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 1.2.8 fixes a very rare bug in decompression. All users are encouraged to upgrade immediately. Version 1.2.8 also has these improvements:

• Add new inflateGetDictionary() function
• Fix bug where gzopen() immediately followed by gzclose() would write an empty file instead of an empty gzip stream.
• Fix bug in gzclose() when gzwrite() runs out of memory

Version 1.2.7 has many portability improvements over 1.2.6, and has these enhancements:

• Fix bug in gzclose_w() when gzwrite() fails to allocate memory
• Add "x" (O_EXCL) and "e" (O_CLOEXEC) modes support to gzopen()
• Add gzopen_w() in Windows for wide character path names
• Fix type mismatch between get_crc_table() and crc_table

Version 1.2.6 has many changes over 1.2.5, including these improvements:

• gzread() can now read a file that is being written concurrently
• gzgetc() is now a macro for increased speed
• Added a 'T' option to gzopen() for transparent writing (no compression)
• Added deflatePending() to return the amount of pending output
• Allow deflateSetDictionary() and inflateSetDictionary() at any time in raw mode
• deflatePrime() can now insert bits in the middle of the stream
• ./configure now creates a configure.log file with all of the results
• Added a ./configure --solo option to compile zlib with no dependency on any libraries
• Fixed a problem with large file support macros
• Fixed a bug in contrib/puff
• Many portability improvements

Version 1.2.5 fixes bugs in gzseek() and gzeof() that were present in version 1.2.4 (March 2010). All users are encouraged to upgrade immediately.

Version 1.2.4 has many changes over 1.2.3, including these improvements:

• Fixed bugs in adler32_combine(), compressBound(), and deflateBound()
• Wholesale replacement of gz* functions with faster versions
• As part of that, added gzbuffer(), gzoffset(), gzclose_r(), and gzclose_w() functions
• Faster Z_HUFFMAN_ONLY and Z_RLE compression for images and other specialized compression
• Added flush options Z_BLOCK to deflate() and Z_TREES to inflate() for finer control
• Added inflateReset2() and inflateMark() functions, the latter to aid in random access applications
• Added LFS (Large File Summit) support for 64-bit file offsets and many other portability improvements
• Updated examples in examples/ and updated third-party contributions in contrib/

If you are using libxml version 2.7.6 or earlier, you will need to update libxml to version 2.7.7 or later before installing zlib version 1.2.4 or later. libxml 2.7.6 and earlier made unnecessary assumptions about the undocumented internal structure of zlib that were changed in zlib 1.2.4 and result in libxml crashing. This was fixed in libxml 2.7.7.

Version 1.2.3 (July 2005) 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

Versions 1.1.4 (March 2002) 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.

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

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 related links. Note that 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).

The current release is publicly available here:

zlib source code, version 1.2.8, tar.gz format (558K, MD5 checksum 44d667c142d7cda120332623eab69f40):

US (zlib.net)

Pick a mirror (prdownloads.sourceforge.net)

Note that if your MD5 checksum does not match, you should check the length of the downloaded .tar.gz file. If it's something like 2.7 MB, then your browser decompressed it without telling you (how nice!), but nevertheless left the .gz in the name. In that case, please don't email me to tell me that my website must have been compromised since the MD5 is wrong, because it isn't and it hasn't.

zlib source code, version 1.2.8, tar.xz format (440K, MD5 checksum 28f1205d8dd2001f26fec1e8c2cebe37):

US (zlib.net)

Pick a mirror (prdownloads.sourceforge.net)

zlib source code, version 1.2.8, zipfile format (678K, MD5 checksum 126f8676442ffbd97884eb4d6f32afb4):

US (zlib.net)

Pick a mirror (prdownloads.sourceforge.net)

zlib compiled DLL, version 1.2.8, zipfile format (139K, MD5 checksum 8fd8dbfb5b01386209872b62e401e639):

US (zlib.net)

Pick a mirror (prdownloads.sourceforge.net)

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:

• 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

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.

Related External Links

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

zlib for Solaris

zlib for Solaris (alternate)

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)

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#

zlib DLL wrapper for .NET in VB

Zip 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 (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 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) for Dr. Dobb's Journal (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

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

Send comments or questions about zlib to the authors at   after checking FAQ and manual.
Please report broken links to (PGP key). Last updated September 13^th, 2016.