Gzip

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File format

The gzip file (.gz) format consists of:

  • a file header
  • optional extra headers, such as the original file name,
  • a body, containing a DEFLATE-compressed payload
  • an 8-byte footer, containing a CRC-32 checksum and the length of the original uncompressed data.

File header

The file header is 10 bytes in size and contains:

Offset Size Value Description
0 2 0x1f 0x8b Signature (or identification byte 1 and 2)
2 1 Compression Method
3 1 Flags
4 4 Last modification time
Contains a POSIX timestamp.
8 1 Extra flags
9 1 Operating system
Value that indicates on which operating system the gzip file was created.

Extra flags

If compression method is 8 the following extra flags can be defined:

  • 0x02 - compressor used maximum compression, slowest algorithm
  • 0x04 - compressor used fastest algorithm

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