\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename lziprecover.info
@documentencoding ISO-8859-15
@settitle Lziprecover Manual
@finalout
@c %**end of header

@set UPDATED 1 October 2024
@set VERSION 1.25-pre1

@dircategory Compression
@direntry
* Lziprecover: (lziprecover).   Data recovery tool for the lzip format
@end direntry


@ifnothtml
@titlepage
@title Lziprecover
@subtitle Data recovery tool for the lzip format
@subtitle for Lziprecover version @value{VERSION}, @value{UPDATED}
@author by Antonio Diaz Diaz

@page
@vskip 0pt plus 1filll
@end titlepage

@contents
@end ifnothtml

@ifnottex
@node Top
@top

This manual is for Lziprecover (version @value{VERSION}, @value{UPDATED}).

@menu
* Introduction::            Purpose and features of lziprecover
* Invoking lziprecover::    Command-line interface
* File format::             Detailed format of the compressed file
* Data safety::             Protecting data from accidental loss
* Fec files::               Forward Error Correction
* Repairing one byte::      Fixing bit flips and similar errors
* Merging files::           Fixing several damaged copies
* Reproducing one sector::  Fixing a missing (zeroed) sector
* Tarlz::                   Options supporting the tar.lz format
* File names::              Names of the files produced by lziprecover
* Trailing data::           Extra data appended to the file
* Examples::                A small tutorial with examples
* Unzcrash::                Testing the robustness of decompressors
* Problems::                Reporting bugs
* Concept index::           Index of concepts
@end menu

@sp 1
Copyright @copyright{} 2009-2024 Antonio Diaz Diaz.

This manual is free documentation: you have unlimited permission to copy,
distribute, and modify it.
@end ifnottex


@node Introduction
@chapter Introduction
@cindex introduction

@uref{http://www.nongnu.org/lzip/lziprecover.html,,Lziprecover}
is a data recovery tool and decompressor for files in the lzip
compressed data format (.lz). Lziprecover also provides Forward Error
Correction (FEC) able to repair any kind of file.

Lziprecover is able to repair slightly damaged lzip files (up to one
single-byte error per member), produce a correct file by merging the good
parts of two or more damaged copies, reproduce a missing (zeroed) sector
using a reference file, extract data from damaged files, decompress files,
and test integrity of files.

Lziprecover can remove the damaged members from multimember files, for
example multimember tar.lz archives.

Lziprecover provides random access to the data in multimember files; it only
decompresses the members containing the desired data.

Lziprecover is not a replacement for regular backups, but a last line of
defense for the case where the backups are also damaged.

Lziprecover is able to provide unique data recovery capabilities because the
lzip format is extraordinarily safe. The simple and safe design of the file
format complements the embedded error detection provided by the LZMA data
stream. 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
possible 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.

The lzip file format is designed for data sharing and long-term archiving,
taking into account both data integrity and decoder availability:

@itemize @bullet
@item
The lzip format provides very safe integrity checking and some data
recovery means. The program lziprecover 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. @xref{Data safety}.

@item
The lzip format is as simple as possible (but not simpler). The lzip
manual provides the source code of a simple decompressor along with a
detailed explanation of how it works, so that with the only help of the
lzip manual it would be possible for a digital archaeologist to extract
the data from a lzip file long after quantum computers eventually
render LZMA obsolete.

@item
Additionally the lzip reference implementation is copylefted, which
guarantees that it will remain free forever.
@end itemize

A nice feature of the lzip format is that a corrupt byte is easier to repair
the nearer it is from the beginning of the file. Therefore, with the help of
lziprecover, losing an entire archive just because of a corrupt byte near
the beginning is a thing of the past.

Compression may be good for long-term archiving. For compressible data,
multiple compressed copies may provide redundancy in a more useful form and
may have a better chance of surviving intact than one uncompressed copy
using the same amount of storage space. This is especially true if the
format provides recovery capabilities like those of lziprecover, which is
able to find and combine the good parts of several damaged copies.

Lziprecover is able to recover or decompress files produced by any of the
compressors in the lzip family: lzip, plzip, minilzip/lzlib, clzip, and
pdlzip.

If the cause of file corruption is a damaged medium, the combination
@w{GNU ddrescue + lziprecover} is the recommended option for recovering data
from damaged lzip files. @xref{ddrescue-example}, and
@ref{ddrescue-example2}, for examples.

If a file is too damaged for lziprecover to repair it, all the recoverable
data in all members of the file can be extracted with the following command
(the resulting file may contain errors and some garbage data may be produced
at the end of each damaged member):

@example
lziprecover -cd --ignore-errors file.lz > file
@end example

When recovering data, lziprecover takes as arguments the names of the
damaged files and writes zero or more recovered files depending on the
operation selected and whether the recovery succeeded or not. The damaged
files themselves are kept unchanged.

When decompressing or testing file integrity, lziprecover behaves like lzip
or lunzip.

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.


@node Invoking lziprecover
@chapter Invoking lziprecover
@cindex invoking
@cindex options
@cindex usage
@cindex version

The format for running lziprecover is:

@example
lziprecover [@var{options}] [@var{files}]
@end example

@noindent
When decompressing or testing, a hyphen @samp{-} used as a @var{file}
argument means standard input. It can be mixed with other @var{files} and is
read just once, the first time it appears in the command line. If no file
names are specified, lziprecover decompresses from standard input to
standard output. Remember to prepend @file{./} to any file name beginning
with a hyphen, or use @samp{--}.

@noindent
lziprecover supports the following
@uref{http://www.nongnu.org/arg-parser/manual/arg_parser_manual.html#Argument-syntax,,options}:
@ifnothtml
@xref{Argument syntax,,,arg_parser}.
@end ifnothtml

@table @code
@item -h
@itemx --help
Print an informative help message describing the options and exit.

@item -V
@itemx --version
Print the version number of lziprecover on the standard output and exit.
This version number should be included in all bug reports.

@anchor{--trailing-error}
@item -a
@itemx --trailing-error
Exit with error status 2 if any remaining input is detected after
decompressing the last member. Such remaining input is usually trailing
garbage that can be safely ignored. @xref{concat-example}.

@item -A
@itemx --alone-to-lz
Convert lzma-alone files to lzip format without recompressing, just
adding a lzip header and trailer. The conversion minimizes the
dictionary size of the resulting file (and therefore the amount of
memory required to decompress it). Only streamed files with default LZMA
properties can be converted; non-streamed lzma-alone files lack the "End
Of Stream" marker required in lzip files.

The name of the converted lzip file is derived from that of the original
lzma-alone file as follows:

@multitable {filename.lzma} {becomes} {anyothername.lz}
@item filename.lzma @tab becomes @tab filename.lz
@item filename.tlz  @tab becomes @tab filename.tar.lz
@item anyothername  @tab becomes @tab anyothername.lz
@end multitable

@item -b @var{bytes}
@itemx --block-size=@var{bytes}
When creating fec files, make the FEC block size a multiple of @var{bytes},
which must be a multiple of 512 not larger than @w{1 GiB}.

@anchor{--byte-repair}
@item -B
@itemx --byte-repair
Try to repair a @var{file} with small errors (up to one single-byte error
per member). If successful, a repaired copy is written to the file
@var{file}_fixed.lz. @var{file} is not modified at all. The exit status is 0
if the file could be repaired, 2 otherwise. @xref{Repairing one byte}, for a
complete description of the byte-repair mode.

@item -c
@itemx --stdout
Write decompressed data to standard output; keep input files unchanged. This
option (or @option{-o}) is needed when reading from a named pipe (fifo) or
from a device. Use it also to recover as much of the decompressed data as
possible when decompressing a corrupt file. @option{-c} overrides @option{-o}.
@option{-c} has no effect when merging, removing members, repairing,
reproducing, splitting, testing or listing.

@item -d
@itemx --decompress
Decompress the files specified. The integrity of the files specified is
checked. If a file does not exist, can't be opened, or the destination file
already exists and @option{--force} has not been specified, lziprecover
continues decompressing the rest of the files and exits with error status 1.
If a file fails to decompress, or is a terminal, lziprecover exits
immediately with error status 2 without decompressing the rest of the files.
A terminal is considered an uncompressed file, and therefore invalid.

@item -D @var{range}
@itemx --range-decompress=@var{range}
Decompress only a range of bytes starting at decompressed byte position
@var{begin} and up to byte position @w{@var{end} - 1}. Byte positions start
at 0. This option provides random access to the data in multimember files;
it only decompresses the members containing the desired data. In order to
guarantee the correctness of the data produced, all members containing any
part of the desired data are decompressed and their integrity is checked.

@anchor{range-format}
Four formats of @var{range} are recognized, @samp{@var{begin}},
@samp{@var{begin}-@var{end}}, @samp{@var{begin},@var{size}}, and
@samp{,@var{size}}. If only @var{begin} is specified, @var{end} is taken as
the end of the file. If only @var{size} is specified, @var{begin} is taken
as the beginning of the file. The bytes produced are sent to standard output
unless the option @option{--output} is used.

@anchor{--reproduce}
@item -e
@itemx --reproduce
Try to recover a missing (zeroed) sector in @var{file} using a reference
file and the same version of lzip that created @var{file}. If successful, a
repaired copy is written to the file @var{file}_fixed.lz. @var{file} is not
modified at all. The exit status is 0 if the member containing the zeroed
sector could be repaired, 2 otherwise. Note that @var{file}_fixed.lz may
still contain errors in the members following the one repaired.
@xref{Reproducing one sector}, for a complete description of the reproduce
mode.

@item --lzip-level=@var{digit}|a|m[@var{length}]
Try only the given compression level or match length limit when reproducing
a zeroed sector. @option{--lzip-level=a} tries all the compression levels
@w{(0 to 9)}, while @option{--lzip-level=m} tries all the match length limits
@w{(5 to 273)}.

@item --lzip-name=@var{name}
Set the name of the lzip executable used by @option{--reproduce}. If
@option{--lzip-name} is not specified, @samp{lzip} is used.

@item --reference-file=@var{file}
Set the reference file used by @option{--reproduce}. It must contain the
uncompressed data corresponding to the missing compressed data of the zeroed
sector, plus some context data before and after them.

@item -f
@itemx --force
Force overwrite of output files.

@item -F create[@var{n}]|repair|test|list
@itemx --fec=create[@var{n}]|repair|test|list
Create fec files, or repair or test files using previously created fec
files, or list the contents of fec files. The argument (create, repair,
test, or list) can be abbreviated even to a single letter. Option
@option{-i} is required to repair or test a file using a corrupt fec file,
or to list a corrupt fec file. @xref{Fec files}.

@var{n} is the number of FEC blocks to be created. The amount of FEC data to
be created may also be specified as a percentage from 0.003% to 100%, or as
a number of bytes followed by a @samp{B} (4096B, 16KiB, etc). If @var{n} is
not specified, it defaults to @samp{8} (8 FEC blocks). (Because, when was
the last time you saw more than 8 bad sectors affecting the same file?)

@option{--fec=create} writes the FEC data created to @var{file}.fec unless
option @option{-c} or @option{-o} is specified. If a fec file can't be
created, lziprecover exits immediately with error status 1 without trying to
create the rest of the files.

@option{--fec=repair} and @option{--fec=test} read the FEC data from
@var{file}.fec unless @option{--fec-file} is specified. @option{--fec=repair}
writes the repaired file to @var{file}_fixed unless option @option{-c} or
@option{-o} is specified. @xref{File names}. If a file fails to repair,
lziprecover exits immediately with error status 2 without repairing the rest
of the files.

@item -0 .. -9
FEC fragmentation level. Defaults to @option{-9}. Level @option{-0} is the
fastest; it creates FEC data using GF(2^8), maybe with large blocks. Levels
@option{-1} to @option{-9} use GF(2^8) or GF(2^16) as required, with
increasing amounts of smaller blocks.

@item --fec-file=@var{file}[/]
When repairing or testing, read FEC data from @var{file}. If @var{file} ends
with a slash, it is interpreted as the name of a directory containing the
fec file(s).

@item -i
@itemx --ignore-errors
Ignore non-fatal errors.@*
Make @option{--decompress}, @option{--test}, and @option{--range-decompress}
ignore format and data errors and continue decompressing the remaining
members in the file; keep input files unchanged. For example, the commands
@w{@samp{lziprecover -cd -i file.lz > file}} or
@w{@samp{lziprecover -D0 -i file.lz > file}} decompress all the recoverable
data in all members of @file{file.lz} without having to split it first. The
@w{@samp{-cd -i}} method resyncs to the next member header after each error,
and is immune to some format errors that make @w{@samp{-D0 -i}} fail. The
range decompressed may be smaller than the range requested, because of the
errors. The exit status is set to 0 unless other errors are found (I/O
errors, for example).

Make @option{--fec=repair} and @option{--fec=test} ignore errors in the fec
file and return with exit status 0 if the repaired/protected file passes the
test, even if corrupt packets or trailing garbage are found in the fec file.
Make @option{--fec=list} ignore errors in the fec files.

Make @option{--list}, @option{--dump}, @option{--remove}, and @option{--strip}
ignore format errors. The sizes of the members with errors (especially the
last) may be wrong.

@item -k
@itemx --keep
Keep (don't delete) input files during decompression.

@item -l
@itemx --list
Print the uncompressed size, compressed size, and percentage saved of the
files specified. 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 @option{-v}, the dictionary
size, the number of members in the file, and the amount of trailing data (if
any) are also printed. With @option{-vv}, the positions and sizes of each
member in multimember files are also printed. With @option{-i}, format errors
are ignored, and with @option{-ivv}, gaps between members are shown. The
member numbers shown coincide with the file numbers produced by @option{--split}.

If any file is damaged, does not exist, can't be opened, or is not regular,
the final exit status is @w{> 0}. @option{-lq} can be used to check quickly
(without decompressing) the structural integrity of the files specified.
(Use @option{--test} to check the data integrity). @option{-alq}
additionally checks that none of the files specified contain trailing data.

@item -m
@itemx --merge
Try to produce a correct file by merging the good parts of two or more
damaged copies. If successful, a repaired copy is written to the file
@var{file}_fixed.lz. The exit status is 0 if a correct file could be
produced, 2 otherwise. @xref{Merging files}, for a complete description of
the merge mode.

@item -n @var{n}
@itemx --threads=@var{n}
Set the maximum number of worker threads for @option{--fec=create},
overriding the system's default. Valid values range from 1 to "as many as
your system can support". If this option is not used, lziprecover tries to
detect the number of processors in the system and use it as default value.
@w{@samp{lziprecover --help}} shows the system's default value.

@item -o @var{file}[/]
@itemx --output=@var{file}[/]
If repairing, place the repaired output into @var{file} instead of into
@var{file}_fixed.lz. If splitting, the names of the files produced are in
the form @file{rec01@var{file}}, @file{rec02@var{file}}, etc.

If creating FEC data and @option{-c} has not been also specified, write the
FEC data to @var{file}. If @var{file} ends with a slash, it is interpreted
as the name of a directory where the fec file(s) will be written to. In this
case, the fec file names are composed by replacing the prefix preceding the
last slash of each file name specified in the command line with @var{file}
(or prepending @var{file} if the file name does not contain a slash), and
appending the extension @file{.fec}.

Else, if @option{-c} has not been also specified, write the (de)compressed
output to @var{file}, automatically creating any missing parent directories;
keep input files unchanged. This option (or @option{-c}) is needed when
reading from a named pipe (fifo) or from a device. @w{@option{-o -}} is
equivalent to @option{-c}. @option{-o} has no effect when testing or listing.

@item -q
@itemx --quiet
Quiet operation. Suppress all messages.

@item -r
@itemx --recursive
When creating or reading fec files (but not when listing), for each directory
operand, read and process all files in that directory, recursively. Follow
symbolic links given in the command line, but skip symbolic links that are
encountered recursively. Ignore files with extension @file{.fec}, and files
and directories named @file{fec}.

@item -R
@itemx --dereference-recursive
When creating or reading fec files (but not when listing), for each directory
operand, read and process all files in that directory, recursively,
following all symbolic links. Ignore files with extension @file{.fec}, and
files and directories named @file{fec}.

@item -s
@itemx --split
Search for members in @var{file} and write each member in its own file. Gaps
between members are detected and each gap is saved in its own file. Trailing
data (if any) are saved alone in the last file. You can then use
@w{@samp{lziprecover -t}} to test the integrity of the resulting files,
decompress those which are undamaged, and try to repair or partially
decompress those which are damaged. Gaps may contain garbage or may be
members with corrupt headers or trailers. If other lziprecover functions
fail to work on a multimember @var{file} because of damage in headers or
trailers, try to split @var{file} and then work on each member individually.

The names of the files produced are in the form @file{rec01@var{file}},
@file{rec02@var{file}}, etc, and are designed so that the use of wildcards
in subsequent processing, for example,
@w{@samp{lziprecover -cd rec*@var{file} > recovered_data}}, processes the
files in the correct order. The number of digits used in the names varies
depending on the number of members in @var{file}.

@item -t
@itemx --test
Check integrity of the files specified, but don't decompress them. This
really performs a trial decompression and throws away the result. Use it
together with @option{-v} to see information about the files. If a file
fails the test, does not exist, can't be opened, or is a terminal, lziprecover
continues testing 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.

@item -v
@itemx --verbose
Verbose mode.@*
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 @option{-v} options show the progress of decompression.@*
In other modes, increasing verbosity levels show final status, progress of
operations, and extra information (for example, the failed areas).

@item --dump=[@var{member_list}][:damaged][:empty][:tdata]
Dump the members listed, the damaged members (if any), the empty members (if
any), or the trailing data (if any) of one or more regular multimember files
to standard output, or to a file if the option @option{--output} is used. If
more than one file is given, the elements dumped from all the files are
concatenated. If a file does not exist, can't be opened, or is not regular,
lziprecover continues processing the rest of the files. If the dump fails in
one file, lziprecover exits immediately without processing the rest of the
files. Only @option{--dump=tdata} can write to a terminal.
@option{--dump=damaged} implies @option{--ignore-errors}.

The argument to @option{--dump} is a colon-separated list of the following
element specifiers; a member list (1,3-6), a reverse member list (r1,3-6),
and the strings "damaged", "empty", and "tdata" (which may be shortened to
'd', 'e', and 't' respectively). A member list selects the members (or gaps)
listed, whose numbers coincide with those shown by @option{--list}. A reverse
member list selects the members listed counting from the last member in the
file (r1). Negated versions of both kinds of lists exist (^1,3-6:r^1,3-6)
which select all the members except those in the list. The strings
"damaged", "empty", and "tdata" select the damaged members, the empty
members (those with a data size = 0), and the trailing data respectively. If
the same member is selected more than once, for example by @samp{1:r1} in a
single-member file, it is dumped just once. See the following examples:

@multitable {@code{3,12:damaged:tdata}} {members 3, 12, damaged members, trailing data}
@headitem @code{--dump} argument @tab Elements dumped
@item @code{1,3-6}               @tab members 1, 3, 4, 5, 6
@item @code{r1-3}                @tab last 3 members in file
@item @code{^13,15}              @tab all but 13th and 15th members in file
@item @code{r^1}                 @tab all but last member in file
@item @code{damaged}             @tab all damaged members in file
@item @code{empty}               @tab all empty members in file
@item @code{tdata}               @tab trailing data
@item @code{1-5:r1:tdata}        @tab members 1 to 5, last member, trailing data
@item @code{damaged:tdata}       @tab damaged members, trailing data
@item @code{3,12:damaged:tdata}  @tab members 3, 12, damaged members, trailing data
@end multitable

@item --remove=[@var{member_list}][:damaged][:empty][:tdata]
Remove the members listed, the damaged members (if any), the empty members
(if any), or the trailing data (if any) from regular multimember files in
place. The date of each file modified is preserved if possible. If all
members in a file are selected to be removed, the file is left unchanged and
the exit status is set to 2. If a file does not exist, can't be opened, is
not regular, or is left unchanged, lziprecover continues processing the rest
of the files. In case of I/O error, lziprecover exits immediately without
processing the rest of the files. See @option{--dump} above for a description
of the argument.

This option may be dangerous even if only the trailing data are being
removed because the file may be corrupt or the trailing data may contain a
forbidden combination of characters. @xref{Trailing data}. It is safer to
send the output of @option{--strip} to a temporary file, check it, and then
copy it over the original file. But if you prefer @option{--remove} because of
its more efficient in-place removal, it is advisable to make a backup before
attempting the removal. At least check that @w{@samp{lzip -cd file.lz | wc -c}}
and the uncompressed size shown by @w{@samp{lzip -l file.lz}} match before
attempting the removal of trailing data.

@item --strip=[@var{member_list}][:damaged][:empty][:tdata]
Copy one or more regular multimember files to standard output (or to a file
if the option @option{--output} is used), stripping the members listed, the
damaged members (if any), the empty members (if any), or the trailing data
(if any) from each file. If all members in a file are selected to be
stripped, the trailing data (if any) are also stripped even if @samp{tdata}
is not specified. If more than one file is given, the files are
concatenated. In this case the trailing data are also stripped from all but
the last file even if @samp{tdata} is not specified. If a file does not
exist, can't be opened, or is not regular, lziprecover continues processing
the rest of the files. If a file fails to copy, lziprecover exits
immediately without processing the rest of the files. See @option{--dump}
above for a description of the argument.

@item --ignore-empty
When decompressing, testing, or listing, ignore empty members in multimember
files. By default lziprecover exits with error status 2 if any empty member
is found in a multimember file.

@item --ignore-nonzero
When decompressing or testing, ignore a nonzero first byte in the LZMA
stream. By default lziprecover exits with error status 2 if the first LZMA
byte is nonzero in any member of the input files.
Use @w{@samp{lziprecover --nonzero-repair}} to repair any such nonzero bytes.

@item --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.

@item --nonzero-repair
Repair in place a nonzero first LZMA byte in the files specified. With
@option{-v}, print the number of members repaired. The date of each file
modified is preserved if possible.

@end table

@noindent
lziprecover also supports the following debug options (for experts):

@table @code
@item -E @var{range}[,@var{sector_size}]
@itemx --debug-reproduce=@var{range}[,@var{sector_size}]
Load the compressed @var{file} into memory, set all bytes in the positions
specified by @var{range} to 0, and try to reproduce a correct compressed
file. @xref{--reproduce}. @xref{range-format}, for a description of
@var{range}. If a @var{sector_size} is specified, set each sector to 0 in
sequence and try to reproduce the file, printing to standard output final
statistics of the number of sectors reproduced successfully. Exit with
nonzero status only in case of fatal error.

@item -F dc@var{n}
@itemx --fec=dc@var{n}
Simulate FEC repair of all combinations of @var{n} zeroed block errors
spread along the whole input file.

@item -F dz@var{range}[:@var{range}]...
@itemx --fec=dz@var{range}[:@var{range}]...
Simulate FEC repair of one or more zeroed block(s) in the input file at the
@var{range}s given. The @var{range}s may be unordered and overlapping.
Lziprecover sorts and joins them as needed. @xref{range-format}, for a
description of @var{range}.

@item -F dZ@var{size}[,@var{delta}]
@itemx --fec=dZ@var{size}[,@var{delta}]
Simulate FEC repair of all possible zeroed blocks of size @var{size} in the
input file. @var{delta} defaults to @var{size}. Values of @var{delta}
smaller than @var{size} result in overlapping blocks.

@item -M
@itemx --md5sum
Print to standard output the MD5 digests of the input @var{files} one per
line in the same format produced by the @command{md5sum} tool. Lziprecover
uses MD5 digests to check the result of some operations. This option can be
used to test the correctness of lziprecover's implementation of the MD5
algorithm.

@item -S[@var{value}]
@itemx --nrep-stats[=@var{value}]
Compare the frequency of sequences of N repeated bytes of a given
@var{value} in the compressed LZMA streams of the input @var{files} with the
frequency expected for random data (1 / 2^(8N)). If @var{value} is not
specified, print the frequency of repeated sequences of all possible byte
values. Print cumulative data for all the files, followed by the name of the
first file with the longest sequence.

@anchor{--unzcrash}
@item -U 1|B@var{size}
@itemx --unzcrash=1|B@var{size}
With argument @samp{1}, test 1-bit errors in the LZMA stream of the
compressed input @var{file} like the command
@w{@samp{unzcrash -b1 -p7 -s-20 'lzip -t' @var{file}}} but in memory, and
therefore much faster (30 to 50 times faster). @xref{Unzcrash}. This option
tests all the members independently in a multimember file, skipping headers
and trailers. If a decompression succeeds, the decompressed output is
compared with the decompressed output of the original @var{file} using MD5
digests. @var{file} must not contain errors and must decompress correctly
for the comparisons to work.

With argument @samp{B}, test zeroed sectors (blocks of bytes) in the LZMA
stream of the compressed input @var{file} like the command
@w{@samp{unzcrash --block=@var{size} -d1 -p7 -s-(@var{size}+20) 'lzip -t' @var{file}}}
but in memory, and therefore much faster. Testing and comparisons work just
like with the argument @samp{1} explained above.

By default @option{--unzcrash} only prints the interesting cases; CRC
mismatches, size mismatches, unsupported marker codes, unexpected EOFs,
apparently successful decompressions, and decoder errors detected 50_000 or
more bytes beyond the byte (or the start of the block) being tested. At
verbosity level 1 (-v) it also prints decoder errors detected 10_000 or more
bytes beyond the byte being tested. At verbosity level 2 (-vv) it prints all
cases for 1-bit errors or the decoder errors detected beyond the end of the
block for zeroed blocks.

@item -W @var{position},@var{value}
@itemx --debug-decompress=@var{position},@var{value}
Load the compressed @var{file} into memory, set the byte at @var{position}
to @var{value}, and decompress the modified compressed data to standard
output. If the damaged member can be decompressed to the end (just fails
with a CRC mismatch), the members following it are also decompressed.

@item -X[@var{position},@var{value}]
@itemx --show-packets[=@var{position},@var{value}]
Load the compressed @var{file} into memory, optionally set the byte at
@var{position} to @var{value}, decompress the modified compressed data
(discarding the output), and print to standard output descriptions of the
LZMA packets being decoded.

@item -Y @var{range}
@itemx --debug-delay=@var{range}
Load the compressed @var{file} into memory and then repeatedly decompress
it, increasing 256 times each byte of the subset of the compressed data
positions specified by @var{range}, so as to test all possible one-byte
errors. For each decompression error find the error detection delay and
print to standard output the maximum delay. The error detection delay is the
difference between the position of the error and the position where the
decoder realized that the data contains an error. @xref{range-format}, for a
description of @var{range}.

@item -Z @var{position},@var{value}
@itemx --debug-byte-repair=@var{position},@var{value}
Load the compressed @var{file} into memory, set the byte at @var{position}
to @var{value}, and then try to repair the byte error. @xref{--byte-repair}.

@item --gf16
Forces the use of GF(2^16) when creating FEC blocks even if the number of
blocks fits in GF(2^8).

@end table

Numbers given as arguments to options may be expressed in decimal,
hexadecimal, or octal (using the same syntax as integer constants in C++),
and may be followed by a multiplier and an optional @samp{B} for "byte".

Table of SI and binary prefixes (unit multipliers):

@multitable {Prefix} {kilobyte   (10^3 = 1000)} {|} {Prefix} {kibibyte  (2^10 = 1024)}
@headitem Prefix @tab Value           @tab | @tab Prefix @tab Value
@item k @tab kilobyte   (10^3 = 1000) @tab | @tab Ki @tab kibibyte  (2^10 = 1024)
@item M @tab megabyte   (10^6)        @tab | @tab Mi @tab mebibyte  (2^20)
@item G @tab gigabyte   (10^9)        @tab | @tab Gi @tab gibibyte  (2^30)
@item T @tab terabyte   (10^12)       @tab | @tab Ti @tab tebibyte  (2^40)
@item P @tab petabyte   (10^15)       @tab | @tab Pi @tab pebibyte  (2^50)
@item E @tab exabyte    (10^18)       @tab | @tab Ei @tab exbibyte  (2^60)
@item Z @tab zettabyte  (10^21)       @tab | @tab Zi @tab zebibyte  (2^70)
@item Y @tab yottabyte  (10^24)       @tab | @tab Yi @tab yobibyte  (2^80)
@item R @tab ronnabyte  (10^27)       @tab | @tab Ri @tab robibyte  (2^90)
@item Q @tab quettabyte (10^30)       @tab | @tab Qi @tab quebibyte (2^100)
@end multitable

@sp 1
Exit status: 0 for a normal exit, 1 for environmental problems
(file not found, invalid command-line options, I/O errors, etc), 2 to
indicate a corrupt or invalid input file, 3 for an internal consistency
error (e.g., bug) which caused lziprecover to panic.


@node File format
@chapter File format
@cindex file format

Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away.@*
--- Antoine de Saint-Exupery

In the diagram below, a box like this:

@verbatim
+---+
|   | <-- the vertical bars might be missing
+---+
@end verbatim

represents one byte; a box like this:

@verbatim
+==============+
|              |
+==============+
@end verbatim

represents a variable number of bytes.

@noindent
A lzip file consists of one or more independent "members" (compressed data
sets). The members simply appear one after another in the file, with no
additional information before, between, or after them. Each member can
encode in compressed form up to @w{16 EiB - 1 byte} of uncompressed data.
The size of a multimember file is unlimited. Empty members (data size = 0)
are not allowed in multimember files.

Each member has the following structure:

@verbatim
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID string | VN | DS | LZMA stream | CRC32 |   Data size   |  Member size  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@end verbatim

All multibyte values are stored in little endian order.

@table @samp
@item ID string (the "magic" bytes)
A four byte string, identifying the lzip format, with the value "LZIP"
(0x4C, 0x5A, 0x49, 0x50).

@item VN (version number, 1 byte)
Just in case something needs to be modified in the future. 1 for now.

@item DS (coded dictionary size, 1 byte)
The dictionary size is calculated by taking a power of 2 (the base size)
and subtracting from it a fraction between 0/16 and 7/16 of the base size.@*
Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).@*
Bits 7-5 contain the numerator of the fraction (0 to 7) to subtract
from the base size to obtain the dictionary size.@*
Example: 0xD3 = 2^19 - 6 * 2^15 = 512 KiB - 6 * 32 KiB = 320 KiB@*
Valid values for dictionary size range from 4 KiB to 512 MiB.

@item LZMA stream
The LZMA stream, finished by an "End Of Stream" marker. Uses default values
for encoder properties.
@ifnothtml
@xref{Stream format,,,lzip},
@end ifnothtml
@ifhtml
See
@uref{http://www.nongnu.org/lzip/manual/lzip_manual.html#Stream-format,,Stream format}
@end ifhtml
for a complete description.

@item CRC32 (4 bytes)
Cyclic Redundancy Check (CRC) of the original uncompressed data.

@item Data size (8 bytes)
Size of the original uncompressed data.

@item Member size (8 bytes)
Total size of the member, including header and trailer. This field acts
as a distributed index, improves the checking of stream integrity, and
facilitates the safe recovery of undamaged members from multimember files.
Lzip limits the member size to @w{2 PiB} to prevent the data size field from
overflowing.
@end table


@node Data safety
@chapter Protecting data from accidental loss
@cindex data safety

It is a fact of life that sometimes data becomes corrupt. Software has
errors. Hardware may misbehave or fail. RAM may be struck by a cosmic ray.
This is why a safe enough integrity checking is needed in compressed
formats, and the reason why a data recovery tool is sometimes needed.

There are 3 main types of data corruption that may cause data loss:
single-byte errors, multibyte errors (generally affecting a whole sector
in a block device), and total device failure.

The two methods most effective to protect data from accidental loss are
backup copies and Forward Error Correction (FEC). Both methods can be used
simultaneously, and both are supported by lziprecover.

Lziprecover protects natively against single-byte errors as long as file
integrity is checked frequently enough that a second single-byte error does
not develop in the same member before the first one is repaired.
@xref{Repairing one byte}.

Lziprecover protects against multibyte errors in 3 cases: if a fec file is
available (@pxref{Fec files}), if at least one backup copy of the file is
available (@pxref{Merging files}), or if the error is a zeroed sector and
the uncompressed data corresponding to the zeroed sector are available
(@pxref{Reproducing one sector}). FEC is best. Else, if you can choose
between merging and reproducing, try merging first because it is usually
faster, easier to use, and has a high probability of success.

Lziprecover can't help in case of device failure. The only remedy for total
device failure is storing backup copies in separate media.

The extraordinary safety of the lzip format allows lziprecover to use the
redundance that occurs naturally when making compressed backups. Lziprecover
can recover data that would not be recoverable from files compressed in
other formats. Let's see two examples of how much better is lzip compared
with gzip and bzip2 with respect to data safety:

@menu
* Merging with a backup::   Recovering a file using a damaged backup
* Reproducing a mailbox::   Recovering new messages using an old backup
@end menu


@node Merging with a backup
@section Recovering a file using a damaged backup
@cindex merging with a backup

Let's suppose that you made a compressed backup of your valuable scientific
data and stored two copies on separate media. Years later you notice that
both copies are corrupt.

If you compressed the data with gzip and both copies suffer any damage in
the data stream, even if it is just one altered bit, the original data can
only be recovered by an expert, if at all.

If you used bzip2, and if the file is large enough to contain more than one
compressed data block (usually larger than @w{900 kB} uncompressed), and if
no block is damaged in both files, then the data can be manually recovered
by splitting the files with bzip2recover, checking every block, and then
copying the right blocks in the right order into another file.

But if you used lzip, the data can be automatically recovered with
@w{@samp{lziprecover --merge}} as long as the damaged areas don't overlap.

Note that each error in a bzip2 file makes a whole block unusable, but each
error in a lzip file only affects the damaged bytes, making it possible to
recover a file with thousands of errors.


@node Reproducing a mailbox
@section Recovering new messages using an old backup
@cindex reproducing a mailbox

Let's suppose that you make periodic backups of your email messages stored
in one or more mailboxes. (A mailbox is a file containing a possibly large
number of email messages). New messages are appended to the end of each
mailbox, therefore the initial part of two consecutive backups is identical
unless some messages have been changed or deleted in the meantime. The new
messages added to each backup are usually a small part of the whole mailbox.

@verbatim
+============================================+
| Older backup containing some messages      |
+============================================+
+============================================+========================+
| Newer backup containing the messages above | plus some new messages |
+============================================+========================+
@end verbatim

One day you discover that your mailbox has disappeared because you deleted
it inadvertently or because of a bug in your email reader. Not only that.
You need to recover a recent message, but the last backup you made of the
mailbox (the newer backup above) has lost the data corresponding to a whole
sector because of an I/O error in the part containing the old messages.

If you compressed the mailbox with gzip, usually none of the new messages
can be recovered even if they are intact because all the data beyond the
missing sector can't be decoded.

If you used bzip2, and if the newer backup is large enough that the new
messages are in a different compressed data block than the one damaged
(usually larger than @w{900 kB} uncompressed), then you can recover the new
messages manually with bzip2recover. If the backups are identical except for
the new messages appended, you may even recover the whole newer backup by
combining the good blocks from both backups.

But if you used lzip, the whole newer backup can be automatically recovered
with @w{@samp{lziprecover --reproduce}} as long as the missing bytes can be
recovered from the older backup, even if other messages in the common part
have been changed or deleted. Mailboxes seem to be especially easy to
reproduce. The probability of reproducing a mailbox
(@pxref{performance-of-reproduce}) is almost as high as that of merging two
identical backups (@pxref{performance-of-merge}).


@node Fec files
@chapter Forward Error Correction
@cindex forward error correction

"Forward Error Correction" (FEC) is any way of protecting data from
corruption by creating redundant data that can be used later to repair
errors in the protected data. Lziprecover uses a Hilbert-based Reed-Solomon
code to create one fec file (with extension @file{.fec}) for each file that
needs to be protected. The fec files created by lziprecover are
reproducible.

Reed-Solomon is the most space-efficient Error Correcting Code (ECC) for
data stored in block devices. It creates redundant FEC blocks in such a way
that X FEC blocks allow the recuperation of any combination of up to X lost
data blocks. All the blocks (data and FEC) are of the same size, which in
fec files must be a multiple of 512 bytes. Reed-Solomon is not optimum for
corruption affecting random single bits in a file because each corrupt bit
invalidates the whole block containing it. But in block devices, scattered
bit flips should not happen.

Usually, a corrupt file does not provide an indication of where the
corruption is located. Therefore, each fec file stores one or two arrays of
CRCs to detect the corrupt blocks in the protected file and mark them as
erasures (missing data blocks). Thus, a fec file creates its own Binary
Erasure Channel (BEC) for the protected file.

Lziprecover's FEC algorithm can repair any kind of file, but its ability to
repair lzip files is greater than for other kinds of files. Lziprecover can
use the statistical properties of lzip data to repair a lzip file rescued
with ddrescue, even if the fec file is so damaged that it has lost both CRC
arrays. Lzip data helps to locate the corrupt parts of the file even without
a BEC. For this to work, at least one chksum packet header must be intact to
provide @samp{prodata_size}, @samp{prodata_md5}, and @samp{gf16}.

@menu
* How Reed-Solomon works::    It is basically an equation system
* Implementation details::    How lziprecover implements Reed-Solomon
* Creating fec files::        How to create fec files
* Testing with fec files::    How to test files using fec files
* Repairing with fec files::  How to repair files using fec files
* Fec file format::           Detailed format of the redundant FEC data
@end menu


@node How Reed-Solomon works
@section How Reed-Solomon works
@cindex Reed-Solomon tutorial

To illustrate how Reed-Solomon works on the BEC, we will use an example with
standard arithmetic on integers. Note that in lziprecover's FEC each
variable is a (potentialy large) block of data, not a single value.

Given variables x, y, and z (the protected data) whose values are known, an
equation system can be created where the values of three FEC variables p, q,
and r can be computed from the values of x, y, and z:

@example
x +  y +  z = p        (1)
x + 2y + 3z = q        (2)
x + 3y + 2z = r        (3)
@end example

If we have that x = 1, y = 2, and z = 3, then p = 6, q = 14, and r = 13:

@example
1 + 2 + 3 =  6        (1a)
1 + 4 + 9 = 14        (2a)
1 + 6 + 6 = 13        (3a)
@end example

Now, if the values of x and y are lost because of data corruption, they can
be recomputed by using any two of the three equations above. For example, if
we replace the known values of z, p, q, and r in equations (1) and (2) we
get:

@example
x +  y + 3 =  6       (1b)
x + 2y + 9 = 14       (2b)
@end example

In order to solve the two equations above, we first reduce them by
subtracting the values of the known data variables from the values of the
FEC variables:

@example
x +  y =  6 - 3       (1c)
x + 2y = 14 - 9       (2c)
@end example

which gives the reduced FEC values P = 3 and Q = 5.

Then we create a square matrix @samp{A} with the coefficients of x and y in
the equations above, and invert it. @samp{A} must be invertible and must not
have any zero element. We also create the column vector D with the missing
data variables x and y, and the column vector F with the reduced FEC values
P and Q:

@example
D = x        A = 1  1        A^-1 =  2 -1        F = P
    y            1  2               -1  1            Q
@end example

Then we multiply the inverse matrix @samp{A^-1} by the column vector F to
obtain the values of x and y (D = A^-1 * F):

@example
x = 2P - Q            (1d)
y = -P + Q            (2d)
@end example

which finally gives us the lost values x = 1 and y = 2:

@example
x = 2 * 3 - 5         (1e)
y = -3 + 5            (2e)
@end example


@node Implementation details
@section How lziprecover implements Reed-Solomon
@cindex Reed-Solomon details

Lziprecover's implementation of Reed-Solomon can manage up to 128 data
blocks + 128 FEC blocks when using a Galois Field of size 256 (GF(2^8)), or
up to 32768 data blocks + 32768 FEC blocks when using a Galois Field of size
65536 (GF(2^16)). GF(2^8) is included because it is faster for files up to
about @w{1 MB}. The number of FEC blocks is currently limited to 2048
because of memory and time limits. Inverting a matrix for 32768 FEC blocks
would take a week and require @w{2 GiB} of RAM.

The file is repaired in memory. Therefore, enough virtual memory
@w{(RAM + swap)} to contain the protected file and the FEC data is required.
The file size is limited to less than @w{2 GiB} on 32-bit systems. The
repaired file is checked with a MD5 digest.

Lziprecover divides the input file in 1 to 32768 data blocks of the same
size, which ranges from 512 bytes to @w{128 TiB}, for a total protected file
size of up to @w{4 EiB}. It then uses a Hilbert matrix @samp{A} to create up
to 2048 FEC blocks of the same size as the data blocks. Lziprecover corrects
errors in the data blocks by first reducing the equation system to M
equations with M unknowns each, where M is the number of missing data
blocks. Then it multiplies the inverse of the relevant submatrix of @samp{A}
by the vector of results of the M equations to recompute the values of the
missing data blocks.

Lziprecover implements GF(2^8) with polynomial 0x11D and GF(2^16) with
polynomial 0x1100B.

A Hilbert matrix is defined as @w{@samp{A[i][j] = 1 / (i + j + 1)}} for i
and j >= 0. But as in a Galois Field addition is exclusive or, applying the
Hilbert definition produces a singular (non invertible) matrix. To avoid
this problem, lziprecover uses a Hilbert matrix starting at row
@w{@samp{gf_size / 2}}. I.e., @w{@samp{A[i][j] = 1 / (i + gf_size / 2 + j)}}
for @w{@samp{0 <= i,j < gf_size / 2}}. (gf_size is the size of the Galois
Field).


@node Creating fec files
@section How to create fec files
@cindex fec create

@noindent
Example 1: Create the fec file @file{archive.tar.lz.fec} and store it in the
same directory where @file{archive.tar.lz} is.

@example
lziprecover -v -Fc archive.tar.lz
@end example

@noindent
Example 2: Create the fec file @file{archive.tar.lz.fec} and store it in the
directory @file{fec}.

@example
lziprecover -v -Fc -o fec/ archive.tar.lz
@end example

@noindent
Example 3: Create recursively one fec file for each file in the directory
@file{datadir} and store them in the tree under the directory @file{fec}.

@example
lziprecover -v -r -Fc -o fec/ datadir
@end example


@node Testing with fec files
@section How to test files using fec files
@cindex fec test

@noindent
Example 1: Test the integrity of @file{archive.tar.lz} using the fec file
@file{archive.tar.lz.fec} from the same directory.

@example
lziprecover -v -Ft archive.tar.lz
@end example

@noindent
Example 2: Test the integrity of the files @file{foo.lz} and @file{bar.lz}
using the corresponding fec files stored in the directory @file{fec}.

@example
lziprecover -v -Ft --fec-file=fec/ foo.lz bar.lz
@end example

@noindent
Example 3: Test recursively the integrity of all the files in the directory
@file{datadir} using the fec files stored in the directory tree under the
directory @file{fec}.

@example
lziprecover -v -r -Ft --fec-file=fec/ datadir
@end example


@node Repairing with fec files
@section How to repair files using fec files
@cindex fec repair

@noindent
Example 1: Repair the file @file{archive.tar.lz} using the fec file
@file{archive.tar.lz.fec} from the same directory. The repaired file is
written to @file{archive_fixed.tar.lz} in the same directory.

@example
lziprecover -v -Fr archive.tar.lz
@end example

@noindent
Example 2: Repair the files @file{foo.lz} and @file{bar.lz} using the
corresponding fec files stored in the directory @file{fec}.

@example
lziprecover -v -Fr --fec-file=fec/ foo.lz bar.lz
@end example

@noindent
Example 3: Repair recursively all the damaged files in the directory
@file{datadir} using the fec files stored in the directory tree under the
directory @file{fec}.

@example
lziprecover -v -r -Fr --fec-file=fec/ datadir
@end example


@node Fec file format
@section Fec file format
@cindex fec file format

A fec file consists of one chksum packet, one or more fec packets, and one
optional second chksum packet. The first chksum packet must be the first
packet in the file, but the second chksum packet does not need to be the
last packet in the file. The essential information is stored in the chksum
packet(s), while the potentially numerous fec packets are kept as simple as
possible:

@verbatim
+=================+===============+=================+
|  Chksum packet  |  Fec packets  |  Chksum packet  |
+=================+===============+=================+
@end verbatim

All multibyte values are stored in little endian order except
@samp{prodata_md5}.

The @samp{fbs} (fec_block_size) field is coded as a little endian 16-bit
floating point unsigned integer with an 11-bit mantissa at bits 0-10 and a
5-bit exponent at bits 11-15. The mantissa is an integer between 0 and 2047.
The exponent is an integer between 9 and 40, stored with a bias of -9; the
exponent 9 is stored as 0, and 40 is stored as 31. Values are stored with
the largest mantissa and smallest exponent; 4096 is stored as m=8, e=0. This
encoding can store values from 0 bytes to @w{2047 TiB} @w{(2^51 - 2^40 bytes)}
with a maximum resolution of 512 bytes, but 0 and the values beyond
@w{128 TiB} are not used:

@verbatim
     5              11
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   exp   |       mantissa      |      The 'fbs' (fec_block_size) field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
15      11 10                  0
@end verbatim

The fec file format is 4-byte aligned for speed because FEC data are created
and decoded 4 bytes at a time. The 4-byte alignment has been achieved by a
careful design, without adding any padding bytes.

The fec file format has an overhead of 8 bytes per protected data block,
plus 16 bytes per FEC block, plus 80 bytes.

@subsection Chksum packet
@cindex chksum packet

A chksum packet contains one CRC for each of the N data blocks in the
protected file, and is structured as shown in the following table. All
lengths and offsets are in decimal:

@multitable {prodata_size} {36 + 4N} {Length (in bytes)}
@headitem Field Name @tab Offset @tab Length (in bytes)
@item magic        @tab  0      @tab  4
@item version      @tab  4      @tab  1
@item flags        @tab  5      @tab  1
@item fbs          @tab  6      @tab  2
@item prodata_size @tab  8      @tab  8
@item prodata_md5  @tab 16      @tab 16
@item header_crc   @tab 32      @tab  4
@item crc_array    @tab 36      @tab 4N
@item payload_crc  @tab 36 + 4N @tab  4
@end multitable

@table @samp
@item magic
A four byte string identifying the chksum packet (and therefore the fec
file), with the value 0xB3, 0xA5, 0xB6, 0xAF. (The complement of "LZIP").

@item version
Just in case something needs to be modified in the future. 0 for now.

@item flags
Bit 0 (is_crc_c): crc_array contains CRC32 (0) or CRC32-C (1).@*
Bit 1 (gf16): Galois field is GF(2^8) (0) or GF(2^16) (1).@*
Bits 2-7: zero.

@anchor{fbs}
@item fbs (coded fec_block_size)
Number of FEC bytes per block. It is a multiple of 512 bytes between 512
bytes and @w{128 TiB}.

@item prodata_size
Size of the protected file. 1 byte to @w{4 EiB}.

@item prodata_md5
Md5sum of the protected file. Stored in big endian order.

@item header_crc
CRC32 of the previous fields, including magic.

@item crc_array
Array of @var{n} CRCs corresponding to the @var{n} blocks in which the
protected file is divided. @var{n} is @w{@samp{ceil( prodata_size / fbs )}}.
The first chksum packet contains an array of CRC32s, while the second chksum
packet (if present) contains an array of CRC32-Cs.

For the expected thousands of bit flips caused by a zeroed sector, a
"symmetric" CRC like CRC32 is probably better than CRC32-C, which detects
all the errors with an odd number of bit flips at the expense of a larger
number of undetected errors with an even number of bit flips.

@item payload_crc
CRC32 of the crc_array.
@end table

@subsection Fec packet
@cindex fec packet

A fec packet contains one FEC block and is structured as shown in the
following table. All lengths and offsets are in decimal:

@multitable {payload_crc} {12 + fbs} {Length (in bytes)}
@headitem Field Name @tab Offset @tab Length (in bytes)
@item magic        @tab  0       @tab 4
@item fbn          @tab  4       @tab 2
@item fbs          @tab  6       @tab 2
@item header_crc   @tab  8       @tab 4
@item fec_block    @tab 12       @tab fbs
@item payload_crc  @tab 12 + fbs @tab 4
@end multitable

@table @samp
@item magic
A four byte string identifying the fec packet, with the value "\xB3FEC"
(0xB3, 0x46, 0x45, 0x43).

@item fbn (fec_block_number)
Number of this FEC block. Required to compute the decode matrix.

@item fbs (coded fec_block_size)
@xref{fbs}.

@item header_crc
CRC32 of the previous fields, including magic.

@item fec_block
The FEC block.

@item payload_crc
CRC32 of the fec_block.
@end table


@node Repairing one byte
@chapter Repairing one byte
@cindex repairing one byte

Lziprecover can repair perfectly most files with small errors (up to one
single-byte error per member), without the need of any extra redundance at
all. If the reparation is successful, the repaired file is identical bit for
bit to the original. This makes lzip files resistant to bit flip, one of the
most common forms of data corruption.

The file is repaired in memory. Therefore, enough virtual memory
@w{(RAM + swap)} to contain the largest damaged member is required. Member
size is limited to @w{2 GiB} on 32-bit systems.

The error may be located anywhere in the file except in the first 5
bytes of each member header or in the @samp{Member size} field of the
trailer (last 8 bytes of each member). If the error is in the header it
can be easily repaired with a text editor like GNU Moe (@pxref{File
format}). If the error is in the member size, it is enough to ignore the
message about @samp{bad member size} when decompressing.

Bit flip happens when one bit in the file is changed from 0 to 1 or vice
versa. It may be caused by bad RAM or even by natural radiation. I have
seen a case of bit flip in a file stored on an USB flash drive.

One byte may seem small, but most file corruptions not produced by
transmission errors or I/O errors just affect one byte, or even one bit,
of the file. Also, unlike magnetic media, where errors usually affect a
whole sector, solid-state storage devices tend to produce single-byte
errors, making of lzip the perfect format for data stored on such devices.

Repairing a file can take some time. Small files or files with the error
located near the beginning can be repaired in a few seconds. But
repairing a large file compressed with a large dictionary size and with
the error located far from the beginning, may take hours.

On the other hand, errors located near the beginning of the file cause
much more loss of data than errors located near the end. So lziprecover
repairs more efficiently the worst errors.


@node Merging files
@chapter Merging files
@cindex merging files

If you have several copies of a file but all of them are too damaged to
repair them individually (@pxref{Repairing one byte}), lziprecover can try
to produce a correct file by merging the good parts of the damaged copies.

The merge may succeed even if some copies of the file have all the headers
and trailers damaged, as long as there is at least one copy of every header
and trailer intact, even if they are in different copies of the file.

The merge fails if the damaged areas overlap (at least one byte is damaged
in all copies), or are adjacent and the boundary can't be determined, or if
the copies have too many damaged areas.

All the copies to be merged must have the same size. If any of them is
larger or smaller than it should, either because it has been truncated or
because it got some garbage data appended at the end, it can be brought to
the correct size with the following command before merging it with the other
copies:

@example
ddrescue -s<correct_size> -x<correct_size> file.lz correct_size_file.lz
@end example

@anchor{performance-of-merge}
To give you an idea of its possibilities, when merging two copies, each of
them with one damaged area affecting 1 percent of the copy, the probability
of obtaining a correct file is about 98 percent. With three such copies the
probability rises to 99.97 percent. For large files (a few MB) with small
errors (one sector damaged per copy), the probability approaches 100 percent
even with only two copies. (Supposing that the errors are randomly located
inside each copy).

Some types of solid-state device (NAND flash, for example) can produce
bursts of scattered single-bit errors. Lziprecover is able to merge
files with thousands of such scattered errors by grouping the errors
into clusters and then merging the files as if each cluster were a
single error.

Here is a real case of successful merging. Two copies of the file
@file{icecat-3.5.3-x86.tar.lz} (compressed size @w{9 MB}) became corrupt
while stored on the same NAND flash device. One of the copies had 76
single-bit errors scattered in an area of 1020 bytes, and the other had
3028 such errors in an area of 31729 bytes. Lziprecover produced a
correct file, identical to the original, in just 5 seconds:

@example
lziprecover -vvm a/icecat-3.5.3-x86.tar.lz b/icecat-3.5.3-x86.tar.lz
Merging member 1 of 1  (2552 errors)
  2552 errors have been grouped in 16 clusters.
  Trying variation 2 of 2, block 2
Input files merged successfully.
@end example

Note that the number of errors reported by lziprecover (2552) is lower
than the number of corrupt bytes (3104) because contiguous corrupt bytes
are counted as a single multibyte error.

@sp 1
@anchor{ddrescue-example}
@noindent
Example 1: Recover a compressed backup from two copies on CD-ROM with
error-checked merging of copies.
@ifnothtml
@xref{Top,GNU ddrescue manual,,ddrescue},
@end ifnothtml
@ifhtml
See the
@uref{http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html,,ddrescue manual}
@end ifhtml
for details about ddrescue.

@example
ddrescue -d -r1 -b2048 /dev/cdrom cdimage1 mapfile1
mount -t iso9660 -o loop,ro cdimage1 /mnt/cdimage
cp /mnt/cdimage/backup.tar.lz rescued1.tar.lz
umount /mnt/cdimage
  (insert second copy in the CD drive)
ddrescue -d -r1 -b2048 /dev/cdrom cdimage2 mapfile2
mount -t iso9660 -o loop,ro cdimage2 /mnt/cdimage
cp /mnt/cdimage/backup.tar.lz rescued2.tar.lz
umount /mnt/cdimage
lziprecover -m -v -o backup.tar.lz rescued1.tar.lz rescued2.tar.lz
  Input files merged successfully.
lziprecover -tv backup.tar.lz
  backup.tar.lz: ok
@end example

@sp 1
@noindent
Example 2: Recover the first volume of those created with the command
@w{@samp{lzip -b 32MiB -S 650MB big_db}} from two copies,
@file{big_db1_00001.lz} and @file{big_db2_00001.lz}, with member 07
damaged in the first copy, member 18 damaged in the second copy, and
member 12 damaged in both copies. The correct file produced is saved in
@file{big_db_00001.lz}.

@example
lziprecover -m -v -o big_db_00001.lz big_db1_00001.lz big_db2_00001.lz
  Input files merged successfully.
lziprecover -tv big_db_00001.lz
  big_db_00001.lz: ok
@end example


@node Reproducing one sector
@chapter Reproducing one sector
@cindex reproducing one sector

Lziprecover can recover a zeroed sector in a lzip file by concatenating the
decompressed contents of the file up to the beginning of the zeroed sector
and the uncompressed data corresponding to the zeroed sector, and then
feeding the concatenated data to the same version of lzip that created the
file. For this to work, a reference file is required containing the
uncompressed data corresponding to the missing compressed data of the zeroed
sector, plus some context data before and after them. It is possible to
recover a large file using just a few kB of reference data.

The difficult part is finding a suitable reference file. It must contain the
exact data required (possibly mixed with other data). Containing similar
data is not enough.

A zeroed sector may be caused by the incomplete recovery of a damaged
storage device (with I/O errors) using, for example, ddrescue. The
reproduction can't be done if the zeroed sector overlaps with the first 15
bytes of a member, or if the zeroed sector is smaller than 8 bytes.

The file is reproduced in memory. Therefore, enough virtual memory
@w{(RAM + swap)} to contain the damaged member is required. Member size is
limited to @w{2 GiB} on 32-bit systems.

To understand how it works, take any lzipped file, say @file{foo.lz},
decompress it (keeping the original), and try to reproduce an artificially
zeroed sector in it by running the following commands:

@example
lzip -kd foo.lz
lziprecover -vv --debug-reproduce=65536,512 --reference-file=foo foo.lz
@end example

@noindent
which should produce an output like the following:

@example
Reproducing:    foo.lz
Reference file: foo
Testing sectors of size 512 at file positions 65536 to 66047
  (master mpos = 65536, dpos = 296892)
foo: Match found at offset 296892
Reproduction succeeded at pos 65536

        1 sectors tested
        1 reproductions returned with zero status
          all comparisons passed
@end example

Using @file{foo} as reference file guarantees that any zeroed sector in
@file{foo.lz} can be reproduced because both files contain the same data. In
real use, the reference file needs to contain the data corresponding to the
zeroed sector, but the rest of the data (if any) may differ between both
files. The reference data may be obtained from the partial decompression of
the damaged file itself if it contains repeated data. For example if the
damaged file is a compressed tarball containing several partially modified
versions of the same file.

The offset reported by lziprecover is the position in the reference file of
the first byte that could not be decompressed. This is the first byte that
will be compressed to reproduce the zeroed sector.

The reproduce mode tries to reproduce the missing compressed data originally
present in the zeroed sector. It is based on the perfect reproducibility of
lzip files (lzip produces identical compressed output from identical input).
Therefore, the same version of lzip that created the file to be reproduced
should be used to reproduce the zeroed sector. Near versions may also work
because the output of lzip changes infrequently. If reproducing a tar.lz
archive created with tarlz, the version of lzip, clzip, or minilzip
corresponding to the version of the lzlib library used by tarlz to create
the archive should be used.

When recovering a tar.lz archive and using as reference a file from the
filesystem, if the zeroed sector encodes (part of) a tar header, the archive
can't be reproduced. Therefore, the less overhead (smaller headers) a tar
archive has, the more probable is that the zeroed sector does not include a
header, and that the archive can be reproduced. The tarlz format has minimum
overhead. It uses basic ustar headers, and only adds extended pax headers
when they are required.

@anchor{performance-of-reproduce}
@section Performance of @option{--reproduce}

Reproduce mode is especially useful when recovering a corrupt backup (or a
corrupt source tarball) that is part of a series. Usually only a small
fraction of the data changes from one backup to the next or from one version
of a source tarball to the next. This makes sometimes possible to reproduce
a given corrupted version using reference data from a near version. The
following two tables show the fraction of reproducible sectors (reproducible
sectors divided by total sectors in archive) for some archives, using sector
sizes of 512 and 4096 bytes. @file{mailbox-aug.tar.lz} is a backup of some
of my mailboxes. @file{backup-feb.tar.lz} and @file{backup-apr.tar.lz} are
real backups of my own working directory:

@multitable {Reference file} {gawk-5.0.1.tar.lz} {4369 / 5844 = 74.76%}
@headitem Reference file @tab File @tab Reproducible (512)
@item backup-feb.tar @tab backup-apr.tar.lz @tab 3273 / 4342 = 75.38%
@item backup-apr.tar @tab backup-feb.tar.lz @tab 3259 / 4161 = 78.32%
@item gawk-5.0.0.tar @tab gawk-5.0.1.tar.lz @tab 4369 / 5844 = 74.76%
@item gawk-5.0.1.tar @tab gawk-5.0.0.tar.lz @tab 4379 / 5603 = 78.15%
@item gmp-6.1.1.tar @tab gmp-6.1.2.tar.lz @tab 2454 / 3787 = 64.8%
@item gmp-6.1.2.tar @tab gmp-6.1.1.tar.lz @tab 2461 / 3782 = 65.07%
@end multitable

@multitable {mailbox-mar.tar} {mailbox-aug.tar.lz} {4036 / 4252 = 94.92%}
@headitem Reference file @tab File @tab Reproducible (4096)
@item mailbox-mar.tar @tab mailbox-aug.tar.lz @tab 4036 / 4252 = 94.92%
@item backup-feb.tar @tab backup-apr.tar.lz @tab 264 / 542 = 48.71%
@item backup-apr.tar @tab backup-feb.tar.lz @tab 264 / 520 = 50.77%
@item gawk-5.0.0.tar @tab gawk-5.0.1.tar.lz @tab 327 / 730 = 44.79%
@item gawk-5.0.1.tar @tab gawk-5.0.0.tar.lz @tab 326 / 700 = 46.57%
@item gmp-6.1.1.tar @tab gmp-6.1.2.tar.lz @tab 175 / 473 = 37%
@item gmp-6.1.2.tar @tab gmp-6.1.1.tar.lz @tab 181 / 472 = 38.35%
@end multitable

Note that the "performance of reproduce" is a probability, not a partial
recovery. The data are either recovered fully (with the probability X shown
in the last column of the tables above) or not recovered at all (with
probability @w{1 - X}).

@noindent
Example 1: Recover a damaged source tarball with a zeroed sector of 512
bytes at file position 1019904, using as reference another source tarball
for a different version of the software.

@example
lziprecover -vv -e --reference-file=gmp-6.1.1.tar gmp-6.1.2.tar.lz
Reproducing bad area in member 1 of 1
  (begin = 1019904, size = 512, value = 0x00)
  (master mpos = 1019904, dpos = 6292134)
warning: gmp-6.1.1.tar: Partial match found at offset 6277798, len 8716.
Reference data may be mixed with other data.
Trying level -9
  Reproducing position 1015808
Member reproduced successfully.
Copy of input file reproduced successfully.
@end example

@sp 1
@anchor{ddrescue-example2}
@noindent
Example 2: Recover a damaged backup with a zeroed sector of 4096 bytes at
file position 1019904, using as reference a previous backup. The damaged
backup comes from a damaged partition copied with ddrescue.

@example
ddrescue -b4096 -r10 /dev/sdc1 hdimage mapfile
mount -o loop,ro hdimage /mnt/hdimage
cp /mnt/hdimage/backup.tar.lz backup.tar.lz
umount /mnt/hdimage
lzip -t backup.tar.lz
  backup.tar.lz: Decoder error at pos 1020530
lziprecover -vv -e --reference-file=old_backup.tar backup.tar.lz
Reproducing bad area in member 1 of 1
  (begin = 1019904, size = 4096, value = 0x00)
  (master mpos = 1019903, dpos = 5857954)
warning: old_backup.tar: Partial match found at offset 5743778, len 9546.
Reference data may be mixed with other data.
Trying level -9
  Reproducing position 1015808
Member reproduced successfully.
Copy of input file reproduced successfully.
@end example

@sp 1
@noindent
Example 3: Recover a damaged backup with a zeroed sector of 4096 bytes at
file position 1019904, using as reference a file from the filesystem. (If
the zeroed sector encodes (part of) a tar header, the tarball can't be
reproduced).

@example
# List the contents of the backup tarball to locate the damaged member.
tarlz -n0 -tvf backup.tar.lz
  [...]
  example.txt
tarlz: Skipping to next header.
tarlz: backup.tar.lz: Archive ends unexpectedly.
# Find in the filesystem the last file listed and use it as reference.
lziprecover -vv -e --reference-file=/somedir/example.txt backup.tar.lz
Reproducing bad area in member 1 of 1
  (begin = 1019904, size = 4096, value = 0x00)
  (master mpos = 1019903, dpos = 5857954)
/somedir/example.txt: Match found at offset 9378
Trying level -9
  Reproducing position 1015808
Member reproduced successfully.
Copy of input file reproduced successfully.
@end example

If @file{backup.tar.lz} is a multimember file with more than one member
damaged and lziprecover shows the message @samp{One member reproduced. Copy
of input file still contains errors.}, the procedure shown in the example
above can be repeated until all the members have been reproduced.

@samp{tarlz --keep-damaged -n0 -xf backup.tar.lz example.txt} produces a
partial copy of the reference file @file{example.txt} that may help locate a
complete copy in the filesystem or in another backup, even if
@file{example.txt} has been renamed.


@node Tarlz
@chapter Options supporting the tar.lz format
@cindex tarlz

@uref{http://www.nongnu.org/lzip/manual/tarlz_manual.html,,Tarlz} is a
massively parallel (multi-threaded) combined implementation of the tar
archiver and the
@uref{http://www.nongnu.org/lzip/manual/lzip_manual.html,,lzip} compressor.

Tarlz creates tar archives using a simplified and safer variant of the POSIX
pax format compressed in lzip format, keeping the alignment between tar
members and lzip members. The resulting multimember tar.lz archive is
backward compatible with standard tar tools like GNU tar, which treat it
like any other tar.lz archive.
@ifnothtml
@xref{Top,tarlz manual,,tarlz}, and @ref{Top,lzip manual,,lzip}.
@end ifnothtml

Multimember tar.lz archives have some safety advantages over solidly
compressed tar.lz archives. For example, in case of corruption, tarlz can
extract all the undamaged members from the tar.lz archive, skipping over the
damaged members, just like the standard (uncompressed) tar. Keeping the
alignment between tar members and lzip members minimizes the amount of data
lost in case of corruption. In this chapter we'll explain the ways in which
lziprecover can recover and process multimember tar.lz archives.

@section Recovering damaged multimember tar.lz archives

If you have several copies of the damaged archive, try merging them first
because merging has a high probability of success. @xref{Merging files}. If
the command below prints something like
@w{@samp{Input files merged successfully.}} you are done and
@file{archive.tar.lz} now contains the recovered archive:

@example
lziprecover -m -v -o archive.tar.lz a/archive.tar.lz b/archive.tar.lz
@end example

If you only have one copy of the damaged archive with a zeroed block of data
caused by an I/O error, you may try to reproduce the archive.
@xref{Reproducing one sector}. If the command below prints something like
@w{@samp{Copy of input file reproduced successfully.}} you are done and
@file{archive_fixed.tar.lz} now contains the recovered archive:

@example
lziprecover -vv -e --reference-file=old_archive.tar archive.tar.lz
@end example

If you only have one copy of the damaged archive, you may try to repair the
archive, but this has a lower probability of success. @xref{Repairing one
byte}. If the command below prints something like
@w{@samp{Copy of input file repaired successfully.}} you are done and
@file{archive_fixed.tar.lz} now contains the recovered archive:

@example
lziprecover -v --byte-repair archive.tar.lz
@end example

If all the above fails, and the archive was created with tarlz, you may save
the damaged members for later and then copy the good members to another
archive. If the two commands below succeed, @file{bad_members.tar.lz} will
contain all the damaged members and @file{archive_cleaned.tar.lz} will
contain a good archive with the damaged members removed:

@example
lziprecover -v --dump=damaged -o bad_members.tar.lz archive.tar.lz
lziprecover -v --strip=damaged -o archive_cleaned.tar.lz archive.tar.lz
@end example

You can then use @samp{tarlz --keep-damaged} to recover as much data as
possible from each damaged member in @file{bad_members.tar.lz}:

@example
mkdir tmp
cd tmp
tarlz --keep-damaged -xvf ../bad_members.tar.lz
@end example

@section Processing multimember tar.lz archives

Lziprecover is able to copy a list of members from a file to another.
For example the command
@w{@samp{lziprecover --dump=1-10:r1:tdata archive.tar.lz > subarch.tar.lz}}
creates a subset archive containing the first ten members, the end-of-file
blocks, and the trailing data (if any) of @file{archive.tar.lz}. The
@samp{r1} part selects the last member, which in an appendable tar.lz
archive contains the end-of-file blocks.


@node File names
@chapter Names of the files produced by lziprecover
@cindex file names

The name of the fixed file produced by @option{--byte-repair} and
@option{--merge} is made by appending the string @file{_fixed.lz} to the
original file name. If the original file name ends with one of the
extensions @file{.tar.lz}, @file{.lz}, or @file{.tlz}, the string
@file{_fixed} is inserted before the extension.

The name of the fixed file produced by @option{--fec=repair} is made by
appending the string @file{_fixed} to the original file name. If the
original file name ends with one of the extensions @file{.tar.lz}, @file{.lz},
or @file{.tlz}, the string @file{_fixed} is inserted before the extension.


@node Trailing data
@chapter Extra data appended to the file
@cindex trailing data

Sometimes extra data are found appended to a lzip file after the last
member. Such trailing data may be:

@itemize @bullet
@item
Padding added to make the file size a multiple of some block size, for
example when writing to a tape. It is safe to append any amount of
padding zero bytes to a lzip file.

@item
Useful data added by the user; an "End Of File" string (to check that the
file has not been truncated), 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 none of the first four bytes of the text matches 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.

@item
Garbage added by some not totally successful copy operation.

@item
Malicious data added to the file in order to make its total size and
hash value (for a chosen hash) coincide with those of another file.

@item
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 lziprecover 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 @option{--trailing-error} guarantees that
any corrupt header is detected.
@end itemize

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
possible in the presence of trailing data.

Trailing data can be safely ignored in most cases. In some cases, like
that of user-added data, they are expected to be ignored. In those cases
where a file containing trailing data must be rejected, the option
@option{--trailing-error} can be used. @xref{--trailing-error}.

Lziprecover facilitates the management of metadata stored as trailing
data in lzip files. See the following examples:

@noindent
Example 1: Add a comment or description to a compressed file.

@example
# First append the comment as trailing data to a lzip file
echo 'This file contains this and that' >> file.lz
# This command prints the comment to standard output
lziprecover --dump=tdata file.lz
# This command outputs file.lz without the comment
lziprecover --strip=tdata file.lz > stripped_file.lz
# This command removes the comment from file.lz
lziprecover --remove=tdata file.lz
@end example

@sp 1
@noindent
Example 2: Add and check a cryptographically secure hash. (This may be
convenient, but a separate copy of the hash must be kept in a safe place
to guarantee that both file and hash have not been maliciously replaced).

@example
sha256sum < file.lz >> file.lz
lziprecover --strip=tdata file.lz | sha256sum -c \
  <(lziprecover --dump=tdata file.lz)
@end example


@node Examples
@chapter A small tutorial with examples
@cindex examples

Example 1: Extract all the files from archive @file{foo.tar.lz}.

@example
  tar -xf foo.tar.lz
or
  lziprecover -cd foo.tar.lz | tar -xf -
@end example

@noindent
Example 2: Restore a regular file from its compressed version
@file{file.lz}. If the operation is successful, @file{file.lz} is removed.

@example
lziprecover -d file.lz
@end example

@noindent
Example 3: Check the integrity of the compressed file @file{file.lz} and
show status.

@example
lziprecover -tv file.lz
@end example

@anchor{concat-example}
@noindent
Example 4: The right way of concatenating the decompressed output of two or
more compressed files. @xref{Trailing data}.

@example
Don't do this
  cat file1.lz file2.lz file3.lz | lziprecover -d -
Do this instead
  lziprecover -cd file1.lz file2.lz file3.lz
You may also concatenate the compressed files like this
  lziprecover --strip=tdata file1.lz file2.lz file3.lz > file123.lz
Or keeping the trailing data of the last file like this
  lziprecover --strip=empty file1.lz file2.lz file3.lz > file123.lz
@end example

@noindent
Example 5: Decompress @file{file.lz} partially until @w{10 KiB} of
decompressed data are produced.

@example
lziprecover -D 0,10KiB file.lz
@end example

@noindent
Example 6: Decompress @file{file.lz} partially from decompressed byte at
offset 10000 to decompressed byte at offset 14999 (5000 bytes are produced).

@example
lziprecover -D 10000-15000 file.lz
@end example

@noindent
Example 7: Repair a corrupt byte in the file @file{file.lz}. (Indented lines
are abridged diagnostic messages from lziprecover).

@example
lziprecover -v --byte-repair file.lz
  Copy of input file repaired successfully.
lziprecover -tv file_fixed.lz
  file_fixed.lz: ok
mv file_fixed.lz file.lz
@end example

@noindent
Example 8: Split the multimember file @file{file.lz} and write each member
in its own @file{recXXXfile.lz} file. Then use @w{@samp{lziprecover -t}} to
test the integrity of the resulting files.

@example
lziprecover -s file.lz
lziprecover -tv rec*file.lz
@end example


@node Unzcrash
@chapter Testing the robustness of decompressors
@cindex unzcrash

@xref{--unzcrash}, for a faster way of testing the robustness of lzip.

The lziprecover package also includes unzcrash, a program written to test
robustness to decompression of corrupted data, inspired by unzcrash.c from
Julian Seward's bzip2. Type @samp{make unzcrash} in the lziprecover source
directory to build it.

By default, unzcrash reads the file specified and then repeatedly
decompresses it, increasing 256 times each byte of the compressed data, so
as to test all possible one-byte errors. Note that it may take years or even
centuries to test all possible one-byte errors in a large file (tens of MB).

If the option @option{--block} is given, unzcrash reads the file specified and
then repeatedly decompresses it, setting all bytes in each successive block
to the value given, so as to test all possible full sector errors.

If the option @option{--truncate} is given, unzcrash reads the file specified
and then repeatedly decompresses it, truncating the file to increasing
lengths, so as to test all possible truncation points.

None of the three test modes described above should cause any invalid memory
accesses. If any of them does, please, report it as a bug to the maintainers
of the decompressor being tested.

Unzcrash really executes as a subprocess the shell command specified in the
first non-option argument, and then writes the file specified in the second
non-option argument to the standard input of the subprocess, modifying the
corresponding byte each time. Therefore unzcrash can be used to test any
decompressor (not only lzip), or even other decoder programs having a
suitable command-line syntax.

If the decompressor returns with zero status, unzcrash compares the output
of the decompressor for the original and corrupt files. If the outputs
differ, it means that the decompressor returned a false negative; it failed
to recognize the corruption and produced garbage output. The only exception
is when a multimember file is truncated just after the last byte of a
member, producing a shorter but valid compressed file. Except in this latter
case, please, report any false negative as a bug.

In order to compare the outputs, unzcrash needs a @samp{zcmp} program able
to understand the format being tested. For example the @samp{zcmp} provided
by @uref{http://www.nongnu.org/zutils/manual/zutils_manual.html#Zcmp,,zutils}.
If the @samp{zcmp} program used does not understand the format being tested,
all the comparisons fail because the compressed files are compared without
being decompressed first. Use @option{--zcmp=false} to disable comparisons.
@ifnothtml
@xref{Zcmp,,,zutils}.
@end ifnothtml

The format for running unzcrash is:

@example
unzcrash [@var{options}] 'lzip -t' @var{file}
@end example

@noindent
The compressed @var{file} must not contain errors and the decompressor being
tested must decompress it correctly for the comparisons to work.

@noindent
unzcrash supports the following options:

@table @code
@item -h
@itemx --help
Print an informative help message describing the options and exit.

@item -V
@itemx --version
Print the version number of unzcrash on the standard output and exit.
This version number should be included in all bug reports.

@item -b @var{range}
@itemx --bits=@var{range}
Test N-bit errors only, instead of testing all the 255 wrong values for
each byte. @samp{N-bit error} means any value differing from the
original value in N bit positions, not a value differing from the
original value in the bit position N.@*
The number of N-bit errors per byte (N = 1 to 8) is:
@w{8 28 56 70 56 28 8 1}

@multitable {Examples of @var{range}} {Tests errors of N-bits}
@headitem Examples of @var{range} @tab Tests errors of N-bits
@item 1       @tab 1
@item 1,2,3   @tab 1, 2, 3
@item 2-4     @tab 2, 3, 4
@item 1,3-5,8 @tab 1, 3, 4, 5, 8
@item 1-3,5-8 @tab 1, 2, 3, 5, 6, 7, 8
@end multitable

@item -B[@var{size}][,@var{value}]
@itemx --block[=@var{size}][,@var{value}]
Test block errors of given @var{size}, simulating a whole sector I/O error.
@var{size} defaults to 512 bytes. @var{value} defaults to 0. By default,
only contiguous, non-overlapping blocks are tested, but this may be changed
with the option @option{--delta}.

@item -d @var{n}
@itemx --delta=@var{n}
Test one byte, block, or truncation size every @var{n} bytes. If
@option{--delta} is not specified, unzcrash tests all the bytes,
non-overlapping blocks, or truncation sizes. Values of @var{n} smaller than
the block size result in overlapping blocks. (Which is convenient for
testing because there are usually too few non-overlapping blocks in a file).

@item -e @var{position},@var{value}
@itemx --set-byte=@var{position},@var{value}
Set byte at @var{position} to @var{value} in the internal buffer after
reading and testing @var{file} but before the first test call to the
decompressor. Byte positions start at 0. If @var{value} is preceded by
@samp{+}, it is added to the original value of the byte at @var{position}.
If @var{value} is preceded by @samp{f} (flip), it is XORed with the original
value of the byte at @var{position}. This option can be used to run tests
with a changed dictionary size, for example.

@item -n
@itemx --no-check
Skip initial test of @var{file} and @samp{zcmp}. May speed up things a lot
when testing many (or large) known good files.

@item -p @var{bytes}
@itemx --position=@var{bytes}
First byte position to test in the file. Defaults to 0. Negative values
are relative to the end of the file.

@item -q
@itemx --quiet
Quiet operation. Suppress all messages.

@item -s @var{bytes}
@itemx --size=@var{bytes}
Number of byte positions to test. If not specified, the rest of the file
is tested (from @option{--position} to end of file). Negative values are
relative to the rest of the file.

@item -t
@itemx --truncate
Test all possible truncation points in the range specified by
@option{--position} and @option{--size}.

@item -v
@itemx --verbose
Verbose mode.

@item -z
@itemx --zcmp=<command>
Set zcmp command name and options. Defaults to @samp{zcmp}. Use
@option{--zcmp=false} to disable comparisons. If testing a decompressor
different from the one used by default by zcmp, it is needed to force
unzcrash and zcmp to use the same decompressor with a command like
@w{@samp{unzcrash --zcmp='zcmp --lz=plzip' 'plzip -t' @var{file}}}

@end table

Exit status: 0 for a normal exit, 1 for environmental problems
(file not found, invalid command-line options, I/O errors, etc), 2 to
indicate a corrupt or invalid input file, 3 for an internal consistency
error (e.g., bug) which caused unzcrash to panic.


@node Problems
@chapter Reporting bugs
@cindex bugs
@cindex getting help

There are probably bugs in lziprecover. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
you don't, no one will ever know about them and they will remain unfixed
for all eternity, if not longer.

If you find a bug in lziprecover, please send electronic mail to
@email{lzip-bug@@nongnu.org}. Include the version number, which you can
find by running @w{@samp{lziprecover --version}}.


@node Concept index
@unnumbered Concept index

@printindex cp

@bye