Zlib
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== Identification == | == Identification == | ||
| − | The first two bytes can help to identify a zlib datastream. The following tests are ordered | + | 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. (Always true) | * If the first two bytes are interpreted as a [[Endianness|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.) | * 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.) | ||
| − | * 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 portable.) | + | * 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.) |
| − | * The first byte is exactly 0x78. (Any other value from the | + | * The first byte is exactly 0x78. (Any other value from the above list means the Deflate decompressor can use a window size less than the standard 32K. The extent to which this feature has been used is unclear.) |
== Specifications == | == Specifications == | ||
Revision as of 16:27, 18 August 2022
zlib format is a variant 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.)
- 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.)
- The first byte is exactly 0x78. (Any other value from the above list means the Deflate decompressor can use a window size less than the standard 32K. The extent to which this feature has been used is unclear.)
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
- zlib data compression library
- CloudFlare fork of zlib
- zlib-ng
- Puff - Lightweight decompression software included with zlib (contrib/puff)
- miniz
- zopfli - zlib compressor highly optimized for file size
- Offzip
- Deark
See also DEFLATE#Software.
