System.FileArchive.GZip

MissingH-1.1.1.0: Large utility library

Portability | portable
Stability | provisional
Maintainer | John Goerzen <jgoerzen@complete.org>

System.FileArchive.GZip

Contents

- GZip Files
- Types
- Whole-File Processing
- Section Processing

Description

GZip file decompression

Copyright (c) 2004 John Goerzen, jgoerzen@complete.org

The GZip format is described in RFC1952.

Synopsis

- data Header = Header {
  - method :: Int
  - flags :: Int
  - extra :: Maybe String
  - filename :: Maybe String
  - comment :: Maybe String
  - mtime :: Word32
  - xfl :: Int
  - os :: Int
  }
- type Section = (Header, String, Footer)
- data GZipError
  - = CRCError
  - | NotGZIPFile
  - | UnknownMethod
  - | UnknownError String
- data Footer = Footer {
  - size :: Word32
  - crc32 :: Word32
  - crc32valid :: Bool
  }
- decompress :: String -> (String, Maybe GZipError)
- hDecompress :: Handle -> Handle -> IO (Maybe GZipError)
- read_sections :: String -> Either GZipError [Section]
- read_header :: String -> Either GZipError (Header, String)
- read_section :: String -> Either GZipError (Section, String)

# GZip Files

GZip files contain one or more `Section`s. Each `Section`, on disk, begins with a GZip `Header`, then stores the compressed data itself, and finally stores a GZip `Footer`.

The `Header` identifies the file as a GZip file, records the original modification date and time, and, in some cases, also records the original filename and comments.

The `Footer` contains a GZip CRC32 checksum over the decompressed data as well as a 32-bit length of the decompressed data. The module `Data.Hash.CRC32.GZip` is used to validate stored CRC32 values.

The vast majority of GZip files contain only one `Section`. Standard tools that work with GZip files create single-section files by default.

Multi-section files can be created by simply concatenating two existing GZip files together. The standard gunzip and zcat tools will simply concatenate the decompressed data when reading these files back. The `decompress` function in this module will do the same.

When reading data from this module, please use caution regarding how you access it. For instance, if you are wanting to write the decompressed stream to disk and validate its CRC32 value, you could use the `decompress` function. However, you should process the entire stream before you check the value of the Bool it returns. Otherwise, you will force Haskell to buffer the entire file in memory just so it can check the CRC32.

# Types

data Header

The data structure representing the GZip header. This occurs at the beginning of each `Section` on disk.

Constructors

Header |
Fields method :: Int Compression method. Only 8 is defined at present. flags :: Int extra :: Maybe String filename :: Maybe String comment :: Maybe String mtime :: Word32 Modification time of the original file xfl :: Int Extra flags os :: Int Creating operating system

Instances

Eq Header |
Show Header |

type Section = (Header, String, Footer)

A section represents a compressed component in a GZip file. Every GZip file has at least one.

data GZipError

Constructors

CRCError | CRC-32 check failed
NotGZIPFile | Couldn't find a GZip header
UnknownMethod | Compressed with something other than method 8 (deflate)
UnknownError String | Other problem arose

Instances

Eq GZipError |
Show GZipError |
Error GZipError |

data Footer

Stored on-disk at the end of each section.

Constructors

Footer |
Fields size :: Word32 The size of the original, decompressed data crc32 :: Word32 The stored GZip CRC-32 of the original, decompressed data crc32valid :: Bool Whether or not the stored CRC-32 matches the calculated CRC-32 of the data

# Whole-File Processing

decompress :: String -> (String, Maybe GZipError)

Read a GZip file, decompressing all sections that are found.

Returns a decompresed data stream and Nothing, or an unreliable string and Just (error). If you get anything other than Nothing, the String returned should be discarded.

hDecompress

Arguments

:: Handle | Input handle
-> Handle | Output handle
-> IO (Maybe GZipError) |

Read a GZip file, decompressing all sections found.

Writes the decompressed data stream to the given output handle.

Returns Nothing if the action was successful, or Just GZipError if there was a problem. If there was a problem, the data written to the output handle should be discarded.

read_sections :: String -> Either GZipError [Section]

Read all sections.

# Section Processing

read_header :: String -> Either GZipError (Header, String)

Read the GZip header. Return (Header, Remainder).

read_section :: String -> Either GZipError (Section, String)

Read one section, returning (ThisSection, Remainder)