path: root/doc/plzip.info

This is plzip.info, produced by makeinfo version 4.13+ from plzip.texi.

INFO-DIR-SECTION Compression
START-INFO-DIR-ENTRY
* Plzip: (plzip).               Massively parallel implementation of lzip
END-INFO-DIR-ENTRY


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

Plzip Manual
************

This manual is for Plzip (version 1.11, 21 January 2024).

* Menu:

* Introduction::           Purpose and features of plzip
* Output::                 Meaning of plzip's output
* Invoking plzip::         Command-line interface
* Program design::         Internal structure of plzip
* Memory requirements::    Memory required to compress and decompress
* Minimum file sizes::     Minimum file sizes required for full speed
* File format::            Detailed format of the compressed file
* Trailing data::          Extra data appended to the file
* Examples::               A small tutorial with examples
* Problems::               Reporting bugs
* Concept index::          Index of concepts


   Copyright (C) 2009-2024 Antonio Diaz Diaz.

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


File: plzip.info,  Node: Introduction,  Next: Output,  Prev: Top,  Up: Top

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

Plzip is a massively parallel (multi-threaded) implementation of lzip,
compatible with lzip 1.4 or newer. Plzip uses the compression library lzlib.

   Lzip is a lossless data compressor with a user interface similar to the
one of gzip or bzip2. Lzip uses a simplified form of the 'Lempel-Ziv-Markov
chain-Algorithm' (LZMA) stream format to maximize interoperability. The
maximum dictionary size is 512 MiB so that any lzip file can be decompressed
on 32-bit machines. Lzip provides accurate and robust 3-factor integrity
checking. Lzip can compress about as fast as gzip (lzip -0) or compress most
files more than bzip2 (lzip -9). Decompression speed is intermediate between
gzip and bzip2. Lzip is better than gzip and bzip2 from a data recovery
perspective. Lzip has been designed, written, and tested with great care to
replace gzip and bzip2 as the standard general-purpose compressed format for
Unix-like systems.

   Plzip can compress/decompress large files on multiprocessor machines much
faster than lzip, at the cost of a slightly reduced compression ratio (0.4
to 2 percent larger compressed files). Note that the number of usable
threads is limited by file size; on files larger than a few GB plzip can use
hundreds of processors, but on files of only a few MB plzip is no faster
than lzip. *Note Minimum file sizes::.

   For creation and manipulation of compressed tar archives tarlz can be
more efficient than using tar and plzip because tarlz is able to keep the
alignment between tar members and lzip members. *Note tarlz manual:
(tarlz)Top.

   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 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. *Note Data safety: (lziprecover)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.

   Plzip uses the same well-defined exit status values used by lzip, which
makes it safer than compressors returning ambiguous warning values (like
gzip) when it is used as a back end for other programs like tar or zutils.

   Plzip automatically uses for each file the largest dictionary size that
does not exceed neither the file size nor the limit given. Keep in mind
that the decompression memory requirement is affected at compression time
by the choice of dictionary size limit. *Note Memory requirements::.

   When compressing, plzip replaces every file given in the command line
with a compressed version of itself, with the name "original_name.lz". When
decompressing, plzip attempts to guess the name for the decompressed file
from that of the compressed file as follows:

filename.lz    becomes   filename
filename.tlz   becomes   filename.tar
anyothername   becomes   anyothername.out

   (De)compressing a file is much like copying or moving it. Therefore plzip
preserves the access and modification dates, permissions, and, if you have
appropriate privileges, ownership of the file just as 'cp -p' does. (If the
user ID or the group ID can't be duplicated, the file permission bits
S_ISUID and S_ISGID are cleared).

   Plzip is able to read from some types of non-regular files if either the
option '-c' or the option '-o' is specified.

   Plzip refuses to read compressed data from a terminal or write compressed
data to a terminal, as this would be entirely incomprehensible and might
leave the terminal in an abnormal state.

   Plzip correctly decompresses a file which is the concatenation of two or
more compressed files. The result is the concatenation of the corresponding
decompressed files. Integrity testing of concatenated compressed files is
also supported.


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

2 Meaning of plzip's output
***************************

The output of plzip looks like this:

     plzip -v foo
       foo:  6.676:1, 14.98% ratio, 85.02% saved, 450560 in, 67493 out.

     plzip -tvvv foo.lz
       foo.lz:  6.676:1, 14.98% ratio, 85.02% saved.  450560 out,  67493 in. ok

   The meaning of each field is as follows:

'N:1'
     The compression ratio (uncompressed_size / compressed_size), shown as
     N to 1.

'ratio'
     The inverse compression ratio (compressed_size / uncompressed_size),
     shown as a percentage. A decimal ratio is easily obtained by moving the
     decimal point two places to the left; 14.98% = 0.1498.

'saved'
     The space saved by compression (1 - ratio), shown as a percentage.

'in'
     Size of the input data. This is the uncompressed size when
     compressing, or the compressed size when decompressing or testing.
     Note that plzip always prints the uncompressed size before the
     compressed size when compressing, decompressing, testing, or listing.

'out'
     Size of the output data. This is the compressed size when compressing,
     or the decompressed size when decompressing or testing.


   When decompressing or testing at verbosity level 4 (-vvvv), the
dictionary size used to compress the file is also shown.

   LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never
have been compressed. Decompressed is used to refer to data which have
undergone the process of decompression.


File: plzip.info,  Node: Invoking plzip,  Next: Program design,  Prev: Output,  Up: Top

3 Invoking plzip
****************

The format for running plzip is:

     plzip [OPTIONS] [FILES]

If no file names are specified, plzip compresses (or decompresses) from
standard input to standard output. A hyphen '-' 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. Remember to prepend
'./' to any file name beginning with a hyphen, or use '--'.

   plzip supports the following options: *Note Argument syntax:
(arg_parser)Argument syntax.

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

'-V'
'--version'
     Print the version number of plzip 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::.

'-B BYTES'
'--data-size=BYTES'
     When compressing, set the size in bytes of the input data blocks. The
     input file is divided in chunks of this size before compression is
     performed. Valid values range from 8 KiB to 1 GiB. Default value is
     two times the dictionary size, except for option '-0' where it
     defaults to 1 MiB. Plzip reduces the dictionary size if it is larger
     than the data size specified. *Note Minimum file sizes::.

'-c'
'--stdout'
     Compress or decompress to standard output; keep input files unchanged.
     If compressing several files, each file is compressed independently.
     (The output consists of a sequence of independently compressed
     members). This option (or '-o') is needed when reading from a named
     pipe (fifo) or from a device. Use 'lziprecover -cd -i' to recover as
     much of the decompressed data as possible when decompressing a corrupt
     file. '-c' overrides '-o'. '-c' has no effect when testing or listing.

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

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

'-F'
'--recompress'
     When compressing, force re-compression of files whose name already has
     the '.lz' or '.tlz' suffix.

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

'-l'
'--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
     '-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.

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

'-m BYTES'
'--match-length=BYTES'
     When compressing, set the match length limit in bytes. After a match
     this long is found, the search is finished. Valid values range from 5
     to 273. Larger values usually give better compression ratios but
     longer compression times.

'-n N'
'--threads=N'
     Set the maximum number of worker threads, overriding the system's
     default. Valid values range from 1 to "as many as your system can
     support". If this option is not used, plzip tries to detect the number
     of processors in the system and use it as default value. When
     compressing on a 32 bit system, plzip tries to limit the memory use to
     under 2.22 GiB (4 worker threads at level -9) by reducing the number
     of threads below the system's default. 'plzip --help' shows the
     system's default value.

     Plzip starts the number of threads required by each file without
     exceeding the value specified. Note that the number of usable threads
     is limited to ceil( file_size / data_size ) during compression (*note
     Minimum file sizes::), and to the number of members in the input
     during decompression. You can find the number of members in a lzip
     file by running 'plzip -lv file.lz'.

'-o FILE'
'--output=FILE'
     If '-c' has not been also specified, write the (de)compressed output
     to FILE, automatically creating any missing parent directories; keep
     input files unchanged. If compressing several files, each file is
     compressed independently. (The output consists of a sequence of
     independently compressed members). This option (or '-c') is needed
     when reading from a named pipe (fifo) or from a device. '-o -' is
     equivalent to '-c'. '-o' has no effect when testing or listing.

     In order to keep backward compatibility with plzip versions prior to
     1.9, when compressing from standard input and no other file names are
     given, the extension '.lz' is appended to FILE unless it already ends
     in '.lz' or '.tlz'. This feature will be removed in a future version
     of plzip. Meanwhile, redirection may be used instead of '-o' to write
     the compressed output to a file without the extension '.lz' in its
     name: 'plzip < file > foo'.

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

'-s BYTES'
'--dictionary-size=BYTES'
     When compressing, set the dictionary size limit in bytes. Plzip uses
     for each file the largest dictionary size that does not exceed neither
     the file size nor this limit. Valid values range from 4 KiB to 512 MiB.
     Values 12 to 29 are interpreted as powers of two, meaning 2^12 to 2^29
     bytes. Dictionary sizes are quantized so that they can be coded in
     just one byte (*note coded-dict-size::). If the size specified does
     not match one of the valid sizes, it is rounded upwards by adding up
     to (BYTES / 8) to it.

     For maximum compression you should use a dictionary size limit as large
     as possible, but keep in mind that the decompression memory requirement
     is affected at compression time by the choice of dictionary size limit.

'-t'
'--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 '-v' to see information about the files. If a file
     fails the test, does not exist, can't be opened, or is a terminal,
     plzip continues testing the rest of the files. A final diagnostic is
     shown at verbosity level 1 or higher if any file fails the test when
     testing multiple files.

'-v'
'--verbose'
     Verbose mode.
     When compressing, show the compression ratio and size for each file
     processed.
     When decompressing or testing, further -v's (up to 4) increase the
     verbosity level, showing status, compression ratio, dictionary size,
     decompressed size, and compressed size.
     Two or more '-v' options show the progress of (de)compression, except
     for single-member files.

'-0 .. -9'
     Compression level. Set the compression parameters (dictionary size and
     match length limit) as shown in the table below. The default
     compression level is '-6', equivalent to '-s8MiB -m36'. Note that '-9'
     can be much slower than '-0'. These options have no effect when
     decompressing, testing, or listing.

     The bidimensional parameter space of LZMA can't be mapped to a linear
     scale optimal for all files. If your files are large, very repetitive,
     etc, you may need to use the options '--dictionary-size' and
     '--match-length' directly to achieve optimal performance.

     If several compression levels or '-s' or '-m' options are given, the
     last setting is used. For example '-9 -s64MiB' is equivalent to
     '-s64MiB -m273'

     Level   Dictionary size (-s)   Match length limit (-m)
     -0      64 KiB                 16 bytes
     -1      1 MiB                  5 bytes
     -2      1.5 MiB                6 bytes
     -3      2 MiB                  8 bytes
     -4      3 MiB                  12 bytes
     -5      4 MiB                  20 bytes
     -6      8 MiB                  36 bytes
     -7      16 MiB                 68 bytes
     -8      24 MiB                 132 bytes
     -9      32 MiB                 273 bytes

'--fast'
'--best'
     Aliases for GNU gzip compatibility.

'--loose-trailing'
     When decompressing, testing, or listing, allow trailing data whose
     first bytes are so similar to the magic bytes of a lzip header that
     they can be confused with a corrupt header. Use this option if a file
     triggers a "corrupt header" error and the cause is not indeed a
     corrupt header.

'--in-slots=N'
     Number of 1 MiB input packets buffered per worker thread when
     decompressing from non-seekable input. Increasing the number of packets
     may increase decompression speed, but requires more memory. Valid
     values range from 1 to 64. The default value is 4.

'--out-slots=N'
     Number of 1 MiB output packets buffered per worker thread when
     decompressing to non-seekable output. Increasing the number of packets
     may increase decompression speed, but requires more memory. Valid
     values range from 1 to 1024. The default value is 64.

'--check-lib'
     Compare the version of lzlib used to compile plzip with the version
     actually being used at run time and exit. Report any differences
     found. Exit with error status 1 if differences are found. A mismatch
     may indicate that lzlib is not correctly installed or that a different
     version of lzlib has been installed after compiling plzip. Exit with
     error status 2 if LZ_API_VERSION and LZ_version_string don't match.
     'plzip -v --check-lib' shows the version of lzlib being used and the
     value of LZ_API_VERSION (if defined). *Note Library version:
     (lzlib)Library version.


   Numbers given as arguments to options may be expressed in decimal,
hexadecimal, or octal (using the same syntax as integer constants in C++),
and may be followed by a multiplier and an optional '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)
R        ronnabyte  (10^27)         |   Ri       robibyte  (2^90)
Q        quettabyte (10^30)         |   Qi       quebibyte (2^100)


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


File: plzip.info,  Node: Program design,  Next: Memory requirements,  Prev: Invoking plzip,  Up: Top

4 Internal structure of plzip
*****************************

When compressing, plzip divides the input file into chunks and compresses as
many chunks simultaneously as worker threads are chosen, creating a
multimember compressed file. Each chunk is compressed in-place (using the
same buffer for input and output), reducing the amount of RAM required.

   When decompressing, plzip decompresses as many members simultaneously as
worker threads are chosen. Files that were compressed with lzip are not
decompressed faster than using lzip (unless the option '-b' was used)
because lzip usually produces single-member files, which can't be
decompressed in parallel.

   For each input file, a splitter thread and several worker threads are
created, acting the main thread as muxer (multiplexer) thread. A "packet
courier" takes care of data transfers among threads and limits the maximum
number of data blocks (packets) being processed simultaneously.

   The splitter reads data blocks from the input file, and distributes them
to the workers. The workers (de)compress the blocks received from the
splitter. The muxer collects processed packets from the workers, and writes
them to the output file.

                             .------------.
                         ,-->| worker   0 |--,
                         |   `------------'  |
.-------.   .----------. |   .------------.  |   .-------.   .--------.
| input |-->| splitter |-+-->| worker   1 |--+-->| muxer |-->| output |
| file  |   `----------' |   `------------'  |   `-------'   |  file  |
`-------'                |        ...        |               `--------'
                         |   .------------.  |
                         `-->| worker N-1 |--'
                             `------------'

   When decompressing from a regular file, the splitter is removed and the
workers read directly from the input file. If the output file is also a
regular file, the muxer is also removed and the workers write directly to
the output file. With these optimizations, the use of RAM is greatly
reduced and the decompression speed of large files with many members is
only limited by the number of processors available and by I/O speed.


File: plzip.info,  Node: Memory requirements,  Next: Minimum file sizes,  Prev: Program design,  Up: Top

5 Memory required to compress and decompress
********************************************

The amount of memory required *per worker thread* for decompression or
testing is approximately the following:

   * For decompression of a regular (seekable) file to another regular file,
     or for testing of a regular file; the dictionary size.

   * For testing of a non-seekable file or of standard input; the dictionary
     size plus 1 MiB plus up to the number of 1 MiB input packets buffered
     (4 by default).

   * For decompression of a regular file to a non-seekable file or to
     standard output; the dictionary size plus up to the number of 1 MiB
     output packets buffered (64 by default).

   * For decompression of a non-seekable file or of standard input; the
     dictionary size plus 1 MiB plus up to the number of 1 MiB input and
     output packets buffered (68 by default).

The amount of memory required *per worker thread* for compression is
approximately the following:

   * For compression at level -0; 1.5 MiB plus 3.375 times the data size
     (*note --data-size::). Default is 4.875 MiB.

   * For compression at other levels; 11 times the dictionary size plus
     3.375 times the data size. Default is 142 MiB.

The following table shows the memory required *per thread* for compression
at a given level, using the default data size for each level:

Level   Memory required
-0      4.875 MiB
-1      17.75 MiB
-2      26.625 MiB
-3      35.5 MiB
-4      53.25 MiB
-5      71 MiB
-6      142 MiB
-7      284 MiB
-8      426 MiB
-9      568 MiB


File: plzip.info,  Node: Minimum file sizes,  Next: File format,  Prev: Memory requirements,  Up: Top

6 Minimum file sizes required for full compression speed
********************************************************

When compressing, plzip divides the input file into chunks and compresses
as many chunks simultaneously as worker threads are chosen, creating a
multimember compressed file.

   For this to work as expected (and roughly multiply the compression speed
by the number of available processors), the uncompressed file must be at
least as large as the number of worker threads times the chunk size (*note
--data-size::). Else some processors do not get any data to compress, and
compression is proportionally slower. The maximum speed increase achievable
on a given file is limited by the ratio (file_size / data_size). For
example, a tarball the size of gcc or linux scales up to 10 or 14
processors at level -9.

   The following table shows the minimum uncompressed file size needed for
full use of N processors at a given compression level, using the default
data size for each level:

Processors   2         4         8         16        64        256
------------------------------------------------------------------
Level                                                          
-0           2 MiB     4 MiB     8 MiB     16 MiB    64 MiB    256 MiB
-1           4 MiB     8 MiB     16 MiB    32 MiB    128 MiB   512 MiB
-2           6 MiB     12 MiB    24 MiB    48 MiB    192 MiB   768 MiB
-3           8 MiB     16 MiB    32 MiB    64 MiB    256 MiB   1 GiB
-4           12 MiB    24 MiB    48 MiB    96 MiB    384 MiB   1.5 GiB
-5           16 MiB    32 MiB    64 MiB    128 MiB   512 MiB   2 GiB
-6           32 MiB    64 MiB    128 MiB   256 MiB   1 GiB     4 GiB
-7           64 MiB    128 MiB   256 MiB   512 MiB   2 GiB     8 GiB
-8           96 MiB    192 MiB   384 MiB   768 MiB   3 GiB     12 GiB
-9           128 MiB   256 MiB   512 MiB   1 GiB     4 GiB     16 GiB


File: plzip.info,  Node: File format,  Next: Trailing data,  Prev: Minimum file sizes,  Up: Top

7 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 one or more independent "members" (compressed
data sets). The members simply appear one after another in the file, with no
additional information before, between, or after them. Each member can
encode in compressed form up to 16 EiB - 1 byte of uncompressed data. The
size of a multimember file is unlimited.

   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)'
     Cyclic Redundancy Check (CRC) of the original uncompressed data.

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

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



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

8 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; an "End Of File" string (to check that
     the file has not been truncated), a cryptographically secure hash, a
     description of file contents, etc. It is safe to append any amount of
     text to a lzip file as long as none of the first four bytes of the
     text matches the corresponding byte in the string "LZIP", and the text
     does not contain any zero bytes (null characters). Nonzero bytes and
     zero bytes can't be safely mixed in trailing data.

   * 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 plzip 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 is 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::.


File: plzip.info,  Node: Examples,  Next: Problems,  Prev: Trailing data,  Up: Top

9 A small tutorial with examples
********************************

WARNING! Even if plzip is bug-free, other causes may result in a corrupt
compressed file (bugs in the system libraries, memory errors, etc).
Therefore, if the data you are going to compress are important, give the
option '--keep' to plzip and don't remove the original file until you check
the compressed file with a command like 'plzip -cd file.lz | cmp file -'.
Most RAM errors happening during compression can only be detected by
comparing the compressed file with the original because the corruption
happens before plzip compresses the RAM contents, resulting in a valid
compressed file containing wrong data.


Example 1: Extract all the files from archive 'foo.tar.lz'.

       tar -xf foo.tar.lz
     or
       plzip -cd foo.tar.lz | tar -xf -


Example 2: Replace a regular file with its compressed version 'file.lz' and
show the compression ratio.

     plzip -v file


Example 3: Like example 2 but the created 'file.lz' has a block size of
1 MiB. The compression ratio is not shown.

     plzip -B 1MiB file


Example 4: Restore a regular file from its compressed version 'file.lz'. If
the operation is successful, 'file.lz' is removed.

     plzip -d file.lz


Example 5: Check the integrity of the compressed file 'file.lz' and show
status.

     plzip -tv file.lz


Example 6: 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 | plzip -d -
     Do this instead
       plzip -cd file1.lz file2.lz file3.lz


Example 7: Decompress 'file.lz' partially until 10 KiB of decompressed data
are produced.

     plzip -cd file.lz | dd bs=1024 count=10


Example 8: Decompress 'file.lz' partially from decompressed byte at offset
10000 to decompressed byte at offset 14999 (5000 bytes are produced).

     plzip -cd file.lz | dd bs=1000 skip=10 count=5


Example 9: Compress a whole device in /dev/sdc and send the output to
'file.lz'.

       plzip -c /dev/sdc > file.lz
     or
       plzip /dev/sdc -o file.lz


File: plzip.info,  Node: Problems,  Next: Concept index,  Prev: Examples,  Up: Top

10 Reporting bugs
*****************

There are probably bugs in plzip. 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 plzip, please send electronic mail to
<lzip-bug@nongnu.org>. Include the version number, which you can find by
running 'plzip --version' and 'plzip -v --check-lib'.


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

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

[index]
* Menu:

* bugs:                                  Problems.                  (line 6)
* examples:                              Examples.                  (line 6)
* file format:                           File format.               (line 6)
* getting help:                          Problems.                  (line 6)
* introduction:                          Introduction.              (line 6)
* invoking:                              Invoking plzip.            (line 6)
* memory requirements:                   Memory requirements.       (line 6)
* minimum file sizes:                    Minimum file sizes.        (line 6)
* options:                               Invoking plzip.            (line 6)
* output:                                Output.                    (line 6)
* program design:                        Program design.            (line 6)
* trailing data:                         Trailing data.             (line 6)
* usage:                                 Invoking plzip.            (line 6)
* version:                               Invoking plzip.            (line 6)



Tag Table:
Node: Top217
Node: Introduction1156
Node: Output5934
Node: Invoking plzip7497
Ref: --trailing-error8372
Ref: --data-size8610
Node: Program design19519
Node: Memory requirements21818
Node: Minimum file sizes23503
Node: File format25506
Ref: coded-dict-size26945
Node: Trailing data28195
Node: Examples30531
Ref: concat-example31964
Node: Problems32721
Node: Concept index33276

End Tag Table


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