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 1.3.1 has these key updates from 1.3:

• Reject overflows of zip header fields in minizip.
• Fix bug in inflateSync() for data held in bit buffer.
• Add LIT_MEM define to use more memory for a small deflate speedup.
• Fix decision on the emission of Zip64 end records in minizip.
• Add bounds checking to ERR_MSG() macro, used by zError().
• Neutralize zip file traversal attacks in miniunz.
• Fix a bug in ZLIB_DEBUG compiles in check_match().

Version 1.3 has these key updates from 1.2.13:

• Building using K&R (pre-ANSI) function definitions is no longer supported.
• Fixed a bug in deflateBound() for level 0 and memLevel 9.
• Fixed a bug when gzungetc() is used immediately after gzopen().
• Fixed a bug when using gzflush() with a very small buffer.
• Fixed a crash when gzsetparams() is attempted for a transparent write.
• Fixed test/example.c to work with FORCE_STORED.
• Fixed minizip to allow it to open an empty zip file.
• Fixed reading disk number start on zip64 files in minizip.
• Fixed a logic error in minizip argument processing.

You can also look at the complete Change Log.

Canonical URL: https://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 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).

The current release is publicly available here:

zlib source code, version 1.3.1, tar.gz format (1477K, SHA-256 hash 9a93b2b7dfdac77ceba5a558a580e74667dd6fede4585b91eefb60f03b72df23):

US (zlib.net) (GPG signature)

zlib source code, version 1.3.1, tar.xz format (1275K, SHA-256 hash 38ef96b8dfe510d42707d9c781877914792541133e1870841463bfa73f883e32):

US (zlib.net) (GPG signature)

zlib source code, version 1.3.1, zipfile format (1616K, SHA-256 hash 72af66d44fcc14c22013b46b814d5d2514673dda3d115e64b690c1ad636e7b17):

US (zlib.net) (GPG signature)

Permalink for the most recent release:

https://zlib.net/current/zlib.tar.gz

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.

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)

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

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 Dec 12^th, 2025.