzip.
Probably the worst defect of the gzip format from the point of view
of data safety is the variable size of its header. If the byte at
offset 3 (flags) of a gzip member gets corrupted, it may become very
offset 3 (flags) of a gzip member gets corrupted, it may become
difficult to recover the data, even if the compressed blocks are
intact, because it can't be known with certainty where the compressed
blocks begin.
@ -399,8 +455,8 @@ error detection. Any distance larger than the dictionary size acts as a
forbidden symbol, allowing the decompressor to detect the approximate
position of errors, and leaving very little work for the check sequence
(CRC and data sizes) in the detection of errors. Lzip is usually able
to detect all posible bit-flips in the compressed data without
resorting to the check sequence. It would be very difficult to write an
to detect all posible bit flips in the compressed data without
resorting to the check sequence. It would be difficult to write an
automatic recovery tool like lziprecover for the gzip format. And, as
far as I know, it has never been written.
@ -409,15 +465,14 @@ decompressed data because it provides more accurate error detection than
CRC64 up to a compressed size of about 16 GiB, a size larger than that
of most files. In the case of lzip, the additional detection capability
of the decompressor reduces the probability of undetected errors more
than a million times, making CRC32 more accurate than CRC64 up to about
20 PiB of compressed size.
than a million times beyond what the CRC32 alone provides.
The lzip format is designed for long-term archiving. Therefore it
excludes any unneeded features that may interfere with the future
extraction of the uncompressed data.
extraction of the decompressed data.
3.1.1 Gzip format (mis)features not present in lzip
4.1.1 Gzip format (mis)features not present in lzip
---------------------------------------------------
'Multiple algorithms'
@ -438,16 +493,22 @@ extraction of the uncompressed data.
compressed blocks.
'Optional CRC for the header'
Using an optional checksum for the header is not only a bad idea,
it is an error; it may prevent the extraction of perfectly good
data. For example, if the checksum is used and the bit enabling it
is reset by a bit-flip, the header will appear to be intact (in
spite of being corrupt) while the compressed blocks will appear to
be totally unrecoverable (in spite of being intact). Very
misleading indeed.
Using an optional CRC for the header is not only a bad idea, it is
an error; it circumvents the HD of the CRC and may prevent the
extraction of perfectly good data. For example, if the CRC is used
and the bit enabling it is reset by a bit flip, the header will
appear to be intact (in spite of being corrupt) while the
compressed blocks will appear to be totally unrecoverable (in
spite of being intact). Very misleading indeed.
'Metadata'
The gzip format stores some metadata, like the modification time
of the original file or the operating system on which compression
took place. This complicates reproducible compression (obtaining
identical compressed output from identical input).
3.1.2 Lzip format improvements over gzip and bzip2
4.1.2 Lzip format improvements over gzip and bzip2
--------------------------------------------------
'64-bit size field'
@ -475,7 +536,7 @@ extraction of the uncompressed data.
total uncompressed size.
3.2 Quality of implementation
4.2 Quality of implementation
=============================
'Accurate and robust error detection'
@ -521,7 +582,7 @@ extraction of the uncompressed data.

File: clzip.info, Node: File format, Next: Algorithm, Prev: Quality assurance, Up: Top
4 File format
5 File format
*************
Perfection is reached, not when there is no longer anything to add, but
@ -592,7 +653,7 @@ additional information before, between, or after them.

File: clzip.info, Node: Algorithm, Next: Stream format, Prev: File format, Up: Top
5 Algorithm
6 Algorithm
***********
In spite of its name (Lempel-Ziv-Markov chain-Algorithm), LZMA is not a
@ -658,7 +719,7 @@ LZMA), and Julian Seward (for bzip2's CLI).

File: clzip.info, Node: Stream format, Next: Trailing data, Prev: Algorithm, Up: Top
6 Format of the LZMA stream in lzip files
7 Format of the LZMA stream in lzip files
*****************************************
The LZMA algorithm has three parameters, called "special LZMA
@ -698,7 +759,7 @@ the lzip download directory. The source code of lzd is included in
appendix A. *Note Reference source code::.
6.1 What is coded
7.1 What is coded
=================
The LZMA stream includes literals, matches and repeated matches (matches
@ -773,7 +834,7 @@ slot + direct_bits distances from 4 to 127
slot + (direct_bits - 4) + 4 bits distances from 128 to 2^32 - 1
6.2 The coding contexts
7.2 The coding contexts
=======================
These contexts ('Bit_model' in the source), are integers or arrays of
@ -863,7 +924,7 @@ difference is found, the rest of the byte is decoded using the normal
bit tree context. (See 'decode_matched' in the source).
6.3 The range decoder
7.3 The range decoder
=====================
The LZMA stream is consumed one byte at a time by the range decoder.
@ -883,7 +944,7 @@ range decoder. This is done by shifting 5 bytes in the initialization of
source).
6.4 Decoding the LZMA stream
7.4 Decoding the LZMA stream
============================
After decoding the member header and obtaining the dictionary size, the
@ -896,7 +957,7 @@ Stream" marker is decoded.

File: clzip.info, Node: Trailing data, Next: Examples, Prev: Stream format, Up: Top
7 Extra data appended to the file
8 Extra data appended to the file
*********************************
Sometimes extra data are found appended to a lzip file after the last
@ -908,10 +969,11 @@ member. Such trailing data may be:
* Useful data added by the user; a cryptographically secure hash, a
description of file contents, etc. It is safe to append any amount
of text to a lzip file as long as the text does not begin with the
string "LZIP", and does not contain any zero bytes (null
characters). Nonzero bytes and zero bytes can't be safely mixed in
trailing data.
of text to a lzip file as long as none of the first four bytes of
the text match the corresponding byte in the string "LZIP", and
the text does not contain any zero bytes (null characters).
Nonzero bytes and zero bytes can't be safely mixed in trailing
data.
* Garbage added by some not totally successful copy operation.
@ -919,12 +981,17 @@ member. Such trailing data may be:
and hash value (for a chosen hash) coincide with those of another
file.
* In very rare cases, trailing data could be the corrupt header of
another member. In multimember or concatenated files the
probability of corruption happening in the magic bytes is 5 times
smaller than the probability of getting a false positive caused by
the corruption of the integrity information itself. Therefore it
can be considered to be below the noise level.
* In rare cases, trailing data could be the corrupt header of another
member. In multimember or concatenated files the probability of
corruption happening in the magic bytes is 5 times smaller than the
probability of getting a false positive caused by the corruption
of the integrity information itself. Therefore it can be
considered to be below the noise level. Additionally, the test
used by clzip to discriminate trailing data from a corrupt header
has a Hamming distance (HD) of 3, and the 3 bit flips must happen
in different magic bytes for the test to fail. In any case, the
option '--trailing-error' guarantees that any corrupt header will
be detected.
Trailing data are in no way part of the lzip file format, but tools
reading lzip files are expected to behave as correctly and usefully as
@ -938,7 +1005,7 @@ cases where a file containing trailing data must be rejected, the option

File: clzip.info, Node: Examples, Next: Problems, Prev: Trailing data, Up: Top
8 A small tutorial with examples
9 A small tutorial with examples
********************************
WARNING! Even if clzip is bug-free, other causes may result in a corrupt
@ -1020,8 +1087,8 @@ file with a member size of 32 MiB.

File: clzip.info, Node: Problems, Next: Reference source code, Prev: Examples, Up: Top
9 Reporting bugs
****************
10 Reporting bugs
*****************
There are probably bugs in clzip. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
@ -1039,7 +1106,7 @@ Appendix A Reference source code
********************************
/* Lzd - Educational decompressor for the lzip format
Copyright (C) 2013-2017 Antonio Diaz Diaz.
Copyright (C) 2013-2018 Antonio Diaz Diaz.
This program is free software. Redistribution and use in source and
binary forms, with or without modification, are permitted provided
@ -1355,9 +1422,9 @@ bool LZ_decoder::decode_member() // Returns false if error
Bit_model bm_align[dis_align_size];
Len_model match_len_model;
Len_model rep_len_model;
unsigned rep0 = 0; // rep[0-3] latest four distances
unsigned rep1 = 0; // used for efficient coding of
unsigned rep2 = 0; // repeated distances
unsigned rep0 = 0; // rep[0-3] latest four distances
unsigned rep1 = 0; // used for efficient coding of
unsigned rep2 = 0; // repeated distances
unsigned rep3 = 0;
State state;
@ -1452,7 +1519,7 @@ int main( const int argc, const char * const argv[] )
"It is not safe to use lzd for any real work.\n"
"\nUsage: %s < file.lz > file\n", argv[0] );
std::printf( "Lzd decompresses from standard input to standard output.\n"
"\nCopyright (C) 2017 Antonio Diaz Diaz.\n"
"\nCopyright (C) 2018 Antonio Diaz Diaz.\n"
"This is free software: you are free to change and redistribute it.\n"
"There is NO WARRANTY, to the extent permitted by law.\n"
"Report bugs to lzip-bug@nongnu.org\n"
@ -1497,7 +1564,7 @@ int main( const int argc, const char * const argv[] )
}
if( std::fclose( stdout ) != 0 )
{ std::fprintf( stderr, "Can't close stdout: %s\n", std::strerror( errno ) );
{ std::fprintf( stderr, "Error closing stdout: %s\n", std::strerror( errno ) );
return 1; }
return 0;
}
@ -1520,6 +1587,7 @@ Concept index
* introduction: Introduction. (line 6)
* invoking: Invoking clzip. (line 6)
* options: Invoking clzip. (line 6)
* output: Output. (line 6)
* quality assurance: Quality assurance. (line 6)
* reference source code: Reference source code. (line 6)
* trailing data: Trailing data. (line 6)
@ -1530,19 +1598,20 @@ Concept index

Tag Table:
Node: Top210
Node: Introduction1154
Node: Invoking clzip6630
Ref: --trailing-error7202
Node: Quality assurance14125
Node: File format22281
Node: Algorithm24686
Node: Stream format27516
Node: Trailing data38257
Node: Examples40159
Ref: concat-example41341
Node: Problems42386
Node: Reference source code42920
Node: Concept index57238
Node: Introduction1210
Node: Output6491
Node: Invoking clzip8011
Ref: --trailing-error8577
Node: Quality assurance16230
Node: File format24640
Node: Algorithm27045
Node: Stream format29875
Node: Trailing data40616
Node: Examples42894
Ref: concat-example44076
Node: Problems45121
Node: Reference source code45657
Node: Concept index59974

End Tag Table