GZ File Format - GNU Zipped Archive File
What is a GZ file?
A GZ file is a compressed archive that is created using the standard gzip (GNU zip) compression algorithm. It may contain multiple compressed files, directories and file stubs. This format was initially developed to replace compression formats on UNIX systems. and is still one of the most common archive types on Linux systems. Applications such as WinZip can open GZ files to view its contents on both Windows and MacOS.
GZ File Format - More Information
Gzip uses the DEFLATE algorithm for compression of archive and differs from the ZIP archive format in applying the compression algorithm on complete archive rather than individual files. The GZIP file format specifications version 4.3 published by Internet Engineering Task Force (IETF) contains detailed information about the file format. The file format consists of:
- File Header
- Optional Headers
- Compressed Data
- File Footer
GZ File Header
The GZ file header consists of 10 bytes as follow:
| Offset | Size | Value | Description |
|---|---|---|---|
| 0 | 2 | 0x1f 0x8b | Magic number identifying file type |
| 2 | 1 | Compression Method * 0-7 (Reserved) * 8 (Deflate) | |
| 3 | 1 | File Flags | |
| 4 | 4 | 32-bit timestamp | |
| 8 | 1 | Compression flags | |
| 9 | 1 | Operating system ID |
File 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 |
Operating System
| Value | 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 |
GZ Optional Headers
The optional extra headers are those as denoted by the file flags and include information such as the original filename, extra fields, comments and header checksum.
Compressed Data
This section contains the compressed data using the DEFLATE compression algorithm.
GZ File Footer
The file footer is 8 bytes in size and contains following information.
| Offset | Size | Description |
|---|---|---|
| 0 | 4 | Checksum (CRC-32) |
| 4 | 4 | Uncompressed data size value in bytes |