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A MASSIVELY SPIFFY YET DELICATELY UNOBTRUSIVE COMPRESSION LIBRARY
(ALSO FREE, NOT TO MENTION UNENCUMBERED BY PATENTS)

(NOT RELATED TO THE LINUX ZLIBC COMPRESSING FILE-I/O LIBRARY)



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Welcome to the zlib home page, web pages originally created by Greg Roelofs and
maintained by Mark Adler. If this page seems suspiciously similar to the PNG
Home Page, rest assured that the similarity is completely coincidental. No,
really.

zlib was written by Jean-loup Gailly (compression) and Mark Adler
(decompression).



Current release:

zlib 1.2.13

October 13, 2022

Version 1.2.13 has these key updates from 1.2.12:

 * Fix a bug when getting a gzip header extra field with inflateGetHeader().
   This remedies CVE-2022-37434.
 * Fix a bug in block type selection when Z_FIXED used. Now the smallest block
   type is selected, for better compression.
 * Fix a configure issue that discarded the provided CC definition.
 * Correct incorrect inputs provided to the CRC functions. This mitigates a bug
   in Java.
 * Repair prototypes and exporting of the new CRC functions.
 * Fix inflateBack to detect invalid input with distances too far.

Due to the first bug fix, any installations of 1.2.12 or earlier should be
replaced with 1.2.13.

Version 1.2.12 has these key improvements over 1.2.11:

 * Fix a deflate bug when using the Z_FIXED strategy that can result in
   out-of-bound accesses.
 * Fix a deflate bug when the window is full in deflate_stored().
 * Speed up CRC-32 computations by a factor of 1.5 to 3.
 * Use the hardware CRC-32 instruction on ARMv8 processors.
 * Speed up crc32_combine() with powers of x tables.
 * Add crc32_combine_gen() and crc32_combine_op() for fast combines.

You can also look at the complete Change Log.



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Canonical URL: https://zlib.net/ (US)



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zlib is designed to be a free, general-purpose, legally unencumbered -- that is,
not covered by any patents -- lossless data-compression library for use on
virtually any computer hardware and operating system. The zlib data format is
itself portable across platforms. Unlike the LZW compression method used in Unix
compress(1) and in the GIF image format, the compression method currently used
in zlib essentially never expands the data. (LZW can double or triple the file
size in extreme cases.) zlib's memory footprint is also independent of the input
data and can be reduced, if necessary, at some cost in compression. A more
precise, technical discussion of both points is available on another page.

zlib was written by Jean-loup Gailly (compression) and Mark Adler
(decompression). Jean-loup is also the primary author of gzip(1), the author of
the comp.compression FAQ list and the former maintainer of Info-ZIP's Zip; Mark
is also the author of gzip's and UnZip's main decompression routines and was the
original author of Zip. Not surprisingly, the compression algorithm used in zlib
is essentially the same as that in gzip and Zip, namely, the `deflate' method
that originated in PKWARE's PKZIP 2.x.

Mark can be reached via e-mail at . Please read the FAQ and the manual before
asking for help. Nearly all of the questions we get already have an answer in
the zlib documentation. Questions on the usage of zlib are best answered on
Stack Overflow. zlib development contributions and issues are best addressed on
zlib's GitHub repository.

The deflate and zlib specifications both achieved official Internet RFC status
in May 1996, and zlib itself was adopted in version 1.1 of the Java Development
Kit (JDK), both as a raw class and as a component of the JAR archive format.

The lovely zlib-vise image above was provided courtesy of Bruce Gardner, art
director of Dr. Dobb's Journal. It appears in Mark Nelson's article in the
January 1997 issue (see below).



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The current release is publicly available here:



zlib source code, version 1.2.13, tar.gz format (1462K, SHA-256 hash
b3a24de97a8fdbc835b9833169501030b8977031bcb54b3b3ac13740f846ab30):



US (zlib.net) (GPG signature) zlib source code, version 1.2.13, tar.xz format
(1266K, SHA-256 hash
d14c38e313afc35a9a8760dadf26042f51ea0f5d154b0630a31da0540107fb98):



US (zlib.net) (GPG signature) zlib source code, version 1.2.13, zipfile format
(1594K, SHA-256 hash
d233fca7cf68db4c16dc5287af61f3cd01ab62495224c66639ca3da537701e42):



US (zlib.net) (GPG signature)

Note that zlib is an integral part of libpng and has been tested extensively as
part of many PNG-supporting applications.



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ZLIB INFORMATION

zlib Frequently Asked Questions Zlib-announce mailing list New versions of zlib
are announced on this list. Zlib-devel mailing list Please do not send questions
or comments about zlib to this mailing list. Send those directly to the authors
at after checking the FAQ and the manual, of course. The zlib-devel list is for
the development of zlib—members are contributors to and testers of new versions
of zlib. zlib Manual zlib Usage Example zlib Technical Details zlib-related
specifications:
 * RFC 1950 ZLIB Compressed Data Format Specification version 3.3
 * RFC 1951 DEFLATE Compressed Data Format Specification version 1.3
 * RFC 1952 GZIP file format specification version 4.3

Deflate stream disassembler. infgen.c produces a readable description of a gzip,
zlib, or raw deflate stream. zlib's Deflate Algorithm zlib's deflate flush modes
zlib License All released versions of zlib zlib on github


CRC (CYCLIC REDUNDANCY CHECK) BONUS INFORMATION

Ross Williams' classic "A Painless Guide to CRC Error Detection Algorithms" Code
to generate any CRC, with a list of CRC descriptions. crcany.c can take a
description of a CRC and compute that CRC efficiently. It includes bit-wise,
table-driven byte-wise, and table-driven word-wise CRC algorithms. Code to
modify a message so that it generates the desired CRC. spoof.c takes an
abbreviated description of the CRC, the exclusive-or of the current CRC of the
message and the desired CRC, the length of the message, and a list of bit
locations in a message, and tells you which of those bits should be inverted in
the message to get the desired CRC. Note that it does not need the message
itself, due to the linearity property of CRCs.


ZIP FILE PROCESSING BONUS SOFTWARE

Code to read a zip file as a stream and extract its contents. sunzip.c will read
a zip file from stdin and extract the files therein that use compression methods
0, 8, 9, or 12 (stored, deflate, deflate64, or bzip2). It accepts Zip64 input.
Code to merge multiple zip files into a single zip file. zipknit.c accepts Zip64
input files, and will create Zip64 output if the combined size of the merged zip
file warrants it. All compression formats are permitted, since no decompression
or recompression is performed. Encrypted entries are permitted, and pass through
unscathed.



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RELATED EXTERNAL LINKS

zlib for Linux, both shared and static plus headers (RPM format, many
architectures) zlib for Solaris (alternate) zlib for macOS (Mac OS X): zlib is
already included as part of macOS zlib for Palm Pilot zlib for Newton OS zlib
for Windows CE zlib for Windows 9x/NT/2000/XP/2003 (DLL version, plus related
utilities) zlib for Windows 9x/NT (DLL and static version) DotNetZip zip file
manipulation for .NET, and more (including replacements for the buggy Microsoft
GZipStream and DeflateStream classes) zlib for .NET in C# zlib DLL wrapper for
.NET in C# Zip for .NET Mark Nelson's ZlibTool article (January 1997) zlib C++
wrapper for the gz* functions. C++ zlib and gzip filters in an iostream
framework. zlib 32-bit OCX (C++ source and binaries for use with Visual
Basic 4.x or Delphi 2.0) (unsupported VB5 binary also available) zlib Delphi 5
interface (includes compiled object files and corresponding C++ Builder 5
project files) zlib Perl interface (source code; look for Compress-Zlib*.tar.gz)
zlib Python interface (online manual; part of the standard library as of
Python 1.5) zlib Tcl interface mkZiplib zlib Haskell interface zlib Java
interface (see also JAR format) zlib reimplementation in pure Java (not tested
by us, but looks like a good alternative to java.util.zip) Mark Nelson's JavaZip
article (with source code) (December 1997) Random access for gzip archives, for
Java Gilles Vollant's zlib-based mini-zip and mini-unzip (see also Info-ZIP's
UnZip, which optionally can be compiled with zlib) Scott Ludwig's zlib-based
CExe executable compressor for Win32 zlib technical issues, including spec
errors Zip Files: History, Explanation and Implementation zlib information in
Japanese zlib information in Russian Real World Scanning and Halftones (second
edition includes a section on zlib) Markus Oberhumer's LZO `real-time' data
compression library (not tested by us, but looks like a good alternative if you
need more speed and less compression) lz4, a very fast compression algorithm
(not tested by us, but looks like an even better alternative if you need more
speed and less compression) Zstandard, a better compression algorithm (not
tested by us, but appears to be a better alternative to zlib in both dimensions
of compression and speed, as well as decompression speed) libbzip2 (not tested
by us, but looks like a good alternative if you need more compression and less
speed) PPP Deflate Protocol (RFC 1979) Info-ZIP Home Page Portable Network
Graphics (PNG) Home Page gzip Home Page pigz (parallel gzip) Home Page
DataCompression.info comp.compression Frequently Asked Questions list



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Send comments or questions about zlib to the authors at   after checking the FAQ
and the manual.
Please report broken links to (PGP key).

This page last updated March 19th, 2023.

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Web page copyright © 1996-2023 Greg Roelofs, Jean-loup Gailly and Mark Adler.
zlib software copyright © 1995-2022 Jean-loup Gailly and Mark Adler. zlib.org
domain name donated by Andrew Green.