| Stable release | 1.2.5 / April 19, 2010 |
|---|---|
| Written in | C |
| Operating system | Cross-platform |
| Type | Data compression |
| License | zlib license |
| Website | http://zlib.net/ |
zlib is a software library used for data compression. zlib was written by Jean-Loup Gailly and Mark Adler and is an abstraction of the DEFLATE compression algorithm used in their gzip file compression program. Zlib is also a crucial component of many software platforms including Linux, Mac OS X, and the iOS. It has also been used in gaming consoles such as the Playstation 3, Wii, and Xbox 360.
The first public version of zlib, 0.9, was released on 1 May 1995 and was originally intended for use with libpng image library. It is free software, distributed under the zlib license.
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zlib compressed data is typically written with a gzip wrapper or a zlib wrapper. The wrapper encapsulates the raw DEFLATE data by adding a header and trailer. This provides stream identification and error detection which are not provided by the raw DEFLATE data.
The gzip header is larger than the zlib header as it stores a file name and other file system information. This is the header format used in the ubiquitous gzip file format.
As of February 2010[update] zlib only supports one algorithm called DEFLATE which is a variation of LZ77 (Lempel–Ziv 1977)
This algorithm provides good compression on a wide variety of data with minimal use of system resources. This is also the algorithm used in the ZIP archive format.
It is unlikely that the zlib format will ever be extended to use any other algorithms, though the header makes allowance for this possibility.
The library provides facilities for control of processor and memory use
A compression level value may be supplied which trades-off speed with compression.
There are also facilities for conserving memory. These are probably only useful in restricted memory environments such as some embedded systems.
The compression can be optimized for specific types of data
If you are using the library to always compress specific types of data then using a specific strategy may improve compression and performance. For example, if your data contains long lengths of repeated bytes then the RLE (run-length encoding) strategy may give good results at higher speed.
For general data, the default strategy is preferred.
Errors may be detected and skipped.
Data corruption can be detected (as long as the data is written with a zlib or gzip header - see above).
Further, if full-flush points are written to the compressed stream then corrupt data can be skipped and the decompression will resynchronise at the next flush point. (No error recovery of the corrupt data is provided.) Full-flush points are useful for large data streams on unreliable channels where some last data loss is unimportant (e.g. multimedia), however creating too many flush points can dramatically affect speed and compression.
There is no limit to the length of data that can be compressed or decompressed.
Repeated calls to the library allow an unlimited numbers of blocks of data to be handled. Some ancillary code (counters) may suffer from overflow for long data streams but this does not affect the actual compression or decompression.
When compressing a long (or infinite) data stream it would be advisable to write regular full-flush points.
Today, zlib is something of a de facto standard, to the point that zlib and DEFLATE are often used interchangeably in standards documents. Thousands of applications rely on it for compression, directly or indirectly,[1] including:
zlib is also used in many embedded devices such as the Apple Inc. iPhone and Sony Playstation 3 because the code is portable, liberally-licensed and has a relatively small memory footprint.