path: root/doc/lziprecover.texi

\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 21 January 2022
@set VERSION 1.23

@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
* Data safety::             Protecting data from accidental loss
* 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
* File format::             Detailed format of the compressed file
* 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{} 2009-2022 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 is able to repair slightly damaged
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 facilitates the management of metadata stored as trailing data
in lzip files.

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

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 specially 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 -i 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.

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 -c
@itemx --stdout
Write decompressed data to standard output; keep input files unchanged. This
option (or @samp{-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. @samp{-c} overrides @samp{-o}.
@samp{-c} has no effect when merging, removing members, repairing,
reproducing, splitting, testing or listing.

@item -d
@itemx --decompress
Decompress the files specified. If a file does not exist, can't be opened,
or the destination file already exists and @samp{--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 verified.

@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 @samp{--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 @samp{@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
@samp{@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. @samp{--lzip-level=a} tries all the compression levels
@w{(0 to 9)}, while @samp{--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 @samp{--reproduce}. If
@samp{--lzip-name} is not specified, @samp{lzip} is used.

@item --reference-file=@var{file}
Set the reference file used by @samp{--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 -i
@itemx --ignore-errors
Make @samp{--decompress}, @samp{--test}, and @samp{--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 @samp{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 @samp{--list}, @samp{--dump}, @samp{--remove}, and @samp{--strip}
ignore format errors. The sizes of the members with errors (specially 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 @samp{-v}, the dictionary
size, the number of members in the file, and the amount of trailing data (if
any) are also printed. With @samp{-vv}, the positions and sizes of each
member in multimember files are also printed. With @samp{-i}, format errors
are ignored, and with @samp{-ivv}, gaps between members are shown. The
member numbers shown coincide with the file numbers produced by @samp{--split}.

If any file is damaged, does not exist, can't be opened, or is not regular,
the final exit status will be @w{> 0}. @samp{-lq} can be used to verify
quickly (without decompressing) the structural integrity of the files
specified. (Use @samp{--test} to verify the data integrity). @samp{-alq}
additionally verifies 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
@samp{@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 -o @var{file}
@itemx --output=@var{file}
Place the output into @var{file} instead of into @samp{@var{file}_fixed.lz}.
If splitting, the names of the files produced are in the form
@samp{rec01@var{file}}, @samp{rec02@var{file}}, etc.

If decompressing, or converting lzma-alone files, and @samp{-c} has not been
also specified, write the decompressed or converted output to @var{file};
keep input files unchanged. This option (or @samp{-c}) is needed when
reading from a named pipe (fifo) or from a device. @w{@samp{-o -}} is
equivalent to @samp{-c}. @samp{-o} has no effect when testing or listing.

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

@anchor{--repair}
@item -R
@itemx --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
@samp{@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 repair mode.

@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 @samp{rec01@var{file}},
@samp{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 @samp{-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 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.

@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 @samp{-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 --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 --dump=[@var{member_list}][:damaged][:tdata]
Dump the members listed, the damaged 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 @samp{--output} is used. If more than
one file is given, the elements dumped from all 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 @samp{--dump=tdata} can write to a terminal.

The argument to @samp{--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" and "tdata" (which may be shortened
to 'd' and 't' respectively). A member list selects the members (or
gaps) listed, whose numbers coincide with those shown by @samp{--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 selects all the members except those in the list.
The strings "damaged" and "tdata" select the damaged members 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{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][:tdata]
Remove the members listed, the damaged members (if any), or the trailing
data (if any) from regular multimember files in place. The date of each
file 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 @samp{--dump} above for a description of the argument.

This option may be dangerous even if only the trailing data is being
removed because the file may be corrupt or the trailing data may contain
a forbidden combination of characters. @xref{Trailing data}. It is
advisable to make a backup before attempting the removal. At least
verify 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][:tdata]
Copy one or more regular multimember files to standard output (or to a
file if the option @samp{--output} is used), stripping the members
listed, the damaged 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 @samp{--dump}
above for a description of the argument.

@end table

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 -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 verify the result of some operations. This option allows
the verification 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 files followed by the name of the
first file with the longest sequence.

@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. @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 @samp{--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 is decompressed fully (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-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 error. @xref{--repair}.

@end table

Numbers given as arguments to options 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)}
@item 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)
@end multitable

@sp 1
Exit status: 0 for a normal exit, 1 for environmental problems (file not
found, invalid flags, 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 Data safety
@chapter Protecting data from accidental loss
@cindex data safety

It is a fact of life that sometimes data will become 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.

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 also protects against multibyte errors if at least one backup
copy of the file is made (@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}). 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 exploit
the redundance that occurrs 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, verifying 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 specially 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 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 will be
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.

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 (@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 will fail 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
@samp{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,
@samp{big_db1_00001.lz} and @samp{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
@samp{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.

To understand how it works, take any lzipped file, say @samp{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 @samp{foo} as reference file guarantees that any zeroed sector in
@samp{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 @samp{--reproduce}
Reproduce mode is specially 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. @samp{mailbox-aug.tar.lz} is a backup of some
of my mailboxes. @samp{backup-feb.tar.lz} and @samp{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 is 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}).

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 @samp{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 @samp{example.txt} that may help locate a
complete copy in the filesystem or in another backup, even if
@samp{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 fully
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.

@sp 1
@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
@samp{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
@samp{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
@samp{archive_fixed.tar.lz} now contains the recovered archive:

@example
lziprecover -v -R 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, @samp{bad_members.tar.lz} will
contain all the damaged members and @samp{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 @samp{bad_members.tar.lz}:

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

@sp 1
@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 @samp{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 @samp{--merge} and @samp{--repair} is
made by appending the string @samp{_fixed.lz} to the original file name. If
the original file name ends with one of the extensions @samp{.tar.lz},
@samp{.lz}, or @samp{.tlz}, the string @samp{_fixed} is inserted before the
extension.


@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

@sp 1
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.

@sp 1
A lzip file consists of a series of 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.

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, allows the verification of stream integrity, and
facilitates the safe recovery of undamaged members from multimember files.
Member size should be limited to @w{2 PiB} to prevent the data size field
from overflowing.

@end table


@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; 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
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.

@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 @samp{--trailing-error} guarantees that
any corrupt header will be 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
@samp{--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 verify 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 @samp{foo.tar.lz}.

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

@sp 1
@noindent
Example 2: Restore a regular file from its compressed version
@samp{file.lz}. If the operation is successful, @samp{file.lz} is removed.

@example
lziprecover -d file.lz
@end example

@sp 1
@noindent
Example 3: Verify the integrity of the compressed file @samp{file.lz} and
show status.

@example
lziprecover -tv file.lz
@end example

@sp 1
@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=damaged file1.lz file2.lz file3.lz > file123.lz
@end example

@sp 1
@noindent
Example 5: Decompress @samp{file.lz} partially until @w{10 KiB} of
decompressed data are produced.

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

@sp 1
@noindent
Example 6: Decompress @samp{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

@sp 1
@noindent
Example 7: Repair small errors in the file @samp{file.lz}. (Indented lines
are abridged diagnostic messages from lziprecover).

@example
lziprecover -v -R 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

@sp 1
@noindent
Example 8: Split the multimember file @samp{file.lz} and write each member
in its own @samp{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

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 @samp{--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 @samp{--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 will fail because the compressed files will be compared
without being decompressed first. Use @samp{--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.

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}
@item 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 @samp{--delta}.

@item -d @var{n}
@itemx --delta=@var{n}
Test one byte, block, or truncation size every @var{n} bytes. If
@samp{--delta} is not specified, unzcrash tests all the bytes,
non-overlapping blocks, or truncation sizes. Values of @var{n} smaller than
the block size will 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-verify
Skip initial verification 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 @samp{--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
@samp{--position} and @samp{--size}.

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

@item -z
@itemx --zcmp=<command>
Set zcmp command name and options. Defaults to @samp{zcmp}. Use
@samp{--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 flags, 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