Zlib

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{{FormatInfo
 
{{FormatInfo
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|name=zlib
 
|formattype=electronic
 
|formattype=electronic
 
|subcat=Compression
 
|subcat=Compression
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|released=1995
 
}}
 
}}
 
{{DISPLAYTITLE:zlib}}
 
{{DISPLAYTITLE:zlib}}
'''zlib''' is a data compression library, and a general-purpose data compression format.
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'''zlib''' format is a container and extension of the [[DEFLATE]] compressed data format. It adds a lightweight container, a small amount of configurability, and error checking.
  
The data format uses [[DEFLATE]] compression. It is used in many other file formats, and sometimes on its own as a file format. It is defined in [http://tools.ietf.org/html/rfc1950 RFC 1950].
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zlib is also the name of a software library that supports the zlib format, and other formats related to DEFLATE.
  
The zlib format is not the same as [[gzip]] format, but they are fairly similar, and both are supported by the zlib library.
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There are many formats and protocols that use, or may use, zlib format. A notable example is [[PNG]].
  
 
== See also ==
 
== See also ==
 
* [[Adler-32]]
 
* [[Adler-32]]
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* [[DEFLATE]]
  
 
== Identification ==
 
== Identification ==
The first byte of a zlib datastream has eight possible values: 0x08, 0x18, 0x28, ..., 0x78.
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The first two bytes can help to identify a zlib datastream. The following tests are ordered starting with those that are most certain to be true.
  
If the first two bytes are interpreted as a [[Endianness|big-endian]] integer, it is always a multiple of 31.
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* If the first two bytes are interpreted as a [[Endianness|big-endian]] integer, it is always a multiple of 31. (Always true)
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* The first byte has eight possible values: <code>0x08</code>, <code>0x18</code>, <code>0x28</code>, ..., <code>0x78</code>. (Always true, unless the zlib format has been extended to support compression methods other than Deflate.)
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* Almost always, the <code>0x20</code> bit of the second byte is 0. (A value of 1 is legal in zlib v3.3+, but indicates use of a preset dictionary, so the zlib stream is not as portable.)
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* Most often, the Deflate window size is 32K, and there is no preset dictionary. With these assumptions, the first byte is <code>0x78</code>, and the second is <code>0x01</code> (if using fastest compression), <code>0x5e</code> (fast), <code>0x9c</code> (default), or <code>0xda</code> (max), with <code>0x9c</code> and <code>0xda</code> being the most common.
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== Specifications ==
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* ZLIB Specification v3.1 - ''No copy found. There are dead links to it in e.g. the [https://zlib.net/fossils/zlib-0.95.tar.gz zlib 0.95] README file.''
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* [https://web.archive.org/web/20030426211434/http://mirror.cs.wisc.edu/pub/mirrors/ghost/gzip_doc/zlib-3.2.doc ZLIB Specification v3.2] (from archive.org)
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* RFC 1950: ZLIB Compressed Data Format Specification version 3.3
  
 
== Software ==
 
== Software ==
* [http://zlib.net/ zlib data compression library]
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* [https://zlib.net zlib data compression library]
** [https://github.com/cloudflare/zlib CloudFlare fork of zlib]
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* [https://github.com/cloudflare/zlib CloudFlare fork of zlib]
* [http://code.google.com/p/miniz/ miniz]
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* [https://github.com/zlib-ng/zlib-ng zlib-ng]
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* Puff - Lightweight decompression software included with [https://zlib.net/ zlib] (contrib/puff)
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* [https://github.com/richgel999/miniz miniz]
 
* [https://github.com/google/zopfli zopfli] - zlib compressor highly optimized for file size
 
* [https://github.com/google/zopfli zopfli] - zlib compressor highly optimized for file size
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* [http://aluigi.altervista.org/mytoolz.htm#offzip Offzip]
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* {{Deark}}
  
== Specifications ==
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See also [[DEFLATE#Software]].
* [http://tools.ietf.org/html/rfc1950 RFC 1950]
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== External links ==
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== Links ==
 
* [[Wikipedia:Zlib|Wikipedia article]]
 
* [[Wikipedia:Zlib|Wikipedia article]]

Revision as of 18:29, 21 August 2022

File Format
Name zlib
Ontology
Released 1995

zlib format is a container and extension of the DEFLATE compressed data format. It adds a lightweight container, a small amount of configurability, and error checking.

zlib is also the name of a software library that supports the zlib format, and other formats related to DEFLATE.

There are many formats and protocols that use, or may use, zlib format. A notable example is PNG.

Contents

See also

Identification

The first two bytes can help to identify a zlib datastream. The following tests are ordered starting with those that are most certain to be true.

  • If the first two bytes are interpreted as a big-endian integer, it is always a multiple of 31. (Always true)
  • The first byte has eight possible values: 0x08, 0x18, 0x28, ..., 0x78. (Always true, unless the zlib format has been extended to support compression methods other than Deflate.)
  • Almost always, the 0x20 bit of the second byte is 0. (A value of 1 is legal in zlib v3.3+, but indicates use of a preset dictionary, so the zlib stream is not as portable.)
  • Most often, the Deflate window size is 32K, and there is no preset dictionary. With these assumptions, the first byte is 0x78, and the second is 0x01 (if using fastest compression), 0x5e (fast), 0x9c (default), or 0xda (max), with 0x9c and 0xda being the most common.

Specifications

  • ZLIB Specification v3.1 - No copy found. There are dead links to it in e.g. the zlib 0.95 README file.
  • ZLIB Specification v3.2 (from archive.org)
  • RFC 1950: ZLIB Compressed Data Format Specification version 3.3

Software

See also DEFLATE#Software.

Links

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