lziprecover/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 4 January 2019
@set VERSION 1.21

@dircategory Data 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

@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 files::        Fixing bit flips and similar errors
* Merging files::          Fixing several damaged copies
* 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{} 2009-2019 Antonio Diaz Diaz.

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


@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,
produce a correct file by merging the good parts of two or more damaged
copies, 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 lziprecover program can repair bit flip errors (one
of the most common forms of data corruption) in lzip files, and provides
data recovery capabilities, including error-checked merging of damaged
copies of a file. @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.

For compressible data, multiple lzip-compressed copies have a better
chance of surviving intact than one uncompressed copy using the same
amount of storage space.

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 damaged media, the combination
@w{GNU ddrescue + lziprecover} is the best option for recovering data from
multiple damaged copies. @xref{ddrescue-example}, for an example.

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 member):

@example
lziprecover -D0 -i -o file -q file.lz
@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, @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.

lziprecover 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 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 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.

@item -d
@itemx --decompress
Decompress the specified files. If a file does not exist or can't be
opened, lziprecover continues decompressing the rest of the files. If a file
fails to decompress, or is a terminal, lziprecover exits immediately without
decompressing the rest of the files.

@item -D @var{range}
@itemx --range-decompress=@var{range}
Decompress only a range of bytes starting at decompressed byte position
@samp{@var{begin}} and up to byte position @w{@samp{@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.

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 produced bytes are sent to
standard output unless the @samp{--output} option is used.

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

@item -i
@itemx --ignore-errors
Make @samp{--range-decompress} ignore errors and continue decompressing
the remaining members in the file. For example,
@w{@samp{lziprecover -D0 -i file.lz > file}} decompresses all the
recoverable data in all members of @samp{file.lz} without having to
split it first.

Make @samp{--list}, @samp{--dump}, @samp{--remove} and @samp{--strip}
ignore format errors.

@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
specified files. Trailing data are ignored. The values produced are
correct even for multimember files. If more than one file is given, a
final line containing the cumulative sizes is printed. With @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}.

@samp{-lq} can be used to verify quickly (without decompressing) the
structural integrity of the specified files. (Use @samp{--test} to
verify the data integrity). @samp{-alq} additionally verifies that none
of the specified files 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 @samp{@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 from standard input and @samp{--stdout} has not
been specified, use @samp{@var{file}} as the name of the decompressed
file. If converting a lzma-alone file from standard input and
@samp{--stdout} has not been specified, use @samp{@var{file}.lz} as the
name of the converted file. (Or plain @samp{@var{file}} if it already
ends in @samp{.lz} or @samp{.tlz}).

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

@item -R
@itemx --repair
Try to repair a 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}. @samp{@var{file}} is not modified at all.
The exit status is 0 if the file could be repaired, 2 otherwise.
@xref{Repairing files}, for a complete description of the repair mode.

@item -s
@itemx --split
Search for members in @samp{@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 @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 @samp{@var{file}}.

@item -t
@itemx --test
Check integrity of the specified files, but don't decompress them. This
really performs a trial decompression and throws away the result. Use it
together with @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 @samp{--output} option 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.

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 @code{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 and 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 @samp{--output} option 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

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 (eg, bug) which
caused lziprecover to panic.


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

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
(@pxref{Repairing files}), 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.

Lziprecover also protects against multibyte errors (@pxref{Merging
files}), if at least one backup copy of the file is made.

The only remedy for total device failure is storing backup copies in
separate media.

How does lzip compare with gzip and bzip2 with respect to data safety?
Let's suppose that you made a backup of your valuable scientific data,
compressed it, and stored two copies on separate media. Years later you
notice that both copies are corrupt.

If you compressed 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 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 Repairing files
@chapter Repairing files
@cindex repairing files

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 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, can 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 files}), 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

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.


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

@uref{http://www.nongnu.org/lzip/manual/tarlz_manual.html,,Tarlz} is an
implementation of the tar archiver which by default creates archives
compressed with lzip on a per file basis. Tarlz can append files to the
end of such compressed archives because each tar member is compressed in
its own lzip member, as well as the end-of-file blocks. Thus tarlz
archives are multimember lzip files, which has 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. In this chapter we'll explain the ways in which
lziprecover can recover and process multimember tar.lz archives.
@ifnothtml
@xref{Top,tarlz manual,,tarlz}.
@end ifnothtml

@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. If the
command below prints something like
@w{@code{Input files merged successfully.}} you are done and
@code{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, you may try to repair
the archive, but this has a lower probability of success. If the command
below prints something like
@w{@code{Copy of input file repaired successfully.}} you are done and
@code{archive_fixed.tar.lz} now contains the recovered archive:

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

If all the above fails, you may save the damaged members for later and
then copy the good members to another archive. If the two commands below
succeed, @code{bad_members.tar.lz} will contain all the damaged members
and @code{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 @code{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{@code{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
@code{archive.tar.lz}. The @code{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 "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 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)
CRC of the uncompressed original data.

@item Data size (8 bytes)
Size of the uncompressed original 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 safe recovery of undamaged members from multimember files.

@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
# 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: 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 2: 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 3: 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 4: 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 5: Decompress @samp{file.lz} partially from decompressed byte
10000 to decompressed byte 15000 (5000 bytes are produced).

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

@sp 1
@noindent
Example 6: 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 7: 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

@sp 1
@anchor{ddrescue-example}
@noindent
Example 8: 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 9: 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 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 specified file 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 @code{--block} option is given, unzcrash reads the specified file
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 @code{--truncate} option is given, unzcrash reads the specified
file 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 @samp{zutils}.
@ifnothtml
@xref{Zcmp,,,zutils}.
@end ifnothtml
@ifhtml
See
@uref{http://www.nongnu.org/zutils/manual/zutils_manual.html#Zcmp,,zcmp}.
@end ifhtml

The format for running unzcrash is:

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

@noindent
@var{file}.lz must not contain errors and must be correctly decompressed
by the decompressor being tested 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}} {1, 2, 3, 5, 6, 7 and 8}
@item Examples of @var{range} @tab Tests errors of N-bit
@item 1                       @tab 1
@item 1,2,3                   @tab 1, 2 and 3
@item 2-4                     @tab 2, 3 and 4
@item 1,3-5,8                 @tab 1, 3, 4, 5 and 8
@item 1-3,5-8                 @tab 1, 2, 3, 5, 6, 7 and 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. Block @var{size} defaults to 512 bytes. @var{value} defaults to
0. By default, only blocks aligned to a @var{size}-byte boundary are
tested, but this may be changed with the @code{--delta} option.

@item -d @var{n}
@itemx --delta=@var{n}
Test only one byte, block, or truncation size every @var{n} bytes,
instead of all of them. If the @code{--block} option is given, @var{n}
defaults to the block size. Else @var{n} defaults to 1. Values of
@var{n} smaller than the block size will result in overlappinng blocks.
(Which is convenient for testing because there are usually too few
non-overlappinng 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}.lz but before the first test call to the
decompressor. 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}.lz 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 @code{--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
@code{--position} and @code{--size}.

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

@item -z
@itemx --zcmp=<command>
Set zcmp command name and options. Defaults to @code{zcmp}. Use
@code{--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{@code{unzcrash --zcmp='zcmp --lz=plzip' 'plzip -t' @var{file}.lz}}

@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 (eg, 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{@code{lziprecover --version}}.


@node Concept index
@unnumbered Concept index

@printindex cp

@bye