MissingH-1.0.3: Large utility librarySource codeContentsIndex
MaintainerJohn Goerzen <jgoerzen@complete.org>
GZip Files
Whole-File Processing
Section Processing

GZip file decompression

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

The GZip format is described in RFC1952.

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 Sections. 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.

data Header Source
The data structure representing the GZip header. This occurs at the beginning of each Section on disk.
method :: IntCompression method. Only 8 is defined at present.
flags :: Int
extra :: Maybe String
filename :: Maybe String
comment :: Maybe String
mtime :: Word32Modification time of the original file
xfl :: IntExtra flags
os :: IntCreating operating system
show/hide Instances
type Section = (Header, String, Footer)Source
A section represents a compressed component in a GZip file. Every GZip file has at least one.
data GZipError Source
CRCErrorCRC-32 check failed
NotGZIPFileCouldn't find a GZip header
UnknownMethodCompressed with something other than method 8 (deflate)
UnknownError StringOther problem arose
show/hide Instances
data Footer Source
Stored on-disk at the end of each section.
size :: Word32The size of the original, decompressed data
crc32 :: Word32The stored GZip CRC-32 of the original, decompressed data
crc32valid :: BoolWhether or not the stored CRC-32 matches the calculated CRC-32 of the data
Whole-File Processing
decompress :: String -> (String, Maybe GZipError)Source

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.

:: HandleInput handle
-> HandleOutput 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]Source
Read all sections.
Section Processing
read_header :: String -> Either GZipError (Header, String)Source
Read the GZip header. Return (Header, Remainder).
read_section :: String -> Either GZipError (Section, String)Source
Read one section, returning (ThisSection, Remainder)
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