Gzip

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Contents

File format

The gzip file (.gz) format consists of:

  • a file header
  • optional headers
    • extra fields
    • original file name
    • comment
    • header checksum
  • a body, containing a DEFLATE-compressed payload
  • a file footer
Characteristics Description
Byte order little-endian
Date and time values Filetime in UTC
Character string ISO 8859-1 (LATIN-1)

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 Compression flags (or extra flags)
9 1 Operating system
Value that indicates on which operating system the gzip file was created.

Compression method

Value Identifier Description
0 - 7 Reserved
8 "deflate" zlib compressed data

Flags

Value Identifier Description
0x01 FTEXT If set the uncompressed data needs to be treated as text instead of binary data.
This flag hints end-of-line conversion for cross-platform text files but does not enforce it.
0x02 FHCRC The file contains a header checksum (CRC-16)
0x04 FEXTRA The file contains extra fields
0x08 FNAME The file contains an original file name string
0x10 FCOMMENT The file contains comment
0x20 Reserved
0x40 Reserved
0x80 Reserved

Note: The FHCRC bit was never set by versions of gzip up to 1.2.4, even though it was documented with a different meaning in gzip 1.2.4.

Compression flags

This value contains flags specific to the compression method.

Compression flags - deflate

If compression method value is 8 (deflate) the following compression flags can be used:

Value Identifier Description
0x02 compressor used maximum compression, slowest algorithm
0x04 compressor used fastest algorithm

Operating System

Value Identifier Description
0 FAT filesystem (MS-DOS, OS/2, NT/Win32)
1 Amiga
2 VMS (or OpenVMS)
3 Unix
4 VM/CMS
5 Atari TOS
6 HPFS filesystem (OS/2, NT)
7 Macintosh
8 Z-System
9 CP/M
10 TOPS-20
11 NTFS filesystem (NT)
12 QDOS
13 Acorn RISCOS
255 unknown

Optional headers

Extra fields

This value is present in the file if the FEXTRA flag is set in the file header flags.

The extra field are variable of size and contains:

Offset Size Value Description
0 2 Extra field data size
Value in bytes.
2 ... Extra field data

Original file name

This value is present in the file if the FNAME flag is set in the file header flags.

This is the original name of the file being compressed, with any directory components removed, and, if the file being compressed is on a file system with case insensitive names, forced to lower case.

Contains an ISO 8859-1 (LATIN-1) string with end-of-string character.

Comment

This value is present in the file if the FCOMMENT flag is set in the file header flags.

Contains an ISO 8859-1 (LATIN-1) string with end-of-string character. Line breaks should be denoted by a single line feed character.

Header checksum

The header checksum contain a CRC-16 that consists of the two least significant bytes of the CRC-32 for all bytes of the gzip header up to and not including the CRC-16.

File footer

The file footer is 8 bytes in size and contains:

Offset Size Value Description
0 4 Checksum (CRC-32)
4 4 Uncompressed data size
Value in bytes.

See Also

External Links