path: root/doc/plzip.texi

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

@set UPDATED 24 January 2022
@set VERSION 1.10

@dircategory Compression
@direntry
* Plzip: (plzip).               Massively parallel implementation of lzip
@end direntry


@ifnothtml
@titlepage
@title Plzip
@subtitle Massively parallel implementation of lzip
@subtitle for Plzip version @value{VERSION}, @value{UPDATED}
@author by Antonio Diaz Diaz

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

@contents
@end ifnothtml

@ifnottex
@node Top
@top

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

@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
@end menu

@sp 1
Copyright @copyright{} 2009-2022 Antonio Diaz Diaz.

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


@node Introduction
@chapter Introduction
@cindex introduction

@uref{http://www.nongnu.org/lzip/plzip.html,,Plzip}
is a massively parallel (multi-threaded) implementation of lzip, fully
compatible with lzip 1.4 or newer. Plzip uses the compression library
@uref{http://www.nongnu.org/lzip/lzlib.html,,lzlib}.

@uref{http://www.nongnu.org/lzip/lzip.html,,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 and provides a 3 factor integrity
checking to maximize interoperability and optimize safety. Lzip can compress
about as fast as gzip @w{(lzip -0)} or compress most files more than bzip2
@w{(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. @xref{Minimum file sizes}.

For creation and manipulation of compressed tar archives
@uref{http://www.nongnu.org/lzip/manual/tarlz_manual.html,,tarlz} can be more
efficient than using tar and plzip because tarlz is able to keep the
alignment between tar members and lzip members.
@ifnothtml
@xref{Top,tarlz manual,,tarlz}.
@end ifnothtml

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

@itemize @bullet
@item
The lzip format provides very safe integrity checking and some data
recovery means. The program
@uref{http://www.nongnu.org/lzip/manual/lziprecover_manual.html#Data-safety,,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.
@ifnothtml
@xref{Data safety,,,lziprecover}.
@end ifnothtml

@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.

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 will automatically use 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. @xref{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:

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

(De)compressing a file is much like copying or moving it. Therefore plzip
preserves the access and modification dates, permissions, and, when
possible, ownership of the file just as @w{@samp{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 @samp{-c} or the option @samp{-o} is specified.

Plzip will refuse 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 will correctly decompress 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.


@node Output
@chapter Meaning of plzip's output
@cindex output

The output of plzip looks like this:

@example
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
@end example

The meaning of each field is as follows:

@table @code
@item N:1
The compression ratio @w{(uncompressed_size / compressed_size)}, shown as
@w{N to 1}.

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

@item saved
The space saved by compression @w{(1 - ratio)}, shown as a percentage.

@item 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.

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

@end table

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.


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

The format for running plzip is:

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

@noindent
If no file names are specified, plzip compresses (or decompresses) from
standard input to standard output. A hyphen @samp{-} used as a @var{file}
argument means standard input. It can be mixed with other @var{files} and is
read just once, the first time it appears in the command line.

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

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

@item -V
@itemx --version
Print the version number of plzip 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}.

@anchor{--data-size}
@item -B @var{bytes}
@itemx --data-size=@var{bytes}
When compressing, set the size in bytes of the input data blocks. The
input file will be divided in chunks of this size before compression is
performed. Valid values range from @w{8 KiB} to @w{1 GiB}. Default value
is two times the dictionary size, except for option @samp{-0} where it
defaults to @w{1 MiB}. Plzip will reduce the dictionary size if it is
larger than the data size specified. @xref{Minimum file sizes}.

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

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

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

@item -F
@itemx --recompress
When compressing, force re-compression of files whose name already has
the @samp{.lz} or @samp{.tlz} suffix.

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

@item -l
@itemx --list
Print the uncompressed size, compressed size, and percentage saved of the
files specified. Trailing data are ignored. The values produced are correct
even for multimember files. If more than one file is given, a final line
containing the cumulative sizes is printed. With @samp{-v}, the dictionary
size, the number of members in the file, and the amount of trailing data (if
any) are also printed. With @samp{-vv}, the positions and sizes of each
member in multimember files are also printed.

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

@item -m @var{bytes}
@itemx --match-length=@var{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.

@item -n @var{n}
@itemx --threads=@var{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 @w{32 bit} system,
plzip tries to limit the memory use to under @w{2.22 GiB} (4 worker threads
at level -9) by reducing the number of threads below the system's default.
@w{@samp{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
@w{ceil( file_size / data_size )} during compression (@pxref{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
@w{@samp{plzip -lv file.lz}}.

@item -o @var{file}
@itemx --output=@var{file}
If @samp{-c} has not been also specified, write the (de)compressed output to
@var{file}; keep input files unchanged. If compressing several files, each
file is compressed independently. This option (or @samp{-c}) is needed when
reading from a named pipe (fifo) or from a device. @w{@samp{-o -}} is
equivalent to @samp{-c}. @samp{-o} has no effect when testing or listing.

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 @samp{.lz} is appended to @var{file} unless it already ends in
@samp{.lz} or @samp{.tlz}. This feature will be removed in a future version
of plzip. Meanwhile, redirection may be used instead of @samp{-o} to write
the compressed output to a file without the extension @samp{.lz} in its
name: @w{@samp{plzip < file > foo}}.

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

@item -s @var{bytes}
@itemx --dictionary-size=@var{bytes}
When compressing, set the dictionary size limit in bytes. Plzip will use
for each file the largest dictionary size that does not exceed neither
the file size nor this limit. Valid values range from @w{4 KiB} to
@w{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 (@pxref{coded-dict-size}). If the size specified
does not match one of the valid sizes, it will be rounded upwards by
adding up to @w{(@var{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.

@item -t
@itemx --test
Check integrity of the files specified, but don't decompress them. This
really performs a trial decompression and throws away the result. Use it
together with @samp{-v} to see information about the files. If a file
fails the test, does not exist, can't be opened, or is a terminal, plzip
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 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 @samp{-v} options show the progress of (de)compression,
except for single-member files.

@item -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 @samp{-6}, equivalent to @w{@samp{-s8MiB -m36}}. Note that
@samp{-9} can be much slower than @samp{-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 @samp{--dictionary-size} and
@samp{--match-length} directly to achieve optimal performance.

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

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

@item --fast
@itemx --best
Aliases for GNU gzip compatibility.

@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 --in-slots=@var{n}
Number of @w{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.

@item --out-slots=@var{n}
Number of @w{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.

@item --check-lib
Compare the
@uref{http://www.nongnu.org/lzip/manual/lzlib_manual.html#Library-version,,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. @w{@samp{plzip -v --check-lib}} shows the version of lzlib being used
and the value of LZ_API_VERSION (if defined).
@ifnothtml
@xref{Library version,,,lzlib}.
@end ifnothtml

@end table

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

Table of SI and binary prefixes (unit multipliers):

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

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


@node Program design
@chapter Internal structure of plzip
@cindex program design

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 will not
be decompressed faster than using lzip (unless the option @samp{-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.

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

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.


@node Memory requirements
@chapter Memory required to compress and decompress
@cindex memory requirements

The amount of memory required @strong{per worker thread} for decompression
or testing is approximately the following:

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

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

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

@item
For decompression of a non-seekable file or of standard input; the
dictionary size plus @w{1 MiB} plus up to the number of @w{1 MiB} input
and output packets buffered (68 by default).
@end itemize

@noindent
The amount of memory required @strong{per worker thread} for compression
is approximately the following:

@itemize @bullet
@item
For compression at level -0; @w{1.5 MiB} plus 3.375 times the data size
(@pxref{--data-size}). Default is @w{4.875 MiB}.

@item
For compression at other levels; 11 times the dictionary size plus 3.375
times the data size. Default is @w{142 MiB}.
@end itemize

@noindent
The following table shows the memory required @strong{per thread} for
compression at a given level, using the default data size for each level:

@multitable {Level} {Memory required}
@item Level @tab Memory required
@item -0 @tab   4.875 MiB
@item -1 @tab  17.75 MiB
@item -2 @tab  26.625 MiB
@item -3 @tab  35.5 MiB
@item -4 @tab  53.25 MiB
@item -5 @tab  71 MiB
@item -6 @tab 142 MiB
@item -7 @tab 284 MiB
@item -8 @tab 426 MiB
@item -9 @tab 568 MiB
@end multitable


@node Minimum file sizes
@chapter Minimum file sizes required for full compression speed
@cindex minimum file sizes

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
(@pxref{--data-size}). Else some processors will not get any data to
compress, and compression will be proportionally slower. The maximum
speed increase achievable on a given file is limited by the ratio
@w{(file_size / data_size)}. For example, a tarball the size of gcc or
linux will scale 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:

@multitable {Processors} {512 MiB} {512 MiB} {512 MiB} {512 MiB} {512 MiB} {512 MiB}
@headitem Processors @tab 2 @tab 4 @tab 8 @tab 16 @tab 64 @tab 256
@item Level
@item -0 @tab   2 MiB @tab   4 MiB @tab   8 MiB @tab  16 MiB @tab  64 MiB @tab 256 MiB
@item -1 @tab   4 MiB @tab   8 MiB @tab  16 MiB @tab  32 MiB @tab 128 MiB @tab 512 MiB
@item -2 @tab   6 MiB @tab  12 MiB @tab  24 MiB @tab  48 MiB @tab 192 MiB @tab 768 MiB
@item -3 @tab   8 MiB @tab  16 MiB @tab  32 MiB @tab  64 MiB @tab 256 MiB @tab   1 GiB
@item -4 @tab  12 MiB @tab  24 MiB @tab  48 MiB @tab  96 MiB @tab 384 MiB @tab 1.5 GiB
@item -5 @tab  16 MiB @tab  32 MiB @tab  64 MiB @tab 128 MiB @tab 512 MiB @tab   2 GiB
@item -6 @tab  32 MiB @tab  64 MiB @tab 128 MiB @tab 256 MiB @tab   1 GiB @tab   4 GiB
@item -7 @tab  64 MiB @tab 128 MiB @tab 256 MiB @tab 512 MiB @tab   2 GiB @tab   8 GiB
@item -8 @tab  96 MiB @tab 192 MiB @tab 384 MiB @tab 768 MiB @tab   3 GiB @tab  12 GiB
@item -9 @tab 128 MiB @tab 256 MiB @tab 512 MiB @tab   1 GiB @tab   4 GiB @tab  16 GiB
@end multitable


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

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

@sp 1
In the diagram below, a box like this:

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

represents one byte; a box like this:

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

represents a variable number of bytes.

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

Each member has the following structure:

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

All multibyte values are stored in little endian order.

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

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

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

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

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

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

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

@end table


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

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

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

@item
Useful data added by the user; a cryptographically secure hash, a
description of file contents, etc. It is safe to append any amount of
text to a lzip file as long as none of the first four bytes of the text
match the corresponding byte in the string "LZIP", and the text does not
contain any zero bytes (null characters). Nonzero bytes and zero bytes
can't be safely mixed in trailing data.

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

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

@item
In rare cases, trailing data could be the corrupt header of another
member. In multimember or concatenated files the probability of
corruption happening in the magic bytes is 5 times smaller than the
probability of getting a false positive caused by the corruption of the
integrity information itself. Therefore it can be considered to be below
the noise level. Additionally, the test used by 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 @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}.


@node Examples
@chapter A small tutorial with examples
@cindex 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 @samp{--keep} to plzip and don't remove the original file until you
verify the compressed file with a command like
@w{@samp{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.

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

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

@sp 1
@noindent
Example 2: Replace a regular file with its compressed version @samp{file.lz}
and show the compression ratio.

@example
plzip -v file
@end example

@sp 1
@noindent
Example 3: Like example 2 but the created @samp{file.lz} has a block size of
@w{1 MiB}. The compression ratio is not shown.

@example
plzip -B 1MiB file
@end example

@sp 1
@noindent
Example 4: Restore a regular file from its compressed version
@samp{file.lz}. If the operation is successful, @samp{file.lz} is removed.

@example
plzip -d file.lz
@end example

@sp 1
@noindent
Example 5: Verify the integrity of the compressed file @samp{file.lz} and
show status.

@example
plzip -tv file.lz
@end example

@sp 1
@anchor{concat-example}
@noindent
Example 6: 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 | plzip -d -
Do this instead
  plzip -cd file1.lz file2.lz file3.lz
@end example

@sp 1
@noindent
Example 7: Decompress @samp{file.lz} partially until @w{10 KiB} of
decompressed data are produced.

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

@sp 1
@noindent
Example 8: Decompress @samp{file.lz} partially from decompressed byte at
offset 10000 to decompressed byte at offset 14999 (5000 bytes are produced).

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

@sp 1
@noindent
Example 9: Compress a whole device in /dev/sdc and send the output to
@samp{file.lz}.

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


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

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
@email{lzip-bug@@nongnu.org}. Include the version number, which you can
find by running @w{@samp{plzip --version}} and
@w{@samp{plzip -v --check-lib}}.


@node Concept index
@unnumbered Concept index

@printindex cp

@bye