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Package gzip

import "compress/gzip"

Overview ▾

Package gzip implements reading and writing of gzip format compressed files, as specified in RFC 1952.

Example (WriterReader)

Name: a-new-hope.txt
Comment: an epic space opera by George Lucas
ModTime: 1977-05-25 00:00:00 +0000 UTC

A long time ago in a galaxy far, far away...


These constants are copied from the flate package, so that code that imports "compress/gzip" does not also have to import "compress/flate".

const (
    NoCompression      = flate.NoCompression
    BestSpeed          = flate.BestSpeed
    BestCompression    = flate.BestCompression
    DefaultCompression = flate.DefaultCompression
    HuffmanOnly        = flate.HuffmanOnly


var (
    // ErrChecksum is returned when reading GZIP data that has an invalid checksum.
    ErrChecksum = errors.New("gzip: invalid checksum")
    // ErrHeader is returned when reading GZIP data that has an invalid header.
    ErrHeader = errors.New("gzip: invalid header")

The gzip file stores a header giving metadata about the compressed file. That header is exposed as the fields of the Writer and Reader structs.

Strings must be UTF-8 encoded and may only contain Unicode code points U+0001 through U+00FF, due to limitations of the GZIP file format.

type Header struct {
    Comment string    // comment
    Extra   []byte    // "extra data"
    ModTime time.Time // modification time
    Name    string    // file name
    OS      byte      // operating system type

type Reader

A Reader is an io.Reader that can be read to retrieve uncompressed data from a gzip-format compressed file.

In general, a gzip file can be a concatenation of gzip files, each with its own header. Reads from the Reader return the concatenation of the uncompressed data of each. Only the first header is recorded in the Reader fields.

Gzip files store a length and checksum of the uncompressed data. The Reader will return an ErrChecksum when Read reaches the end of the uncompressed data if it does not have the expected length or checksum. Clients should treat data returned by Read as tentative until they receive the io.EOF marking the end of the data.

type Reader struct {
    Header // valid after NewReader or Reader.Reset
    // contains filtered or unexported fields

func NewReader

func NewReader(r io.Reader) (*Reader, error)

NewReader creates a new Reader reading the given reader. If r does not also implement io.ByteReader, the decompressor may read more data than necessary from r.

It is the caller's responsibility to call Close on the Reader when done.

The Reader.Header fields will be valid in the Reader returned.

func (*Reader) Close

func (z *Reader) Close() error

Close closes the Reader. It does not close the underlying io.Reader. In order for the GZIP checksum to be verified, the reader must be fully consumed until the io.EOF.

func (*Reader) Multistream 1.4

func (z *Reader) Multistream(ok bool)

Multistream controls whether the reader supports multistream files.

If enabled (the default), the Reader expects the input to be a sequence of individually gzipped data streams, each with its own header and trailer, ending at EOF. The effect is that the concatenation of a sequence of gzipped files is treated as equivalent to the gzip of the concatenation of the sequence. This is standard behavior for gzip readers.

Calling Multistream(false) disables this behavior; disabling the behavior can be useful when reading file formats that distinguish individual gzip data streams or mix gzip data streams with other data streams. In this mode, when the Reader reaches the end of the data stream, Read returns io.EOF. The underlying reader must implement io.ByteReader in order to be left positioned just after the gzip stream. To start the next stream, call z.Reset(r) followed by z.Multistream(false). If there is no next stream, z.Reset(r) will return io.EOF.


Name: file-1.txt
Comment: file-header-1
ModTime: 2006-02-01 03:04:05 +0000 UTC

Hello Gophers - 1

Name: file-2.txt
Comment: file-header-2
ModTime: 2007-03-02 04:05:06 +0000 UTC

Hello Gophers - 2

func (*Reader) Read

func (z *Reader) Read(p []byte) (n int, err error)

Read implements io.Reader, reading uncompressed bytes from its underlying Reader.

func (*Reader) Reset 1.3

func (z *Reader) Reset(r io.Reader) error

Reset discards the Reader z's state and makes it equivalent to the result of its original state from NewReader, but reading from r instead. This permits reusing a Reader rather than allocating a new one.

type Writer

A Writer is an io.WriteCloser. Writes to a Writer are compressed and written to w.

type Writer struct {
    Header // written at first call to Write, Flush, or Close
    // contains filtered or unexported fields

func NewWriter

func NewWriter(w io.Writer) *Writer

NewWriter returns a new Writer. Writes to the returned writer are compressed and written to w.

It is the caller's responsibility to call Close on the Writer when done. Writes may be buffered and not flushed until Close.

Callers that wish to set the fields in Writer.Header must do so before the first call to Write, Flush, or Close.

func NewWriterLevel

func NewWriterLevel(w io.Writer, level int) (*Writer, error)

NewWriterLevel is like NewWriter but specifies the compression level instead of assuming DefaultCompression.

The compression level can be DefaultCompression, NoCompression, HuffmanOnly or any integer value between BestSpeed and BestCompression inclusive. The error returned will be nil if the level is valid.

func (*Writer) Close

func (z *Writer) Close() error

Close closes the Writer by flushing any unwritten data to the underlying io.Writer and writing the GZIP footer. It does not close the underlying io.Writer.

func (*Writer) Flush 1.1

func (z *Writer) Flush() error

Flush flushes any pending compressed data to the underlying writer.

It is useful mainly in compressed network protocols, to ensure that a remote reader has enough data to reconstruct a packet. Flush does not return until the data has been written. If the underlying writer returns an error, Flush returns that error.

In the terminology of the zlib library, Flush is equivalent to Z_SYNC_FLUSH.

func (*Writer) Reset 1.2

func (z *Writer) Reset(w io.Writer)

Reset discards the Writer z's state and makes it equivalent to the result of its original state from NewWriter or NewWriterLevel, but writing to w instead. This permits reusing a Writer rather than allocating a new one.

func (*Writer) Write

func (z *Writer) Write(p []byte) (int, error)

Write writes a compressed form of p to the underlying io.Writer. The compressed bytes are not necessarily flushed until the Writer is closed.