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INFO-DIR-SECTION Data Compression
START-INFO-DIR-ENTRY
* Lziprecover: (lziprecover).   Data recovery tool for the lzip format
END-INFO-DIR-ENTRY


File: lziprecover.info,  Node: Top,  Next: Introduction,  Up: (dir)

Lziprecover Manual
******************

This manual is for Lziprecover (version 1.21, 4 January 2019).

* 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


   Copyright (C) 2009-2019 Antonio Diaz Diaz.

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


File: lziprecover.info,  Node: Introduction,  Next: Invoking lziprecover,  Prev: Top,  Up: Top

1 Introduction
**************

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:

   * 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. *Note Data safety::.

   * 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.

   * Additionally the lzip reference implementation is copylefted, which
     guarantees that it will remain free forever.

   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
GNU ddrescue + lziprecover is the best option for recovering data from
multiple damaged copies. *Note 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):

     lziprecover -D0 -i -o file -q file.lz

   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.


File: lziprecover.info,  Node: Invoking lziprecover,  Next: Data safety,  Prev: Introduction,  Up: Top

2 Invoking lziprecover
**********************

The format for running lziprecover is:

     lziprecover [OPTIONS] [FILES]

When decompressing or testing, '-' used as a FILE argument means
standard input. It can be mixed with other FILES and is read just once,
the first time it appears in the command line.

   lziprecover supports the following options:

'-h'
'--help'
     Print an informative help message describing the options and exit.

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

'-a'
'--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. *Note
     concat-example::.

'-A'
'--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:

     filename.lzma   becomes   filename.lz
     filename.tlz    becomes   filename.tar.lz
     anyothername    becomes   anyothername.lz

'-c'
'--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.

'-d'
'--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.

'-D RANGE'
'--range-decompress=RANGE'
     Decompress only a range of bytes starting at decompressed byte
     position 'BEGIN' and up to byte position '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 RANGE are recognized, 'BEGIN', 'BEGIN-END',
     'BEGIN,SIZE', and ',SIZE'. If only BEGIN is specified, END is taken
     as the end of the file. If only SIZE is specified, BEGIN is taken
     as the beginning of the file. The produced bytes are sent to
     standard output unless the '--output' option is used.

'-f'
'--force'
     Force overwrite of output files.

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

     Make '--list', '--dump', '--remove' and '--strip' ignore format
     errors.

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

'-l'
'--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 '-v', the dictionary size, the number of members in
     the file, and the amount of trailing data (if any) are also
     printed. With '-vv', the positions and sizes of each member in
     multimember files are also printed. With '-i', format errors are
     ignored, and with '-ivv', gaps between members are shown. The
     member numbers shown coincide with the file numbers produced by
     '--split'.

     '-lq' can be used to verify quickly (without decompressing) the
     structural integrity of the specified files. (Use '--test' to
     verify the data integrity). '-alq' additionally verifies that none
     of the specified files contain trailing data.

'-m'
'--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 'FILE_fixed.lz'. The exit status is 0 if a correct file
     could be produced, 2 otherwise. *Note Merging files::, for a
     complete description of the merge mode.

'-o FILE'
'--output=FILE'
     Place the output into 'FILE' instead of into 'FILE_fixed.lz'. If
     splitting, the names of the files produced are in the form
     'rec01FILE', 'rec02FILE', etc. If decompressing from standard
     input and '--stdout' has not been specified, use 'FILE' as the
     name of the decompressed file. If converting a lzma-alone file
     from standard input and '--stdout' has not been specified, use
     'FILE.lz' as the name of the converted file. (Or plain 'FILE' if
     it already ends in '.lz' or '.tlz').

'-q'
'--quiet'
     Quiet operation. Suppress all messages.

'-R'
'--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 'FILE_fixed.lz'. 'FILE' is not modified at all.  The exit
     status is 0 if the file could be repaired, 2 otherwise.  *Note
     Repairing files::, for a complete description of the repair mode.

'-s'
'--split'
     Search for members in '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 '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 FILE because of damage in headers or trailers, try to
     split FILE and then work on each member individually.

     The names of the files produced are in the form 'rec01FILE',
     'rec02FILE', etc, and are designed so that the use of wildcards in
     subsequent processing, for example,
     'lziprecover -cd rec*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 'FILE'.

'-t'
'--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 '-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.

'-v'
'--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 '-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).

'--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.

'--dump=[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 '--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 '--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 '--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 '1:r1' in a single-member file, it is dumped just once.
     See the following examples:

     '--dump' argument      Elements dumped
     ---------------------------------------------------------------------
     '1,3-6'                members 1, 3, 4, 5 and 6
     'r1-3'                 last 3 members in file
     '^13,15'               all but 13th and 15th members in file
     'r^1'                  all but last member in file
     'damaged'              all damaged members in file
     'tdata'                trailing data
     '1-5:r1:tdata'         members 1 to 5, last member, trailing data
     'damaged:tdata'        damaged members, trailing data
     '3,12:damaged:tdata'   members 3, 12, damaged members, trailing data

'--remove=[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 '--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. *Note Trailing
     data::. It is advisable to make a backup before attempting the
     removal. At least verify that 'lzip -cd file.lz | wc -c' and the
     uncompressed size shown by 'lzip -l file.lz' match before
     attempting the removal of trailing data.

'--strip=[MEMBER_LIST][:damaged][:tdata]'
     Copy one or more regular multimember files to standard output (or
     to a file if the '--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
     '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 '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 '--dump' above for a
     description of the argument.


   Numbers given as arguments to options may be followed by a multiplier
and an optional 'B' for "byte".

   Table of SI and binary prefixes (unit multipliers):

Prefix   Value                     |   Prefix   Value
k        kilobyte  (10^3 = 1000)   |   Ki       kibibyte (2^10 = 1024)
M        megabyte  (10^6)          |   Mi       mebibyte (2^20)
G        gigabyte  (10^9)          |   Gi       gibibyte (2^30)
T        terabyte  (10^12)         |   Ti       tebibyte (2^40)
P        petabyte  (10^15)         |   Pi       pebibyte (2^50)
E        exabyte   (10^18)         |   Ei       exbibyte (2^60)
Z        zettabyte (10^21)         |   Zi       zebibyte (2^70)
Y        yottabyte (10^24)         |   Yi       yobibyte (2^80)


   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.


File: lziprecover.info,  Node: Data safety,  Next: Repairing files,  Prev: Invoking lziprecover,  Up: Top

3 Protecting data from accidental loss
**************************************

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 (*note
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 (*note 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 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.


File: lziprecover.info,  Node: Repairing files,  Next: Merging files,  Prev: Data safety,  Up: Top

4 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 '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 (*note File
format::). If the error is in the member size, it is enough to ignore
the message about '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.


File: lziprecover.info,  Node: Merging files,  Next: Tarlz,  Prev: Repairing files,  Up: Top

5 Merging files
***************

If you have several copies of a file but all of them are too damaged to
repair them (*note 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:

     ddrescue -s<correct_size> -x<correct_size> file.lz correct_size_file.lz

   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
'icecat-3.5.3-x86.tar.lz' (compressed size 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:

     $ 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.

   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.


File: lziprecover.info,  Node: Tarlz,  Next: File names,  Prev: Merging files,  Up: Top

6 Options supporting the tar.lz format
**************************************

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.
*Note tarlz manual: (tarlz)Top.


6.1 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 'Input files merged successfully.' you are
done and 'archive.tar.lz' now contains the recovered archive:

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

   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
'Copy of input file repaired successfully.' you are done and
'archive_fixed.tar.lz' now contains the recovered archive:

     lziprecover -v -R archive.tar.lz

   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, 'bad_members.tar.lz' will contain all the damaged members
and 'archive_cleaned.tar.lz' will contain a good archive with the
damaged members removed:

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

   You can then use 'tarlz --keep-damaged' to recover as much data as
possible from each damaged member in 'bad_members.tar.lz':

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


6.2 Processing multimember tar.lz archives
==========================================

Lziprecover is able to copy a list of members from a file to another.
For example the command
'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 'archive.tar.lz'.
The 'r1' part selects the last member, which in an appendable tar.lz
archive contains the end-of-file blocks.


File: lziprecover.info,  Node: File names,  Next: File format,  Prev: Tarlz,  Up: Top

7 Names of the files produced by lziprecover
********************************************

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


File: lziprecover.info,  Node: File format,  Next: Trailing data,  Prev: File names,  Up: Top

8 File format
*************

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


   In the diagram below, a box like this:
+---+
|   | <-- the vertical bars might be missing
+---+

   represents one byte; a box like this:
+==============+
|              |
+==============+

   represents a variable number of bytes.


   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:
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID string | VN | DS | LZMA stream | CRC32 |   Data size   |  Member size  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All multibyte values are stored in little endian order.

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

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

'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.

'LZMA stream'
     The LZMA stream, finished by an end of stream marker. Uses default
     values for encoder properties.  *Note Stream format: (lzip)Stream
     format, for a complete description.

'CRC32 (4 bytes)'
     CRC of the uncompressed original data.

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

'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.



File: lziprecover.info,  Node: Trailing data,  Next: Examples,  Prev: File format,  Up: Top

9 Extra data appended to the file
*********************************

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

   * 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.

   * 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.

   * Garbage added by some not totally successful copy operation.

   * 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.

   * 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 '--trailing-error' guarantees that any corrupt header
     will be detected.

   Trailing data are in no way part of the lzip file format, but tools
reading lzip files are expected to behave as correctly and usefully as
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
'--trailing-error' can be used. *Note --trailing-error::.

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

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

     # 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


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).

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


File: lziprecover.info,  Node: Examples,  Next: Unzcrash,  Prev: Trailing data,  Up: Top

10 A small tutorial with examples
*********************************

Example 1: Restore a regular file from its compressed version
'file.lz'. If the operation is successful, 'file.lz' is removed.

     lziprecover -d file.lz


Example 2: Verify the integrity of the compressed file 'file.lz' and
show status.

     lziprecover -tv file.lz


Example 3: The right way of concatenating the decompressed output of two
or more compressed files. *Note Trailing data::.

     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


Example 4: Decompress 'file.lz' partially until 10 KiB of decompressed
data are produced.

     lziprecover -D 0,10KiB file.lz


Example 5: Decompress 'file.lz' partially from decompressed byte 10000
to decompressed byte 15000 (5000 bytes are produced).

     lziprecover -D 10000-15000 file.lz


Example 6: Repair small errors in the file 'file.lz'. (Indented lines
are abridged diagnostic messages from lziprecover).

     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


Example 7: Split the multimember file 'file.lz' and write each member
in its own 'recXXXfile.lz' file. Then use 'lziprecover -t' to test the
integrity of the resulting files.

     lziprecover -s file.lz
     lziprecover -tv rec*file.lz


Example 8: Recover a compressed backup from two copies on CD-ROM with
error-checked merging of copies.  *Note GNU ddrescue manual:
(ddrescue)Top, for details about ddrescue.

     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


Example 9: Recover the first volume of those created with the command
'lzip -b 32MiB -S 650MB big_db' from two copies, 'big_db1_00001.lz' and
'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 'big_db_00001.lz'.

     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


File: lziprecover.info,  Node: Unzcrash,  Next: Problems,  Prev: Examples,  Up: Top

11 Testing the robustness of decompressors
******************************************

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 '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 '--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 '--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 'zcmp' program able
to understand the format being tested. For example the 'zcmp' provided
by 'zutils'.  *Note Zcmp: (zutils)Zcmp.

   The format for running unzcrash is:

     unzcrash [OPTIONS] 'lzip -t' FILE.lz

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:

'-h'
'--help'
     Print an informative help message describing the options and exit.

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

'-b RANGE'
'--bits=RANGE'
     Test N-bit errors only, instead of testing all the 255 wrong
     values for each byte. '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:
     8 28 56 70 56 28 8 1

     Examples of RANGE   Tests errors of N-bit
     1                   1
     1,2,3               1, 2 and 3
     2-4                 2, 3 and 4
     1,3-5,8             1, 3, 4, 5 and 8
     1-3,5-8             1, 2, 3, 5, 6, 7 and 8

'-B[SIZE][,VALUE]'
'--block[=SIZE][,VALUE]'
     Test block errors of given SIZE, simulating a whole sector I/O
     error. Block SIZE defaults to 512 bytes. VALUE defaults to 0. By
     default, only blocks aligned to a SIZE-byte boundary are tested,
     but this may be changed with the '--delta' option.

'-d N'
'--delta=N'
     Test only one byte, block, or truncation size every N bytes,
     instead of all of them. If the '--block' option is given, N
     defaults to the block size. Else N defaults to 1. Values of 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).

'-e POSITION,VALUE'
'--set-byte=POSITION,VALUE'
     Set byte at POSITION to VALUE in the internal buffer after reading
     and testing FILE.lz but before the first test call to the
     decompressor. If VALUE is preceded by '+', it is added to the
     original value of the byte at POSITION. If VALUE is preceded by
     'f' (flip), it is XORed with the original value of the byte at
     POSITION. This option can be used to run tests with a changed
     dictionary size, for example.

'-n'
'--no-verify'
     Skip initial verification of FILE.lz and 'zcmp'. May speed up
     things a lot when testing many (or large) known good files.

'-p BYTES'
'--position=BYTES'
     First byte position to test in the file. Defaults to 0. Negative
     values are relative to the end of the file.

'-q'
'--quiet'
     Quiet operation. Suppress all messages.

'-s BYTES'
'--size=BYTES'
     Number of byte positions to test. If not specified, the rest of
     the file is tested (from '--position' to end of file). Negative
     values are relative to the rest of the file.

'-t'
'--truncate'
     Test all possible truncation points in the range specified by
     '--position' and '--size'.

'-v'
'--verbose'
     Verbose mode.

'-z'
'--zcmp=<command>'
     Set zcmp command name and options. Defaults to 'zcmp'. Use
     '--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 'unzcrash --zcmp='zcmp --lz=plzip' 'plzip -t' FILE.lz'


   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.


File: lziprecover.info,  Node: Problems,  Next: Concept index,  Prev: Unzcrash,  Up: Top

12 Reporting bugs
*****************

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
<lzip-bug@nongnu.org>. Include the version number, which you can find
by running 'lziprecover --version'.


File: lziprecover.info,  Node: Concept index,  Prev: Problems,  Up: Top

Concept index
*************

[index]
* Menu:

* bugs:                                  Problems.              (line 6)
* data safety:                           Data safety.           (line 6)
* examples:                              Examples.              (line 6)
* file format:                           File format.           (line 6)
* file names:                            File names.            (line 6)
* getting help:                          Problems.              (line 6)
* introduction:                          Introduction.          (line 6)
* invoking:                              Invoking lziprecover.  (line 6)
* merging files:                         Merging files.         (line 6)
* options:                               Invoking lziprecover.  (line 6)
* repairing files:                       Repairing files.       (line 6)
* tarlz:                                 Tarlz.                 (line 6)
* trailing data:                         Trailing data.         (line 6)
* unzcrash:                              Unzcrash.              (line 6)
* usage:                                 Invoking lziprecover.  (line 6)
* version:                               Invoking lziprecover.  (line 6)



Tag Table:
Node: Top231
Node: Introduction1335
Node: Invoking lziprecover4918
Ref: --trailing-error5628
Node: Data safety18371
Node: Repairing files20322
Node: Merging files22245
Node: Tarlz25002
Node: File names27857
Node: File format28313
Node: Trailing data30739
Node: Examples33974
Ref: concat-example34407
Ref: ddrescue-example35778
Node: Unzcrash37066
Node: Problems43130
Node: Concept index43682

End Tag Table


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