Documentation

    Table of Content
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    • Compression File Formats
      • Compression File Formats
      • 7Z
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    1. Home
    2. Compression File Formats
    3. GZ

    What's on this Page

      • What is a GZ file?
      • File Header
        • File Flags
        • Operating System
      • Optional Headers
      • Compressed Data
      • File Footer
      • References

    What is a GZ file?

    Files with .gz extension are compressed files created with gzip compression application. It can contain multiple compressed files and is commonly used on UNIX and Linux systems. GZIP was introduced as a free utility for replacing the Compress program used in Unix systems. Such files can be opened and extracted with a several applications such as WinZip which is available on both Windows and MacOS. While the format is similar to .ZIP compression in archiving, it differs in terms of compression applied to the archive instead of individual file.

    GZ File Format

    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

    File Header

    The file header consists of 10 bytes as follow:

    OffsetSizeValueDescription
    020x1f 0x8bMagic number identifying file type
    21Compression Method * 0-7 (Reserved) * 8 (Deflate)
    31File Flags
    4432-bit timestamp
    81Compression flags
    91Operating system ID

    File Flags

    ValueIdentifierDescription
    0x01FTEXTIf 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.
    0x02FHCRCThe file contains a header checksum (CRC-16)
    0x04FEXTRAThe file contains extra fields
    0x08FNAMEThe file contains an original file name string
    0x10FCOMMENTThe file contains comment
    0x20Reserved
    0x40Reserved
    0x80Reserved

    Operating System

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

    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.

    File Footer

    The file footer is 8 bytes in size and contains following information.

    OffsetSizeDescription
    04Checksum (CRC-32)
    44Uncompressed data size value in bytes

    References

    • gzip - Wikipedia
    • RFC1952: GZIP file format specification, by IETF