Difference between revisions of "Gzip"

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The gzip file (.gz) format consists of:
 
The gzip file (.gz) format consists of:
 
* a file header
 
* a file header
* optional extra headers, such as the original file name,
+
* optional headers
 +
** extra fields
 +
** original file name
 +
** comment
 +
** header checksum
 
* a body, containing a DEFLATE-compressed payload
 
* a body, containing a DEFLATE-compressed payload
* an 8-byte footer, containing a CRC-32 checksum and the length of the original uncompressed data.
+
* a file footer
  
 
=== File header ===
 
=== File header ===
Line 72: Line 76:
 
| 0x01
 
| 0x01
 
| FTEXT
 
| FTEXT
|  
+
| If set the uncompressed data needs to be treated as text instead of binary data. <br> This flag hints end-of-line conversion for cross-platform text files but does not enforce it.
 
|-
 
|-
 
| 0x02
 
| 0x02
 
| FHCRC
 
| FHCRC
|  
+
| The file contains a header checksum (CRC-16)
 
|-
 
|-
 
| 0x04
 
| 0x04
 
| FEXTRA
 
| FEXTRA
|  
+
| The file contains extra fields
 
|-
 
|-
 
| 0x08
 
| 0x08
 
| FNAME
 
| FNAME
|  
+
| The file contains an original file name string
 
|-
 
|-
 
| 0x10
 
| 0x10
 
| FCOMMENT
 
| FCOMMENT
|  
+
| The file contains comment
 
|-
 
|-
 
| 0x20
 
| 0x20
Line 102: Line 106:
 
| Reserved
 
| Reserved
 
|}
 
|}
 +
 +
<b>Note:</b> 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.
  
 
==== Extra flags ====
 
==== Extra flags ====
 
If compression method is 8 the following extra flags can be defined:
 
If compression method is 8 the following extra flags can be defined:
* 0x02 - compressor used maximum compression, slowest algorithm
+
{| class="wikitable"
* 0x04 - compressor used fastest algorithm
+
! align="left"| Value
 +
! Identifier
 +
! Description
 +
|-
 +
| 0x02
 +
|
 +
| compressor used maximum compression, slowest algorithm
 +
|-
 +
| 0x04
 +
|
 +
| compressor used fastest algorithm
 +
|}
 +
 
 +
==== Operating System ====
 +
{| class="wikitable"
 +
! align="left"| 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 ====
 +
<b>TODO: add description</b>
 +
 
 +
The extra field are variable of size and contains:
 +
{| class="wikitable"
 +
! align="left"| Offset
 +
! Size
 +
! Value
 +
! Description
 +
|-
 +
| 0
 +
| 2
 +
|
 +
| Extra field data size <br> Value in bytes.
 +
|-
 +
| 2
 +
| ...
 +
|
 +
| Extra field data
 +
|}
 +
 
 +
==== Original file name ====
 +
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 ====
 +
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:
 +
{| class="wikitable"
 +
! align="left"| Offset
 +
! Size
 +
! Value
 +
! Description
 +
|-
 +
| 0
 +
| 4
 +
|
 +
| Checksum (CRC-32)
 +
|-
 +
| 4
 +
| 4
 +
|
 +
| Uncompressed data size <br> Value in bytes.
 +
|}
  
 
== External Links ==
 
== External Links ==

Revision as of 02:05, 28 November 2013

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Please help to improve this article by expanding it.
Further information might be found on the discussion page.

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

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.

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.

Extra flags

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

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

TODO: add description

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 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

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.

External Links