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Bzip2 is not research work, in the sense that it doesn't present any
new ideas. Rather, it's an engineering exercise based on existing
ideas.
Four documents describe essentially all the ideas behind bzip2:
Michael Burrows and D. J. Wheeler:
"A block-sorting lossless data compression algorithm"
10th May 1994.
Digital SRC Research Report 124.
ftp://ftp.digital.com/pub/DEC/SRC/research-reports/SRC-124.ps.gz
Daniel S. Hirschberg and Debra A. LeLewer
"Efficient Decoding of Prefix Codes"
Communications of the ACM, April 1990, Vol 33, Number 4.
You might be able to get an electronic copy of this
from the ACM Digital Library.
David J. Wheeler
Program bred3.c and accompanying document bred3.ps.
This contains the idea behind the multi-table Huffman
coding scheme.
ftp://ftp.cl.cam.ac.uk/pub/user/djw3/
Jon L. Bentley and Robert Sedgewick
"Fast Algorithms for Sorting and Searching Strings"
Available from Sedgewick's web page,
www.cs.princeton.edu/~rs
The following paper gives valuable additional insights into the
algorithm, but is not immediately the basis of any code
used in bzip2.
Peter Fenwick:
Block Sorting Text Compression
Proceedings of the 19th Australasian Computer Science Conference,
Melbourne, Australia. Jan 31 - Feb 2, 1996.
ftp://ftp.cs.auckland.ac.nz/pub/peter-f/ACSC96paper.ps
All three are well written, and make fascinating reading. If you want
to modify bzip2 in any non-trivial way, I strongly suggest you obtain,
read and understand these papers.
I am much indebted to the various authors for their help, support and
advice.

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GNU GENERAL PUBLIC LICENSE
Version 2, June 1991
Copyright (C) 1989, 1991 Free Software Foundation, Inc.
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Also add information on how to contact you by electronic and paper mail.
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Gnomovision version 69, Copyright (C) 19yy name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
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The hypothetical commands `show w' and `show c' should show the appropriate
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mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your
school, if any, to sign a "copyright disclaimer" for the program, if
necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.
<signature of Ty Coon>, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your program into
proprietary programs. If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library. If this is what you want to do, use the GNU Library General
Public License instead of this License.

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CC = gcc
SH = /bin/sh
CFLAGS = -O3 -fomit-frame-pointer -funroll-loops -Wall -Winline -W
all:
cat words0
$(CC) $(CFLAGS) -o bzip2 bzip2.c
$(CC) $(CFLAGS) -o bzip2recover bzip2recover.c
rm -f bunzip2
ln -s ./bzip2 ./bunzip2
cat words1
./bzip2 -1 < sample1.ref > sample1.rb2
./bzip2 -2 < sample2.ref > sample2.rb2
./bunzip2 < sample1.bz2 > sample1.tst
./bunzip2 < sample2.bz2 > sample2.tst
cat words2
cmp sample1.bz2 sample1.rb2
cmp sample2.bz2 sample2.rb2
cmp sample1.tst sample1.ref
cmp sample2.tst sample2.ref
cat words3
clean:
rm -f bzip2 bunzip2 bzip2recover sample*.tst sample*.rb2

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GREETINGS!
This is the README for bzip2, my block-sorting file compressor,
version 0.1.
bzip2 is distributed under the GNU General Public License version 2;
for details, see the file LICENSE. Pointers to the algorithms used
are in ALGORITHMS. Instructions for use are in bzip2.1.preformatted.
Please read this file carefully.
HOW TO BUILD
-- for UNIX:
Type `make'. (tough, huh? :-)
This creates binaries "bzip2", and "bunzip2",
which is a symbolic link to "bzip2".
It also runs four compress-decompress tests to make sure
things are working properly. If all goes well, you should be up &
running. Please be sure to read the output from `make'
just to be sure that the tests went ok.
To install bzip2 properly:
-- Copy the binary "bzip2" to a publically visible place,
possibly /usr/bin, /usr/common/bin or /usr/local/bin.
-- In that directory, make "bunzip2" be a symbolic link
to "bzip2".
-- Copy the manual page, bzip2.1, to the relevant place.
Probably the right place is /usr/man/man1/.
-- for Windows 95 and NT:
For a start, do you *really* want to recompile bzip2?
The standard distribution includes a pre-compiled version
for Windows 95 and NT, `bzip2.exe'.
This executable was created with Jacob Navia's excellent
port to Win32 of Chris Fraser & David Hanson's excellent
ANSI C compiler, "lcc". You can get to it at the pages
of the CS department of Princeton University,
www.cs.princeton.edu.
I have not tried to compile this version of bzip2 with
a commercial C compiler such as MS Visual C, as I don't
have one available.
Note that lcc is designed primarily to be portable and
fast. Code quality is a secondary aim, so bzip2.exe
runs perhaps 40% slower than it could if compiled with
a good optimising compiler.
I compiled a previous version of bzip (0.21) with Borland
C 5.0, which worked fine, and with MS VC++ 2.0, which
didn't. Here is an comment from the README for bzip-0.21.
MS VC++ 2.0's optimising compiler has a bug which, at
maximum optimisation, gives an executable which produces
garbage compressed files. Proceed with caution.
I do not know whether or not this happens with later
versions of VC++.
Edit the defines starting at line 86 of bzip.c to
select your platform/compiler combination, and then compile.
Then check that the resulting executable (assumed to be
called bzip.exe) works correctly, using the SELFTEST.BAT file.
Bearing in mind the previous paragraph, the self-test is
important.
Note that the defines which bzip-0.21 had, to support
compilation with VC 2.0 and BC 5.0, are gone. Windows
is not my preferred operating system, and I am, for the
moment, content with the modestly fast executable created
by lcc-win32.
A manual page is supplied, unformatted (bzip2.1),
preformatted (bzip2.1.preformatted), and preformatted
and sanitised for MS-DOS (bzip2.txt).
COMPILATION NOTES
bzip2 should work on any 32 or 64-bit machine. It is known to work
[meaning: it has compiled and passed self-tests] on the
following platform-os combinations:
Intel i386/i486 running Linux 2.0.21
Sun Sparcs (various) running SunOS 4.1.4 and Solaris 2.5
Intel i386/i486 running Windows 95 and NT
DEC Alpha running Digital Unix 4.0
Following the release of bzip-0.21, many people mailed me
from around the world to say they had made it work on all sorts
of weird and wonderful machines. Chances are, if you have
a reasonable ANSI C compiler and a 32-bit machine, you can
get it to work.
The #defines starting at around line 82 of bzip2.c supply some
degree of platform-independance. If you configure bzip2 for some
new far-out platform which is not covered by the existing definitions,
please send me the relevant definitions.
I recommend GNU C for compilation. The code is standard ANSI C,
except for the Unix-specific file handling, so any ANSI C compiler
should work. Note however that the many routines marked INLINE
should be inlined by your compiler, else performance will be very
poor. Asking your compiler to unroll loops gives some
small improvement too; for gcc, the relevant flag is
-funroll-loops.
On a 386/486 machines, I'd recommend giving gcc the
-fomit-frame-pointer flag; this liberates another register for
allocation, which measurably improves performance.
I used the abovementioned lcc compiler to develop bzip2.
I would highly recommend this compiler for day-to-day development;
it is fast, reliable, lightweight, has an excellent profiler,
and is generally excellent. And it's fun to retarget, if you're
into that kind of thing.
If you compile bzip2 on a new platform or with a new compiler,
please be sure to run the four compress-decompress tests, either
using the Makefile, or with the test.bat (MSDOS) or test.cmd (OS/2)
files. Some compilers have been seen to introduce subtle bugs
when optimising, so this check is important. Ideally you should
then go on to test bzip2 on a file several megabytes or even
tens of megabytes long, just to be 110% sure. ``Professional
programmers are paranoid programmers.'' (anon).
VALIDATION
Correct operation, in the sense that a compressed file can always be
decompressed to reproduce the original, is obviously of paramount
importance. To validate bzip2, I used a modified version of
Mark Nelson's churn program. Churn is an automated test driver
which recursively traverses a directory structure, using bzip2 to
compress and then decompress each file it encounters, and checking
that the decompressed data is the same as the original. As test
material, I used several runs over several filesystems of differing
sizes.
One set of tests was done on my base Linux filesystem,
410 megabytes in 23,000 files. There were several runs over
this filesystem, in various configurations designed to break bzip2.
That filesystem also contained some specially constructed test
files designed to exercise boundary cases in the code.
This included files of zero length, various long, highly repetitive
files, and some files which generate blocks with all values the same.
The other set of tests was done just with the "normal" configuration,
but on a much larger quantity of data.
Tests are:
Linux FS, 410M, 23000 files
As above, with --repetitive-fast
As above, with -1
Low level disk image of a disk containing
Windows NT4.0; 420M in a single huge file
Linux distribution, incl Slackware,
all GNU sources. 1900M in 2300 files.
Approx ~100M compiler sources and related
programming tools, running under Purify.
About 500M of data in 120 files of around
4 M each. This is raw data from a
biomagnetometer (SQUID-based thing).
Overall, total volume of test data is about
3300 megabytes in 25000 files.
The distribution does four tests after building bzip. These tests
include test decompressions of pre-supplied compressed files, so
they not only test that bzip works correctly on the machine it was
built on, but can also decompress files compressed on a different
machine. This guards against unforseen interoperability problems.
Please read and be aware of the following:
WARNING:
This program (attempts to) compress data by performing several
non-trivial transformations on it. Unless you are 100% familiar
with *all* the algorithms contained herein, and with the
consequences of modifying them, you should NOT meddle with the
compression or decompression machinery. Incorrect changes can and
very likely *will* lead to disastrous loss of data.
DISCLAIMER:
I TAKE NO RESPONSIBILITY FOR ANY LOSS OF DATA ARISING FROM THE
USE OF THIS PROGRAM, HOWSOEVER CAUSED.
Every compression of a file implies an assumption that the
compressed file can be decompressed to reproduce the original.
Great efforts in design, coding and testing have been made to
ensure that this program works correctly. However, the complexity
of the algorithms, and, in particular, the presence of various
special cases in the code which occur with very low but non-zero
probability make it impossible to rule out the possibility of bugs
remaining in the program. DO NOT COMPRESS ANY DATA WITH THIS
PROGRAM UNLESS YOU ARE PREPARED TO ACCEPT THE POSSIBILITY, HOWEVER
SMALL, THAT THE DATA WILL NOT BE RECOVERABLE.
That is not to say this program is inherently unreliable. Indeed,
I very much hope the opposite is true. bzip2 has been carefully
constructed and extensively tested.
End of nasty legalities.
I hope you find bzip2 useful. Feel free to contact me at
jseward@acm.org
if you have any suggestions or queries. Many people mailed me with
comments, suggestions and patches after the releases of 0.15 and 0.21,
and the changes in bzip2 are largely a result of this feedback.
I thank you for your comments.
Julian Seward
Manchester, UK
18 July 1996 (version 0.15)
25 August 1996 (version 0.21)
Guildford, Surrey, UK
7 August 1997 (bzip2, version 0.0)

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Windows 95 & Windows NT users:
1. There's a pre-built executable, bzip2.exe, which
should work. You don't need to compile anything.
You can run the `test.bat' batch file to check
the executable is working ok, if you want.
2. The control-C signal catcher seems pretty dodgy
under Windows, at least for the executable supplied.
When it catches a control-C, bzip2 tries to delete
its output file, so you don't get left with a half-
baked file. But this sometimes seems to fail
under Windows. Caveat Emptor! I think I am doing
something not-quite-right in the signal catching.
Windows-&-C gurus got any suggestions?
Control-C handling all seems to work fine under Unix.
7 Aug 97

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.PU
.TH bzip2 1
.SH NAME
bzip2, bunzip2 \- a block-sorting file compressor, v0.1
.br
bzip2recover \- recovers data from damaged bzip2 files
.SH SYNOPSIS
.ll +8
.B bzip2
.RB [ " \-cdfkstvVL123456789 " ]
[
.I "filenames \&..."
]
.ll -8
.br
.B bunzip2
.RB [ " \-kvsVL " ]
[
.I "filenames \&..."
]
.br
.B bzip2recover
.I "filename"
.SH DESCRIPTION
.I Bzip2
compresses files using the Burrows-Wheeler block-sorting
text compression algorithm, and Huffman coding.
Compression is generally considerably
better than that
achieved by more conventional LZ77/LZ78-based compressors,
and approaches the performance of the PPM family of statistical
compressors.
The command-line options are deliberately very similar to
those of
.I GNU Gzip,
but they are not identical.
.I Bzip2
expects a list of file names to accompany the command-line flags.
Each file is replaced by a compressed version of itself,
with the name "original_name.bz2".
Each compressed file has the same modification date and permissions
as the corresponding original, so that these properties can be
correctly restored at decompression time. File name handling is
naive in the sense that there is no mechanism for preserving
original file names, permissions and dates in filesystems
which lack these concepts, or have serious file name length
restrictions, such as MS-DOS.
.I Bzip2
and
.I bunzip2
will not overwrite existing files; if you want this to happen,
you should delete them first.
If no file names are specified,
.I bzip2
compresses from standard input to standard output.
In this case,
.I bzip2
will decline to write compressed output to a terminal, as
this would be entirely incomprehensible and therefore pointless.
.I Bunzip2
(or
.I bzip2 \-d
) decompresses and restores all specified files whose names
end in ".bz2".
Files without this suffix are ignored.
Again, supplying no filenames
causes decompression from standard input to standard output.
You can also compress or decompress files to
the standard output by giving the \-c flag.
You can decompress multiple files like this, but you may
only compress a single file this way, since it would otherwise
be difficult to separate out the compressed representations of
the original files.
Compression is always performed, even if the compressed file is
slightly larger than the original. Files of less than about
one hundred bytes tend to get larger, since the compression
mechanism has a constant overhead in the region of 50 bytes.
Random data (including the output of most file compressors)
is coded at about 8.05 bits per byte, giving an expansion of
around 0.5%.
As a self-check for your protection,
.I bzip2
uses 32-bit CRCs to make sure that the decompressed
version of a file is identical to the original.
This guards against corruption of the compressed data,
and against undetected bugs in
.I bzip2
(hopefully very unlikely).
The chances of data corruption going undetected is
microscopic, about one chance in four billion
for each file processed. Be aware, though, that the check
occurs upon decompression, so it can only tell you that
that something is wrong. It can't help you recover the
original uncompressed data.
You can use
.I bzip2recover
to try to recover data from damaged files.
Return values:
0 for a normal exit,
1 for environmental
problems (file not found, invalid flags, I/O errors, &c),
2 to indicate a corrupt compressed file,
3 for an internal consistency error (eg, bug) which caused
.I bzip2
to panic.
.SH MEMORY MANAGEMENT
.I Bzip2
compresses large files in blocks. The block size affects both the
compression ratio achieved, and the amount of memory needed both for
compression and decompression. The flags \-1 through \-9
specify the block size to be 100,000 bytes through 900,000 bytes
(the default) respectively. At decompression-time, the block size used for
compression is read from the header of the compressed file, and
.I bunzip2
then allocates itself just enough memory to decompress the file.
Since block sizes are stored in compressed files, it follows that the flags
\-1 to \-9
are irrelevant to and so ignored during decompression.
Compression and decompression requirements, in bytes, can be estimated as:
Compression: 400k + ( 7 x block size )
Decompression: 100k + ( 5 x block size ), or
.br
100k + ( 2.5 x block size )
Larger block sizes give rapidly diminishing marginal returns; most
of the
compression comes from the first two or three hundred k of block size,
a fact worth bearing in mind when using
.I bzip2
on small machines. It is also important to appreciate that the
decompression memory requirement is set at compression-time by the
choice of block size.
For files compressed with the default 900k block size,
.I bunzip2
will require about 4600 kbytes to decompress.
To support decompression of any file on a 4 megabyte machine,
.I bunzip2
has an option to decompress using approximately half this
amount of memory, about 2300 kbytes. Decompression speed is
also halved, so you should use this option only where necessary.
The relevant flag is \-s.
In general, try and use the largest block size
memory constraints allow, since that maximises the compression
achieved. Compression and decompression
speed are virtually unaffected by block size.
Another significant point applies to files which fit in a single
block -- that means most files you'd encounter using a large
block size. The amount of real memory touched is proportional
to the size of the file, since the file is smaller than a block.
For example, compressing a file 20,000 bytes long with the flag
\-9
will cause the compressor to allocate around
6700k of memory, but only touch 400k + 20000 * 7 = 540
kbytes of it. Similarly, the decompressor will allocate 4600k but
only touch 100k + 20000 * 5 = 200 kbytes.
Here is a table which summarises the maximum memory usage for
different block sizes. Also recorded is the total compressed
size for 14 files of the Calgary Text Compression Corpus
totalling 3,141,622 bytes. This column gives some feel for how
compression varies with block size. These figures tend to understate
the advantage of larger block sizes for larger files, since the
Corpus is dominated by smaller files.
Compress Decompress Decompress Corpus
Flag usage usage -s usage Size
-1 1100k 600k 350k 914704
-2 1800k 1100k 600k 877703
-3 2500k 1600k 850k 860338
-4 3200k 2100k 1100k 846899
-5 3900k 2600k 1350k 845160
-6 4600k 3100k 1600k 838626
-7 5400k 3600k 1850k 834096
-8 6000k 4100k 2100k 828642
-9 6700k 4600k 2350k 828642
.SH OPTIONS
.TP
.B \-c --stdout
Compress or decompress to standard output. \-c will decompress
multiple files to stdout, but will only compress a single file to
stdout.
.TP
.B \-d --decompress
Force decompression.
.I Bzip2
and
.I bunzip2
are really the same program, and the decision about whether to
compress or decompress is done on the basis of which name is
used. This flag overrides that mechanism, and forces
.I bzip2
to decompress.
.TP
.B \-f --compress
The complement to \-d: forces compression, regardless of the invokation
name.
.TP
.B \-t --test
Check integrity of the specified file(s), but don't decompress them.
This really performs a trial decompression and throws away the result,
using the low-memory decompression algorithm (see \-s).
.TP
.B \-k --keep
Keep (don't delete) input files during compression or decompression.
.TP
.B \-s --small
Reduce memory usage, both for compression and decompression.
Files are decompressed using a modified algorithm which only
requires 2.5 bytes per block byte. This means any file can be
decompressed in 2300k of memory, albeit somewhat more slowly than
usual.
During compression, -s selects a block size of 200k, which limits
memory use to around the same figure, at the expense of your
compression ratio. In short, if your machine is low on memory
(8 megabytes or less), use -s for everything. See
MEMORY MANAGEMENT above.
.TP
.B \-v --verbose
Verbose mode -- show the compression ratio for each file processed.
Further \-v's increase the verbosity level, spewing out lots of
information which is primarily of interest for diagnostic purposes.
.TP
.B \-L --license
Display the software version, license terms and conditions.
.TP
.B \-V --version
Same as \-L.
.TP
.B \-1 to \-9
Set the block size to 100 k, 200 k .. 900 k when
compressing. Has no effect when decompressing.
See MEMORY MANAGEMENT above.
.TP
.B \--repetitive-fast
.I bzip2
injects some small pseudo-random variations
into very repetitive blocks to limit
worst-case performance during compression.
If sorting runs into difficulties, the block
is randomised, and sorting is restarted.
Very roughly,
.I bzip2
persists for three times as long as a well-behaved input
would take before resorting to randomisation.
This flag makes it give up much sooner.
.TP
.B \--repetitive-best
Opposite of \--repetitive-fast; try a lot harder before
resorting to randomisation.
.SH RECOVERING DATA FROM DAMAGED FILES
.I bzip2
compresses files in blocks, usually 900kbytes long.
Each block is handled independently. If a media or
transmission error causes a multi-block .bz2
file to become damaged,
it may be possible to recover data from the undamaged blocks
in the file.
The compressed representation of each block is delimited by
a 48-bit pattern, which makes it possible to find the block
boundaries with reasonable certainty. Each block also carries
its own 32-bit CRC, so damaged blocks can be
distinguished from undamaged ones.
.I bzip2recover
is a simple program whose purpose is to search for
blocks in .bz2 files, and write each block out into
its own .bz2 file. You can then use
.I bzip2 -t
to test the integrity of the resulting files,
and decompress those which are undamaged.
.I bzip2recover
takes a single argument, the name of the damaged file,
and writes a number of files "rec0001file.bz2", "rec0002file.bz2",
etc, containing the extracted blocks. The output filenames
are designed so that the use of wildcards in subsequent processing
-- for example, "bzip2 -dc rec*file.bz2 > recovered_data" --
lists the files in the "right" order.
.I bzip2recover
should be of most use dealing with large .bz2 files, as
these will contain many blocks. It is clearly futile to
use it on damaged single-block files, since a damaged
block cannot be recovered. If you wish to minimise
any potential data loss through media or transmission
errors, you might consider compressing with a smaller
block size.
.SH PERFORMANCE NOTES
The sorting phase of compression gathers together similar strings
in the file. Because of this, files containing very long
runs of repeated symbols, like "aabaabaabaab ..." (repeated
several hundred times) may compress extraordinarily slowly.
You can use the
\-vvvvv
option to monitor progress in great detail, if you want.
Decompression speed is unaffected.
Such pathological cases
seem rare in practice, appearing mostly in artificially-constructed
test files, and in low-level disk images. It may be inadvisable to
use
.I bzip2
to compress the latter.
If you do get a file which causes severe slowness in compression,
try making the block size as small as possible, with flag \-1.
Incompressible or virtually-incompressible data may decompress
rather more slowly than one would hope. This is due to
a naive implementation of the move-to-front coder.
.I bzip2
usually allocates several megabytes of memory to operate in,
and then charges all over it in a fairly random fashion. This
means that performance, both for compressing and decompressing,
is largely determined by the speed
at which your machine can service cache misses.
Because of this, small changes
to the code to reduce the miss rate have been observed to give
disproportionately large performance improvements.
I imagine
.I bzip2
will perform best on machines with very large caches.
Test mode (\-t) uses the low-memory decompression algorithm
(\-s). This means test mode does not run as fast as it could;
it could run as fast as the normal decompression machinery.
This could easily be fixed at the cost of some code bloat.
.SH CAVEATS
I/O error messages are not as helpful as they could be.
.I Bzip2
tries hard to detect I/O errors and exit cleanly, but the
details of what the problem is sometimes seem rather misleading.
This manual page pertains to version 0.1 of
.I bzip2.
It may well happen that some future version will
use a different compressed file format. If you try to
decompress, using 0.1, a .bz2 file created with some
future version which uses a different compressed file format,
0.1 will complain that your file "is not a bzip2 file".
If that happens, you should obtain a more recent version
of
.I bzip2
and use that to decompress the file.
Wildcard expansion for Windows 95 and NT
is flaky.
.I bzip2recover
uses 32-bit integers to represent bit positions in
compressed files, so it cannot handle compressed files
more than 512 megabytes long. This could easily be fixed.
.I bzip2recover
sometimes reports a very small, incomplete final block.
This is spurious and can be safely ignored.
.SH RELATIONSHIP TO bzip-0.21
This program is a descendant of the
.I bzip
program, version 0.21, which I released in August 1996.
The primary difference of
.I bzip2
is its avoidance of the possibly patented algorithms
which were used in 0.21.
.I bzip2
also brings various useful refinements (\-s, \-t),
uses less memory, decompresses significantly faster, and
has support for recovering data from damaged files.
Because
.I bzip2
uses Huffman coding to construct the compressed bitstream,
rather than the arithmetic coding used in 0.21,
the compressed representations generated by the two programs
are incompatible, and they will not interoperate. The change
in suffix from .bz to .bz2 reflects this. It would have been
helpful to at least allow
.I bzip2
to decompress files created by 0.21, but this would
defeat the primary aim of having a patent-free compressor.
Huffman coding necessarily involves some coding inefficiency
compared to arithmetic coding. This means that
.I bzip2
compresses about 1% worse than 0.21, an unfortunate but
unavoidable fact-of-life. On the other hand, decompression
is approximately 50% faster for the same reason, and the
change in file format gave an opportunity to add data-recovery
features. So it is not all bad.
.SH AUTHOR
Julian Seward, jseward@acm.org.
The ideas embodied in
.I bzip
and
.I bzip2
are due to (at least) the following people:
Michael Burrows and David Wheeler (for the block sorting
transformation), David Wheeler (again, for the Huffman coder),
Peter Fenwick (for the structured coding model in 0.21,
and many refinements),
and
Alistair Moffat, Radford Neal and Ian Witten (for the arithmetic
coder in 0.21). I am much indebted for their help, support and advice.
See the file ALGORITHMS in the source distribution for pointers to
sources of documentation.
Christian von Roques encouraged me to look for faster
sorting algorithms, so as to speed up compression.
Bela Lubkin encouraged me to improve the worst-case
compression performance.
Many people sent patches, helped with portability problems,
lent machines, gave advice and were generally helpful.

462
bzip2.1.preformatted Normal file
View File

@ -0,0 +1,462 @@
bzip2(1) bzip2(1)
NNAAMMEE
bzip2, bunzip2 - a block-sorting file compressor, v0.1
bzip2recover - recovers data from damaged bzip2 files
SSYYNNOOPPSSIISS
bbzziipp22 [ --ccddffkkssttvvVVLL112233445566778899 ] [ _f_i_l_e_n_a_m_e_s _._._. ]
bbuunnzziipp22 [ --kkvvssVVLL ] [ _f_i_l_e_n_a_m_e_s _._._. ]
bbzziipp22rreeccoovveerr _f_i_l_e_n_a_m_e
DDEESSCCRRIIPPTTIIOONN
_B_z_i_p_2 compresses files using the Burrows-Wheeler block-
sorting text compression algorithm, and Huffman coding.
Compression is generally considerably better than that
achieved by more conventional LZ77/LZ78-based compressors,
and approaches the performance of the PPM family of sta-
tistical compressors.
The command-line options are deliberately very similar to
those of _G_N_U _G_z_i_p_, but they are not identical.
_B_z_i_p_2 expects a list of file names to accompany the com-
mand-line flags. Each file is replaced by a compressed
version of itself, with the name "original_name.bz2".
Each compressed file has the same modification date and
permissions as the corresponding original, so that these
properties can be correctly restored at decompression
time. File name handling is naive in the sense that there
is no mechanism for preserving original file names, per-
missions and dates in filesystems which lack these con-
cepts, or have serious file name length restrictions, such
as MS-DOS.
_B_z_i_p_2 and _b_u_n_z_i_p_2 will not overwrite existing files; if
you want this to happen, you should delete them first.
If no file names are specified, _b_z_i_p_2 compresses from
standard input to standard output. In this case, _b_z_i_p_2
will decline to write compressed output to a terminal, as
this would be entirely incomprehensible and therefore
pointless.
_B_u_n_z_i_p_2 (or _b_z_i_p_2 _-_d ) decompresses and restores all spec-
ified files whose names end in ".bz2". Files without this
suffix are ignored. Again, supplying no filenames causes
decompression from standard input to standard output.
You can also compress or decompress files to the standard
output by giving the -c flag. You can decompress multiple
files like this, but you may only compress a single file
this way, since it would otherwise be difficult to sepa-
rate out the compressed representations of the original
files.
1
bzip2(1) bzip2(1)
Compression is always performed, even if the compressed
file is slightly larger than the original. Files of less
than about one hundred bytes tend to get larger, since the
compression mechanism has a constant overhead in the
region of 50 bytes. Random data (including the output of
most file compressors) is coded at about 8.05 bits per
byte, giving an expansion of around 0.5%.
As a self-check for your protection, _b_z_i_p_2 uses 32-bit
CRCs to make sure that the decompressed version of a file
is identical to the original. This guards against corrup-
tion of the compressed data, and against undetected bugs
in _b_z_i_p_2 (hopefully very unlikely). The chances of data
corruption going undetected is microscopic, about one
chance in four billion for each file processed. Be aware,
though, that the check occurs upon decompression, so it
can only tell you that that something is wrong. It can't
help you recover the original uncompressed data. You can
use _b_z_i_p_2_r_e_c_o_v_e_r to try to recover data from damaged
files.
Return values: 0 for a normal exit, 1 for environmental
problems (file not found, invalid flags, I/O errors, &c),
2 to indicate a corrupt compressed file, 3 for an internal
consistency error (eg, bug) which caused _b_z_i_p_2 to panic.
MMEEMMOORRYY MMAANNAAGGEEMMEENNTT
_B_z_i_p_2 compresses large files in blocks. The block size
affects both the compression ratio achieved, and the
amount of memory needed both for compression and decom-
pression. The flags -1 through -9 specify the block size
to be 100,000 bytes through 900,000 bytes (the default)
respectively. At decompression-time, the block size used
for compression is read from the header of the compressed
file, and _b_u_n_z_i_p_2 then allocates itself just enough memory
to decompress the file. Since block sizes are stored in
compressed files, it follows that the flags -1 to -9 are
irrelevant to and so ignored during decompression. Com-
pression and decompression requirements, in bytes, can be
estimated as:
Compression: 400k + ( 7 x block size )
Decompression: 100k + ( 5 x block size ), or
100k + ( 2.5 x block size )
Larger block sizes give rapidly diminishing marginal
returns; most of the compression comes from the first two
or three hundred k of block size, a fact worth bearing in
mind when using _b_z_i_p_2 on small machines. It is also
important to appreciate that the decompression memory
requirement is set at compression-time by the choice of
block size.
2
bzip2(1) bzip2(1)
For files compressed with the default 900k block size,
_b_u_n_z_i_p_2 will require about 4600 kbytes to decompress. To
support decompression of any file on a 4 megabyte machine,
_b_u_n_z_i_p_2 has an option to decompress using approximately
half this amount of memory, about 2300 kbytes. Decompres-
sion speed is also halved, so you should use this option
only where necessary. The relevant flag is -s.
In general, try and use the largest block size memory con-
straints allow, since that maximises the compression
achieved. Compression and decompression speed are virtu-
ally unaffected by block size.
Another significant point applies to files which fit in a
single block -- that means most files you'd encounter
using a large block size. The amount of real memory
touched is proportional to the size of the file, since the
file is smaller than a block. For example, compressing a
file 20,000 bytes long with the flag -9 will cause the
compressor to allocate around 6700k of memory, but only
touch 400k + 20000 * 7 = 540 kbytes of it. Similarly, the
decompressor will allocate 4600k but only touch 100k +
20000 * 5 = 200 kbytes.
Here is a table which summarises the maximum memory usage
for different block sizes. Also recorded is the total
compressed size for 14 files of the Calgary Text Compres-
sion Corpus totalling 3,141,622 bytes. This column gives
some feel for how compression varies with block size.
These figures tend to understate the advantage of larger
block sizes for larger files, since the Corpus is domi-
nated by smaller files.
Compress Decompress Decompress Corpus
Flag usage usage -s usage Size
-1 1100k 600k 350k 914704
-2 1800k 1100k 600k 877703
-3 2500k 1600k 850k 860338
-4 3200k 2100k 1100k 846899
-5 3900k 2600k 1350k 845160
-6 4600k 3100k 1600k 838626
-7 5400k 3600k 1850k 834096
-8 6000k 4100k 2100k 828642
-9 6700k 4600k 2350k 828642
OOPPTTIIOONNSS
--cc ----ssttddoouutt
Compress or decompress to standard output. -c will
decompress multiple files to stdout, but will only
compress a single file to stdout.
3
bzip2(1) bzip2(1)
--dd ----ddeeccoommpprreessss
Force decompression. _B_z_i_p_2 and _b_u_n_z_i_p_2 are really
the same program, and the decision about whether to
compress or decompress is done on the basis of
which name is used. This flag overrides that mech-
anism, and forces _b_z_i_p_2 to decompress.
--ff ----ccoommpprreessss
The complement to -d: forces compression, regard-
less of the invokation name.
--tt ----tteesstt
Check integrity of the specified file(s), but don't
decompress them. This really performs a trial
decompression and throws away the result, using the
low-memory decompression algorithm (see -s).
--kk ----kkeeeepp
Keep (don't delete) input files during compression
or decompression.
--ss ----ssmmaallll
Reduce memory usage, both for compression and
decompression. Files are decompressed using a mod-
ified algorithm which only requires 2.5 bytes per
block byte. This means any file can be decom-
pressed in 2300k of memory, albeit somewhat more
slowly than usual.
During compression, -s selects a block size of
200k, which limits memory use to around the same
figure, at the expense of your compression ratio.
In short, if your machine is low on memory (8
megabytes or less), use -s for everything. See
MEMORY MANAGEMENT above.
--vv ----vveerrbboossee
Verbose mode -- show the compression ratio for each
file processed. Further -v's increase the ver-
bosity level, spewing out lots of information which
is primarily of interest for diagnostic purposes.
--LL ----lliicceennssee
Display the software version, license terms and
conditions.
--VV ----vveerrssiioonn
Same as -L.
--11 ttoo --99
Set the block size to 100 k, 200 k .. 900 k when
compressing. Has no effect when decompressing.
See MEMORY MANAGEMENT above.
4
bzip2(1) bzip2(1)
----rreeppeettiittiivvee--ffaasstt
_b_z_i_p_2 injects some small pseudo-random variations
into very repetitive blocks to limit worst-case
performance during compression. If sorting runs
into difficulties, the block is randomised, and
sorting is restarted. Very roughly, _b_z_i_p_2 persists
for three times as long as a well-behaved input
would take before resorting to randomisation. This
flag makes it give up much sooner.
----rreeppeettiittiivvee--bbeesstt
Opposite of --repetitive-fast; try a lot harder
before resorting to randomisation.
RREECCOOVVEERRIINNGG DDAATTAA FFRROOMM DDAAMMAAGGEEDD FFIILLEESS
_b_z_i_p_2 compresses files in blocks, usually 900kbytes long.
Each block is handled independently. If a media or trans-
mission error causes a multi-block .bz2 file to become
damaged, it may be possible to recover data from the
undamaged blocks in the file.
The compressed representation of each block is delimited
by a 48-bit pattern, which makes it possible to find the
block boundaries with reasonable certainty. Each block
also carries its own 32-bit CRC, so damaged blocks can be
distinguished from undamaged ones.
_b_z_i_p_2_r_e_c_o_v_e_r is a simple program whose purpose is to
search for blocks in .bz2 files, and write each block out
into its own .bz2 file. You can then use _b_z_i_p_2 _-_t to test
the integrity of the resulting files, and decompress those
which are undamaged.
_b_z_i_p_2_r_e_c_o_v_e_r takes a single argument, the name of the dam-
aged file, and writes a number of files "rec0001file.bz2",
"rec0002file.bz2", etc, containing the extracted blocks.
The output filenames are designed so that the use of wild-
cards in subsequent processing -- for example, "bzip2 -dc
rec*file.bz2 > recovered_data" -- lists the files in the
"right" order.
_b_z_i_p_2_r_e_c_o_v_e_r should be of most use dealing with large .bz2
files, as these will contain many blocks. It is clearly
futile to use it on damaged single-block files, since a
damaged block cannot be recovered. If you wish to min-
imise any potential data loss through media or transmis-
sion errors, you might consider compressing with a smaller
block size.
PPEERRFFOORRMMAANNCCEE NNOOTTEESS
The sorting phase of compression gathers together similar
5
bzip2(1) bzip2(1)
strings in the file. Because of this, files containing
very long runs of repeated symbols, like "aabaabaabaab
..." (repeated several hundred times) may compress
extraordinarily slowly. You can use the -vvvvv option to
monitor progress in great detail, if you want. Decompres-
sion speed is unaffected.
Such pathological cases seem rare in practice, appearing
mostly in artificially-constructed test files, and in low-
level disk images. It may be inadvisable to use _b_z_i_p_2 to
compress the latter. If you do get a file which causes
severe slowness in compression, try making the block size
as small as possible, with flag -1.
Incompressible or virtually-incompressible data may decom-
press rather more slowly than one would hope. This is due
to a naive implementation of the move-to-front coder.
_b_z_i_p_2 usually allocates several megabytes of memory to
operate in, and then charges all over it in a fairly ran-
dom fashion. This means that performance, both for com-
pressing and decompressing, is largely determined by the
speed at which your machine can service cache misses.
Because of this, small changes to the code to reduce the
miss rate have been observed to give disproportionately
large performance improvements. I imagine _b_z_i_p_2 will per-
form best on machines with very large caches.
Test mode (-t) uses the low-memory decompression algorithm
(-s). This means test mode does not run as fast as it
could; it could run as fast as the normal decompression
machinery. This could easily be fixed at the cost of some
code bloat.
CCAAVVEEAATTSS
I/O error messages are not as helpful as they could be.
_B_z_i_p_2 tries hard to detect I/O errors and exit cleanly,
but the details of what the problem is sometimes seem
rather misleading.
This manual page pertains to version 0.1 of _b_z_i_p_2_. It may
well happen that some future version will use a different
compressed file format. If you try to decompress, using
0.1, a .bz2 file created with some future version which
uses a different compressed file format, 0.1 will complain
that your file "is not a bzip2 file". If that happens,
you should obtain a more recent version of _b_z_i_p_2 and use
that to decompress the file.
Wildcard expansion for Windows 95 and NT is flaky.
_b_z_i_p_2_r_e_c_o_v_e_r uses 32-bit integers to represent bit posi-
tions in compressed files, so it cannot handle compressed
6
bzip2(1) bzip2(1)
files more than 512 megabytes long. This could easily be
fixed.
_b_z_i_p_2_r_e_c_o_v_e_r sometimes reports a very small, incomplete
final block. This is spurious and can be safely ignored.
RREELLAATTIIOONNSSHHIIPP TTOO bbzziipp--00..2211
This program is a descendant of the _b_z_i_p program, version
0.21, which I released in August 1996. The primary dif-
ference of _b_z_i_p_2 is its avoidance of the possibly patented
algorithms which were used in 0.21. _b_z_i_p_2 also brings
various useful refinements (-s, -t), uses less memory,
decompresses significantly faster, and has support for
recovering data from damaged files.
Because _b_z_i_p_2 uses Huffman coding to construct the com-
pressed bitstream, rather than the arithmetic coding used
in 0.21, the compressed representations generated by the
two programs are incompatible, and they will not interop-
erate. The change in suffix from .bz to .bz2 reflects
this. It would have been helpful to at least allow _b_z_i_p_2
to decompress files created by 0.21, but this would defeat
the primary aim of having a patent-free compressor.
Huffman coding necessarily involves some coding ineffi-
ciency compared to arithmetic coding. This means that
_b_z_i_p_2 compresses about 1% worse than 0.21, an unfortunate
but unavoidable fact-of-life. On the other hand, decom-
pression is approximately 50% faster for the same reason,
and the change in file format gave an opportunity to add
data-recovery features. So it is not all bad.
AAUUTTHHOORR
Julian Seward, jseward@acm.org.
The ideas embodied in _b_z_i_p and _b_z_i_p_2 are due to (at least)
the following people: Michael Burrows and David Wheeler
(for the block sorting transformation), David Wheeler
(again, for the Huffman coder), Peter Fenwick (for the
structured coding model in 0.21, and many refinements),
and Alistair Moffat, Radford Neal and Ian Witten (for the
arithmetic coder in 0.21). I am much indebted for their
help, support and advice. See the file ALGORITHMS in the
source distribution for pointers to sources of documenta-
tion. Christian von Roques encouraged me to look for
faster sorting algorithms, so as to speed up compression.
Bela Lubkin encouraged me to improve the worst-case com-
pression performance. Many people sent patches, helped
with portability problems, lent machines, gave advice and
were generally helpful.
7

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bzip2(1) bzip2(1)
NAME
bzip2, bunzip2 - a block-sorting file compressor, v0.1
bzip2recover - recovers data from damaged bzip2 files
SYNOPSIS
bzip2 [ -cdfkstvVL123456789 ] [ filenames ... ]
bunzip2 [ -kvsVL ] [ filenames ... ]
bzip2recover filename
DESCRIPTION
Bzip2 compresses files using the Burrows-Wheeler block-
sorting text compression algorithm, and Huffman coding.
Compression is generally considerably better than that
achieved by more conventional LZ77/LZ78-based compressors,
and approaches the performance of the PPM family of sta-
tistical compressors.
The command-line options are deliberately very similar to
those of GNU Gzip, but they are not identical.
Bzip2 expects a list of file names to accompany the com-
mand-line flags. Each file is replaced by a compressed
version of itself, with the name "original_name.bz2".
Each compressed file has the same modification date and
permissions as the corresponding original, so that these
properties can be correctly restored at decompression
time. File name handling is naive in the sense that there
is no mechanism for preserving original file names, per-
missions and dates in filesystems which lack these con-
cepts, or have serious file name length restrictions, such
as MS-DOS.
Bzip2 and bunzip2 will not overwrite existing files; if
you want this to happen, you should delete them first.
If no file names are specified, bzip2 compresses from
standard input to standard output. In this case, bzip2
will decline to write compressed output to a terminal, as
this would be entirely incomprehensible and therefore
pointless.
Bunzip2 (or bzip2 -d ) decompresses and restores all spec-
ified files whose names end in ".bz2". Files without this
suffix are ignored. Again, supplying no filenames causes
decompression from standard input to standard output.
You can also compress or decompress files to the standard
output by giving the -c flag. You can decompress multiple
files like this, but you may only compress a single file
this way, since it would otherwise be difficult to sepa-
rate out the compressed representations of the original
files.
1
bzip2(1) bzip2(1)
Compression is always performed, even if the compressed
file is slightly larger than the original. Files of less
than about one hundred bytes tend to get larger, since the
compression mechanism has a constant overhead in the
region of 50 bytes. Random data (including the output of
most file compressors) is coded at about 8.05 bits per
byte, giving an expansion of around 0.5%.
As a self-check for your protection, bzip2 uses 32-bit
CRCs to make sure that the decompressed version of a file
is identical to the original. This guards against corrup-
tion of the compressed data, and against undetected bugs
in bzip2 (hopefully very unlikely). The chances of data
corruption going undetected is microscopic, about one
chance in four billion for each file processed. Be aware,
though, that the check occurs upon decompression, so it
can only tell you that that something is wrong. It can't
help you recover the original uncompressed data. You can
use bzip2recover to try to recover data from damaged
files.
Return values: 0 for a normal exit, 1 for environmental
problems (file not found, invalid flags, I/O errors, &c),
2 to indicate a corrupt compressed file, 3 for an internal
consistency error (eg, bug) which caused bzip2 to panic.
MEMORY MANAGEMENT
Bzip2 compresses large files in blocks. The block size
affects both the compression ratio achieved, and the
amount of memory needed both for compression and decom-
pression. The flags -1 through -9 specify the block size
to be 100,000 bytes through 900,000 bytes (the default)
respectively. At decompression-time, the block size used
for compression is read from the header of the compressed
file, and bunzip2 then allocates itself just enough memory
to decompress the file. Since block sizes are stored in
compressed files, it follows that the flags -1 to -9 are
irrelevant to and so ignored during decompression. Com-
pression and decompression requirements, in bytes, can be
estimated as:
Compression: 400k + ( 7 x block size )
Decompression: 100k + ( 5 x block size ), or
100k + ( 2.5 x block size )
Larger block sizes give rapidly diminishing marginal
returns; most of the compression comes from the first two
or three hundred k of block size, a fact worth bearing in
mind when using bzip2 on small machines. It is also
important to appreciate that the decompression memory
requirement is set at compression-time by the choice of
block size.
2
bzip2(1) bzip2(1)
For files compressed with the default 900k block size,
bunzip2 will require about 4600 kbytes to decompress. To
support decompression of any file on a 4 megabyte machine,
bunzip2 has an option to decompress using approximately
half this amount of memory, about 2300 kbytes. Decompres-
sion speed is also halved, so you should use this option
only where necessary. The relevant flag is -s.
In general, try and use the largest block size memory con-
straints allow, since that maximises the compression
achieved. Compression and decompression speed are virtu-
ally unaffected by block size.
Another significant point applies to files which fit in a
single block -- that means most files you'd encounter
using a large block size. The amount of real memory
touched is proportional to the size of the file, since the
file is smaller than a block. For example, compressing a
file 20,000 bytes long with the flag -9 will cause the
compressor to allocate around 6700k of memory, but only
touch 400k + 20000 * 7 = 540 kbytes of it. Similarly, the
decompressor will allocate 4600k but only touch 100k +
20000 * 5 = 200 kbytes.
Here is a table which summarises the maximum memory usage
for different block sizes. Also recorded is the total
compressed size for 14 files of the Calgary Text Compres-
sion Corpus totalling 3,141,622 bytes. This column gives
some feel for how compression varies with block size.
These figures tend to understate the advantage of larger
block sizes for larger files, since the Corpus is domi-
nated by smaller files.
Compress Decompress Decompress Corpus
Flag usage usage -s usage Size
-1 1100k 600k 350k 914704
-2 1800k 1100k 600k 877703
-3 2500k 1600k 850k 860338
-4 3200k 2100k 1100k 846899
-5 3900k 2600k 1350k 845160
-6 4600k 3100k 1600k 838626
-7 5400k 3600k 1850k 834096
-8 6000k 4100k 2100k 828642
-9 6700k 4600k 2350k 828642
OPTIONS
-c --stdout
Compress or decompress to standard output. -c will
decompress multiple files to stdout, but will only
compress a single file to stdout.
3
bzip2(1) bzip2(1)
-d --decompress
Force decompression. Bzip2 and bunzip2 are really
the same program, and the decision about whether to
compress or decompress is done on the basis of
which name is used. This flag overrides that mech-
anism, and forces bzip2 to decompress.
-f --compress
The complement to -d: forces compression, regard-
less of the invokation name.
-t --test
Check integrity of the specified file(s), but don't
decompress them. This really performs a trial
decompression and throws away the result, using the
low-memory decompression algorithm (see -s).
-k --keep
Keep (don't delete) input files during compression
or decompression.
-s --small
Reduce memory usage, both for compression and
decompression. Files are decompressed using a mod-
ified algorithm which only requires 2.5 bytes per
block byte. This means any file can be decom-
pressed in 2300k of memory, albeit somewhat more
slowly than usual.
During compression, -s selects a block size of
200k, which limits memory use to around the same
figure, at the expense of your compression ratio.
In short, if your machine is low on memory (8
megabytes or less), use -s for everything. See
MEMORY MANAGEMENT above.
-v --verbose
Verbose mode -- show the compression ratio for each
file processed. Further -v's increase the ver-
bosity level, spewing out lots of information which
is primarily of interest for diagnostic purposes.
-L --license
Display the software version, license terms and
conditions.
-V --version
Same as -L.
-1 to -9
Set the block size to 100 k, 200 k .. 900 k when
compressing. Has no effect when decompressing.
See MEMORY MANAGEMENT above.
4
bzip2(1) bzip2(1)
--repetitive-fast
bzip2 injects some small pseudo-random variations
into very repetitive blocks to limit worst-case
performance during compression. If sorting runs
into difficulties, the block is randomised, and
sorting is restarted. Very roughly, bzip2 persists
for three times as long as a well-behaved input
would take before resorting to randomisation. This
flag makes it give up much sooner.
--repetitive-best
Opposite of --repetitive-fast; try a lot harder
before resorting to randomisation.
RECOVERING DATA FROM DAMAGED FILES
bzip2 compresses files in blocks, usually 900kbytes long.
Each block is handled independently. If a media or trans-
mission error causes a multi-block .bz2 file to become
damaged, it may be possible to recover data from the
undamaged blocks in the file.
The compressed representation of each block is delimited
by a 48-bit pattern, which makes it possible to find the
block boundaries with reasonable certainty. Each block
also carries its own 32-bit CRC, so damaged blocks can be
distinguished from undamaged ones.
bzip2recover is a simple program whose purpose is to
search for blocks in .bz2 files, and write each block out
into its own .bz2 file. You can then use bzip2 -t to test
the integrity of the resulting files, and decompress those
which are undamaged.
bzip2recover takes a single argument, the name of the dam-
aged file, and writes a number of files "rec0001file.bz2",
"rec0002file.bz2", etc, containing the extracted blocks.
The output filenames are designed so that the use of wild-
cards in subsequent processing -- for example, "bzip2 -dc
rec*file.bz2 > recovered_data" -- lists the files in the
"right" order.
bzip2recover should be of most use dealing with large .bz2
files, as these will contain many blocks. It is clearly
futile to use it on damaged single-block files, since a
damaged block cannot be recovered. If you wish to min-
imise any potential data loss through media or transmis-
sion errors, you might consider compressing with a smaller
block size.
PERFORMANCE NOTES
The sorting phase of compression gathers together similar
5
bzip2(1) bzip2(1)
strings in the file. Because of this, files containing
very long runs of repeated symbols, like "aabaabaabaab
..." (repeated several hundred times) may compress
extraordinarily slowly. You can use the -vvvvv option to
monitor progress in great detail, if you want. Decompres-
sion speed is unaffected.
Such pathological cases seem rare in practice, appearing
mostly in artificially-constructed test files, and in low-
level disk images. It may be inadvisable to use bzip2 to
compress the latter. If you do get a file which causes
severe slowness in compression, try making the block size
as small as possible, with flag -1.
Incompressible or virtually-incompressible data may decom-
press rather more slowly than one would hope. This is due
to a naive implementation of the move-to-front coder.
bzip2 usually allocates several megabytes of memory to
operate in, and then charges all over it in a fairly ran-
dom fashion. This means that performance, both for com-
pressing and decompressing, is largely determined by the
speed at which your machine can service cache misses.
Because of this, small changes to the code to reduce the
miss rate have been observed to give disproportionately
large performance improvements. I imagine bzip2 will per-
form best on machines with very large caches.
Test mode (-t) uses the low-memory decompression algorithm
(-s). This means test mode does not run as fast as it
could; it could run as fast as the normal decompression
machinery. This could easily be fixed at the cost of some
code bloat.
CAVEATS
I/O error messages are not as helpful as they could be.
Bzip2 tries hard to detect I/O errors and exit cleanly,
but the details of what the problem is sometimes seem
rather misleading.
This manual page pertains to version 0.1 of bzip2. It may
well happen that some future version will use a different
compressed file format. If you try to decompress, using
0.1, a .bz2 file created with some future version which
uses a different compressed file format, 0.1 will complain
that your file "is not a bzip2 file". If that happens,
you should obtain a more recent version of bzip2 and use
that to decompress the file.
Wildcard expansion for Windows 95 and NT is flaky.
bzip2recover uses 32-bit integers to represent bit posi-
tions in compressed files, so it cannot handle compressed
6
bzip2(1) bzip2(1)
files more than 512 megabytes long. This could easily be
fixed.
bzip2recover sometimes reports a very small, incomplete
final block. This is spurious and can be safely ignored.
RELATIONSHIP TO bzip-0.21
This program is a descendant of the bzip program, version
0.21, which I released in August 1996. The primary dif-
ference of bzip2 is its avoidance of the possibly patented
algorithms which were used in 0.21. bzip2 also brings
various useful refinements (-s, -t), uses less memory,
decompresses significantly faster, and has support for
recovering data from damaged files.
Because bzip2 uses Huffman coding to construct the com-
pressed bitstream, rather than the arithmetic coding used
in 0.21, the compressed representations generated by the
two programs are incompatible, and they will not interop-
erate. The change in suffix from .bz to .bz2 reflects
this. It would have been helpful to at least allow bzip2
to decompress files created by 0.21, but this would defeat
the primary aim of having a patent-free compressor.
Huffman coding necessarily involves some coding ineffi-
ciency compared to arithmetic coding. This means that
bzip2 compresses about 1% worse than 0.21, an unfortunate
but unavoidable fact-of-life. On the other hand, decom-
pression is approximately 50% faster for the same reason,
and the change in file format gave an opportunity to add
data-recovery features. So it is not all bad.
AUTHOR
Julian Seward, jseward@acm.org.
The ideas embodied in bzip and bzip2 are due to (at least)
the following people: Michael Burrows and David Wheeler
(for the block sorting transformation), David Wheeler
(again, for the Huffman coder), Peter Fenwick (for the
structured coding model in 0.21, and many refinements),
and Alistair Moffat, Radford Neal and Ian Witten (for the
arithmetic coder in 0.21). I am much indebted for their
help, support and advice. See the file ALGORITHMS in the
source distribution for pointers to sources of documenta-
tion. Christian von Roques encouraged me to look for
faster sorting algorithms, so as to speed up compression.
Bela Lubkin encouraged me to improve the worst-case com-
pression performance. Many people sent patches, helped
with portability problems, lent machines, gave advice and
were generally helpful.
7

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/*-----------------------------------------------------------*/
/*--- Block recoverer program for bzip2 ---*/
/*--- bzip2recover.c ---*/
/*-----------------------------------------------------------*/
/*--
This program is bzip2recover, a program to attempt data
salvage from damaged files created by the accompanying
bzip2 program.
Copyright (C) 1996, 1997 by Julian Seward.
Guildford, Surrey, UK
email: jseward@acm.org
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
The GNU General Public License is contained in the file LICENSE.
--*/
#include <stdio.h>
#include <errno.h>
#include <malloc.h>
#include <stdlib.h>
#include <strings.h> /*-- or try string.h --*/
#define UInt32 unsigned int
#define Int32 int
#define UChar unsigned char
#define Char char
#define Bool unsigned char
#define True 1
#define False 0
Char inFileName[2000];
Char outFileName[2000];
Char progName[2000];
UInt32 bytesOut = 0;
UInt32 bytesIn = 0;
/*---------------------------------------------------*/
/*--- I/O errors ---*/
/*---------------------------------------------------*/
/*---------------------------------------------*/
void readError ( void )
{
fprintf ( stderr,
"%s: I/O error reading `%s', possible reason follows.\n",
progName, inFileName );
perror ( progName );
fprintf ( stderr, "%s: warning: output file(s) may be incomplete.\n",
progName );
exit ( 1 );
}
/*---------------------------------------------*/
void writeError ( void )
{
fprintf ( stderr,
"%s: I/O error reading `%s', possible reason follows.\n",
progName, inFileName );
perror ( progName );
fprintf ( stderr, "%s: warning: output file(s) may be incomplete.\n",
progName );
exit ( 1 );
}
/*---------------------------------------------*/
void mallocFail ( Int32 n )
{
fprintf ( stderr,
"%s: malloc failed on request for %d bytes.\n",
progName, n );
fprintf ( stderr, "%s: warning: output file(s) may be incomplete.\n",
progName );
exit ( 1 );
}
/*---------------------------------------------------*/
/*--- Bit stream I/O ---*/
/*---------------------------------------------------*/
typedef
struct {
FILE* handle;
Int32 buffer;
Int32 buffLive;
Char mode;
}
BitStream;
/*---------------------------------------------*/
BitStream* bsOpenReadStream ( FILE* stream )
{
BitStream *bs = malloc ( sizeof(BitStream) );
if (bs == NULL) mallocFail ( sizeof(BitStream) );
bs->handle = stream;
bs->buffer = 0;
bs->buffLive = 0;
bs->mode = 'r';
return bs;
}
/*---------------------------------------------*/
BitStream* bsOpenWriteStream ( FILE* stream )
{
BitStream *bs = malloc ( sizeof(BitStream) );
if (bs == NULL) mallocFail ( sizeof(BitStream) );
bs->handle = stream;
bs->buffer = 0;
bs->buffLive = 0;
bs->mode = 'w';
return bs;
}
/*---------------------------------------------*/
void bsPutBit ( BitStream* bs, Int32 bit )
{
if (bs->buffLive == 8) {
Int32 retVal = putc ( (UChar) bs->buffer, bs->handle );
if (retVal == EOF) writeError();
bytesOut++;
bs->buffLive = 1;
bs->buffer = bit & 0x1;
} else {
bs->buffer = ( (bs->buffer << 1) | (bit & 0x1) );
bs->buffLive++;
};
}
/*---------------------------------------------*/
/*--
Returns 0 or 1, or 2 to indicate EOF.
--*/
Int32 bsGetBit ( BitStream* bs )
{
if (bs->buffLive > 0) {
bs->buffLive --;
return ( ((bs->buffer) >> (bs->buffLive)) & 0x1 );
} else {
Int32 retVal = getc ( bs->handle );
if ( retVal == EOF ) {
if (errno != 0) readError();
return 2;
}
bs->buffLive = 7;
bs->buffer = retVal;
return ( ((bs->buffer) >> 7) & 0x1 );
}
}
/*---------------------------------------------*/
void bsClose ( BitStream* bs )
{
Int32 retVal;
if ( bs->mode == 'w' ) {
while ( bs->buffLive < 8 ) {
bs->buffLive++;
bs->buffer <<= 1;
};
retVal = putc ( (UChar) (bs->buffer), bs->handle );
if (retVal == EOF) writeError();
bytesOut++;
retVal = fflush ( bs->handle );
if (retVal == EOF) writeError();
}
retVal = fclose ( bs->handle );
if (retVal == EOF)
if (bs->mode == 'w') writeError(); else readError();
free ( bs );
}
/*---------------------------------------------*/
void bsPutUChar ( BitStream* bs, UChar c )
{
Int32 i;
for (i = 7; i >= 0; i--)
bsPutBit ( bs, (((UInt32) c) >> i) & 0x1 );
}
/*---------------------------------------------*/
void bsPutUInt32 ( BitStream* bs, UInt32 c )
{
Int32 i;
for (i = 31; i >= 0; i--)
bsPutBit ( bs, (c >> i) & 0x1 );
}
/*---------------------------------------------*/
Bool endsInBz2 ( Char* name )
{
Int32 n = strlen ( name );
if (n <= 4) return False;
return
(name[n-4] == '.' &&
name[n-3] == 'b' &&
name[n-2] == 'z' &&
name[n-1] == '2');
}
/*---------------------------------------------------*/
/*--- ---*/
/*---------------------------------------------------*/
#define BLOCK_HEADER_HI 0x00003141UL
#define BLOCK_HEADER_LO 0x59265359UL
#define BLOCK_ENDMARK_HI 0x00001772UL
#define BLOCK_ENDMARK_LO 0x45385090UL
Int32 main ( Int32 argc, Char** argv )
{
FILE* inFile;
FILE* outFile;
BitStream* bsIn, *bsWr;
Int32 currBlock, b, wrBlock;
UInt32 bitsRead;
UInt32 bStart[20000];
UInt32 bEnd[20000];
UInt32 buffHi, buffLo, blockCRC;
Char* p;
strcpy ( progName, argv[0] );
inFileName[0] = outFileName[0] = 0;
fprintf ( stderr, "bzip2recover: extracts blocks from damaged .bz2 files.\n" );
if (argc != 2) {
fprintf ( stderr, "%s: usage is `%s damaged_file_name'.\n",
progName, progName );
exit(1);
}
strcpy ( inFileName, argv[1] );
inFile = fopen ( inFileName, "rb" );
if (inFile == NULL) {
fprintf ( stderr, "%s: can't read `%s'\n", progName, inFileName );
exit(1);
}
bsIn = bsOpenReadStream ( inFile );
fprintf ( stderr, "%s: searching for block boundaries ...\n", progName );
bitsRead = 0;
buffHi = buffLo = 0;
currBlock = 0;
bStart[currBlock] = 0;
while (True) {
b = bsGetBit ( bsIn );
bitsRead++;
if (b == 2) {
if (bitsRead >= bStart[currBlock] &&
(bitsRead - bStart[currBlock]) >= 40) {
bEnd[currBlock] = bitsRead-1;
if (currBlock > 0)
fprintf ( stderr, " block %d runs from %d to %d (incomplete)\n",
currBlock, bStart[currBlock], bEnd[currBlock] );
} else
currBlock--;
break;
}
buffHi = (buffHi << 1) | (buffLo >> 31);
buffLo = (buffLo << 1) | (b & 1);
if ( ( (buffHi & 0x0000ffff) == BLOCK_HEADER_HI
&& buffLo == BLOCK_HEADER_LO)
||
( (buffHi & 0x0000ffff) == BLOCK_ENDMARK_HI
&& buffLo == BLOCK_ENDMARK_LO)
) {
if (bitsRead > 49)
bEnd[currBlock] = bitsRead-49; else
bEnd[currBlock] = 0;
if (currBlock > 0)
fprintf ( stderr, " block %d runs from %d to %d\n",
currBlock, bStart[currBlock], bEnd[currBlock] );
currBlock++;
bStart[currBlock] = bitsRead;
}
}
bsClose ( bsIn );
/*-- identified blocks run from 1 to currBlock inclusive. --*/
if (currBlock < 1) {
fprintf ( stderr,
"%s: sorry, I couldn't find any block boundaries.\n",
progName );
exit(1);
};
fprintf ( stderr, "%s: splitting into blocks\n", progName );
inFile = fopen ( inFileName, "rb" );
if (inFile == NULL) {
fprintf ( stderr, "%s: can't open `%s'\n", progName, inFileName );
exit(1);
}
bsIn = bsOpenReadStream ( inFile );
/*-- placate gcc's dataflow analyser --*/
blockCRC = 0; bsWr = 0;
bitsRead = 0;
outFile = NULL;
wrBlock = 1;
while (True) {
b = bsGetBit(bsIn);
if (b == 2) break;
buffHi = (buffHi << 1) | (buffLo >> 31);
buffLo = (buffLo << 1) | (b & 1);
if (bitsRead == 47+bStart[wrBlock])
blockCRC = (buffHi << 16) | (buffLo >> 16);
if (outFile != NULL && bitsRead >= bStart[wrBlock]
&& bitsRead <= bEnd[wrBlock]) {
bsPutBit ( bsWr, b );
}
bitsRead++;
if (bitsRead == bEnd[wrBlock]+1) {
if (outFile != NULL) {
bsPutUChar ( bsWr, 0x17 ); bsPutUChar ( bsWr, 0x72 );
bsPutUChar ( bsWr, 0x45 ); bsPutUChar ( bsWr, 0x38 );
bsPutUChar ( bsWr, 0x50 ); bsPutUChar ( bsWr, 0x90 );
bsPutUInt32 ( bsWr, blockCRC );
bsClose ( bsWr );
}
if (wrBlock >= currBlock) break;
wrBlock++;
} else
if (bitsRead == bStart[wrBlock]) {
outFileName[0] = 0;
sprintf ( outFileName, "rec%4d", wrBlock );
for (p = outFileName; *p != 0; p++) if (*p == ' ') *p = '0';
strcat ( outFileName, inFileName );
if ( !endsInBz2(outFileName)) strcat ( outFileName, ".bz2" );
fprintf ( stderr, " writing block %d to `%s' ...\n",
wrBlock, outFileName );
outFile = fopen ( outFileName, "wb" );
if (outFile == NULL) {
fprintf ( stderr, "%s: can't write `%s'\n",
progName, outFileName );
exit(1);
}
bsWr = bsOpenWriteStream ( outFile );
bsPutUChar ( bsWr, 'B' ); bsPutUChar ( bsWr, 'Z' );
bsPutUChar ( bsWr, 'h' ); bsPutUChar ( bsWr, '9' );
bsPutUChar ( bsWr, 0x31 ); bsPutUChar ( bsWr, 0x41 );
bsPutUChar ( bsWr, 0x59 ); bsPutUChar ( bsWr, 0x26 );
bsPutUChar ( bsWr, 0x53 ); bsPutUChar ( bsWr, 0x59 );
}
}
fprintf ( stderr, "%s: finished\n", progName );
return 0;
}
/*-----------------------------------------------------------*/
/*--- end bzip2recover.c ---*/
/*-----------------------------------------------------------*/

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test.bat Normal file
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@rem
@rem MSDOS test driver for bzip2
@rem
type words1
.\bzip2 -1 < sample1.ref > sample1.rbz
.\bzip2 -2 < sample2.ref > sample2.rbz
.\bzip2 -dvv < sample1.bz2 > sample1.tst
.\bzip2 -dvv < sample2.bz2 > sample2.tst
type words3sh

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test.cmd Normal file
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@rem
@rem OS/2 test driver for bzip2
@rem
type words1
.\bzip2 -1 < sample1.ref > sample1.rbz
.\bzip2 -2 < sample2.ref > sample2.rbz
.\bzip2 -dvv < sample1.bz2 > sample1.tst
.\bzip2 -dvv < sample2.bz2 > sample2.tst
type words3sh

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***-------------------------------------------------***
***--------- IMPORTANT: READ WHAT FOLLOWS! ---------***
***--------- viz: pay attention :-) ---------***
***-------------------------------------------------***
Compiling bzip2 ...

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words1 Normal file
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Doing 4 tests (2 compress, 2 uncompress) ...
If there's a problem, things might stop at this point.

6
words2 Normal file
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Checking test results. If any of the four "cmp"s which follow
report any differences, something is wrong. If you can't easily
figure out what, please let me know (jseward@acm.org).

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If you got this far and the "cmp"s didn't find anything amiss, looks
like you're in business. You should install bzip2 and bunzip2:
copy bzip2 to a public place, maybe /usr/bin.
In that public place, make bunzip2 a symbolic link
to the bzip2 you just copied there.
Put the manual page, bzip2.1, somewhere appropriate;
perhaps in /usr/man/man1.
Complete instructions for use are in the preformatted
manual page, in the file bzip2.1.preformatted.
You can also do "bzip2 --help" to see some helpful information.
"bzip2 -L" displays the software license.
Please read the README file carefully.
Finally, note that bzip2 comes with ABSOLUTELY NO WARRANTY.
Happy compressing!

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If you got this far and the "bzip2 -dvv"s give identical
stored vs computed CRCs, you're probably in business.
Complete instructions for use are in the preformatted manual page,
in the file bzip2.txt.
You can also do "bzip2 --help" to see some helpful information.
"bzip2 -L" displays the software license.
Please read the README file carefully.
Finally, note that bzip2 comes with ABSOLUTELY NO WARRANTY.
Happy compressing!