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
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.
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. |
