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

Lzip is a lossless data compressor with a user interface similar to the one
of gzip or bzip2. Lzip uses a simplified form of LZMA (Lempel-Ziv-Markov
chain-Algorithm) and is designed to achieve complete interoperability
between implementations. 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 -0' compresses about as fast as
gzip, while 'lzip -9' compresses most files more than bzip2. Decompression
speed is intermediate between gzip and bzip2. Lzip provides better data
recovery capabilities than gzip and bzip2. Lzip has been designed, written,
and tested with great care to replace gzip and bzip2 as general-purpose
compressed format for Unix-like systems.

For compressing/decompressing large files on multiprocessor machines plzip
can be much faster than lzip at the cost of a slightly reduced compression
ratio. *Note plzip manual: (plzip)Top.

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 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.

The member trailer stores the 32-bit CRC of the original data, the size of
the original data, and the size of the member. These values, together with
the 'End Of Stream' marker, provide a 3-factor integrity checking that
guards against corruption of the compressed data and against undetected bugs
in lzip (hopefully very unlikely). The chances of data corruption going
undetected are microscopic. Be aware, though, that the check occurs upon
decompression, so it can only tell you that something is wrong. It can't
help you recover the original uncompressed data.

Lzip uses the same well-defined exit status values used by bzip2, 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.

Lzip automatically uses for each file the largest dictionary size that does
not exceed neither the file size nor the limit given. The dictionary size
used for decompression is the same dictionary size used for compression.

The amount of memory required for compression is about 1 or 2 times the
dictionary size limit (1 if input file size is less than dictionary size
limit, else 2) plus 9 times the dictionary size really used. The option
'-0' is special and only requires about 1.5 MiB at most. The amount of
memory required for decompression is about 46 kB larger than the dictionary
size really used.

When compressing, lzip replaces each file given in the command line with a
compressed version named 'original_name.lz'. When decompressing, lzip
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 lzip
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).

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

Lzip 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.

Lzip 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.

Lzip can produce multimember files, and lziprecover can safely recover the
undamaged members in case of file damage. Lzip can also split the compressed
output in volumes of a given size, even when reading from standard input.

Lzip is able to compress and decompress streams of unlimited size by
automatically creating multimember output. The members so created are large,
about 2 PiB each.