Difference between revisions of "Gzip"

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(File format)
(Flags)
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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
+
* optional headers
 +
** extra fields
 
** original file name
 
** original file name
 +
** comment
 
** header checksum
 
** header checksum
 
* a body, containing a DEFLATE-compressed payload
 
* a body, containing a DEFLATE-compressed payload
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| 0x01
 
| 0x01
 
| FTEXT
 
| FTEXT
| If set the uncompressed data needs to be treated as text instead of binary data. <br>
+
| 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.
| This flag hints end-of-line conversion for cross-platform text files but does not enforce it.
+
 
|-
 
|-
 
| 0x02
 
| 0x02
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| 0x04
 
| 0x04
 
| FEXTRA
 
| FEXTRA
|  
+
| The file contains extra fields
 
|-
 
|-
 
| 0x08
 
| 0x08
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| 0x10
 
| 0x10
 
| FCOMMENT
 
| FCOMMENT
|  
+
| The file contains comment
 
|-
 
|-
 
| 0x20
 
| 0x20
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=== Optional headers ===
 
=== 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 ====
 
==== Header checksum ====
 
The CRC16 of the header checksum consists of the two least significant bytes of the CRC32 for all bytes of the gzip header up to and not including the CRC16.
 
The CRC16 of the header checksum consists of the two least significant bytes of the CRC32 for all bytes of the gzip header up to and not including the CRC16.

Revision as of 02:01, 28 November 2013

Information icon.png

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
  • 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.

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 (CRC16)
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 CRC16 of the header checksum consists of the two least significant bytes of the CRC32 for all bytes of the gzip header up to and not including the CRC16.

External Links