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)

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).

Version 1.2.3 eliminates potential security vulnerabilities in zlib 1.2.1 and 1.2.2, so all users of those versions should upgrade immediately. The following important fixes are provided in zlib 1.2.3 over 1.2.1 and 1.2.2:

• Eliminate a potential security vulnerability when decoding invalid compressed data
• Eliminate a potential security vulnerability when decoding specially crafted compressed data
• Fix a bug when decompressing dynamic blocks with no distance codes
• Fix crc check bug in gzread() after gzungetc()
• Do not return an error when using gzread() on an empty file

Version 1.2.x adds many improvements and features to zlib. Here are some highlights:

• inflate is about 20% faster and minimizes memory allocation
• crc32 is about 50% faster
• New, improved, and supported DLL interface and DLL FAQ
• New functions and functionality:
  • deflate() and inflate() will now optionally encode and decode gzip streams
  • raw deflate() and inflate() now documented and supported
  • New inflateBack() functions for call-back interface -- faster than inflate()
  • Z_BLOCK flush option and new return information added to inflate to facilitate appending to deflate streams
  • raw deflate() now accepts a dictionary -- for appending
  • deflatePrime() for starting deflate output mid-byte -- for appending
  • Z_RLE strategy for run-length encoding -- option for PNG compression
  • gzclearerr() and gzungetc(), analogous to stdio functions
  • inflateCopy() added, analogous to deflateCopy()
  • deflateBound() and compressBound() for maximum size of compressed data
  • zlibCompileFlags() to provide compilation information
• More supported architectures and improved shared library support
• Many more FAQ entries
• Several new and updated contributions in the contrib directory

Versions 1.1.4 and later eliminate an earlier potential security vulnerability, see details here. Any software that is linked against or derived from an earlier version of zlib should be upgraded immediately. A partial list of over 500 applications using zlib is given here (uncompressed).

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

Mirror sites:

• http://www.gzip.org/zlib/ (France)
• http://zlib.ipinfo.de/ (Germany)

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/maintainer 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 and Jean-loup can be reached by e-mail at . Please read the FAQ and the manual before asking us for help. We are getting too many questions which already have an answer in the zlib documentation.

Greg, Mark and/or Jean-loup will add some more stuff here when they think of something to add. For now this page is mainly a pointer to zlib itself and to the official zlib and deflate documentation. Note that the 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).

The current release is publicly available here:

zlib source code, version 1.2.3, tar.gz format (485K, MD5 checksum debc62758716a169df9f62e6ab2bc634):

US (www.zlib.net)

France (www.gzip.org)

Pick a mirror (prdownloads.sourceforge.net)

zlib source code, version 1.2.3, tar.bz2 format (415K, MD5 checksum dee233bf288ee795ac96a98cc2e369b6):

US (www.zlib.net)

France (www.gzip.org)

Pick a mirror (prdownloads.sourceforge.net)

zlib source code, version 1.2.3, zipfile format (570K, MD5 checksum abbd0f2b456206da5e3ffd179324413a):

US (www.zlib.net)

France (www.gzip.org)

Pick a mirror (prdownloads.sourceforge.net)

zlib compiled DLL, version 1.2.3, zipfile format (79K, MD5 checksum cc7fa97f9c19386bb701acc79d0abbca):

US (www.zlib.net)

France (www.gzip.org)

Pick a mirror (prdownloads.sourceforge.net)

NOTE: zlib does not currently support window sizes of 256 bytes (windowBits == 8). A 512-byte window is the smallest the encoder can use. (Most applications use the default 32,768-byte window size for best compression.)

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

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

zlib's Deflate Algorithm

zlib's deflate flush modes

zlib License

Related External Links

unofficial (contributed) patches and binaries (not tested by zlib team)

zlib for Linux, both shared and static plus headers (RPM format, many architectures)

zlib for HP-UX 10.20 and 11.00 (shared library and headers)

(alternatively here: HP-UX 10.20 and HP-UX 11.00)

zlib for SGI Irix 6.x (shared library and headers)

zlib for Solaris

zlib for Solaris (alternate)

zlib for Digital Unix 4.0

zlib for SCO Open Server 5.0

zlib for BeOS R5

zlib for Mac OS X: zlib is already included as part of Mac OS X

zlib for Mac OS

zlib for OS/2 (DLL and static version for emx 0.9c, 46k)

(click here if link breaks)

zlib for Palm Pilot

zlib for Newton OS

zlib for Windows CE

zlib for RIM BlackBerry

zlib for Windows 9x/NT/2000/XP/2003 (DLL version, plus related utilities)

zlib for Windows 9x/NT (DLL and static version)

zlib for .NET in C#

zlib DLL wrapper for .NET in VB

Zip for .NET

zip file manipulation for .NET

Mark Nelson's ZlibTool article and Win32 source code for Dr. Dobb's Journal (January 1997)

zlib C++ wrapper for the gz* functions.

C++ zlib and gzip filters in an iostream framework.

zlib 32-bit OCX and 16-bit DLL (Visual Basic interface, source code and binaries, 84k)

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 Pascal port (Pascal source, tested with Turbo Pascal 7.0 and Delphi 3.02)

(not tested by us, but looks complete and well-maintained)

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 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) for Dr. Dobb's Journal (December 1997)

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

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)

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

Send comments or questions about zlib to the authors at   after checking FAQ and manual.
Please report broken links to (PGP key). Last updated August 7th, 2009.