Gzip Filters

Overview

Acknowledgments

Headers

Synopsis

Reference

namespace boost::iostreams::gzip

Class gzip_params

Class template gzip_compressor

Class template gzip_decompressor

Class gzip_error

Examples

Installation

Overview

The class templates basic_gzip_compressor and basic_gzip_decompressor perform compression and decompression based on the GZIP format ([Deutsch3]) using Jean-loup Gailly's and Mark Adler's zlib compression library ([Gailly]). They are implementation as derived classes of the Zlib Filters. The difference between the GZIP and ZLIB formats is that data in the GZIP contains more header information and a different checksum ([Deutsch1], [Deutsch3]).

Currently, basic_gzip_compressor is a DualUseFilters, but basic_gzip_compressor is only an InputFilter.

Acknowledgments

The gzip Filters were influences by the work of Jeff Garland ([Garland]) and Jonathan de Halleux ([de Halleux]).

Headers

<boost/iostreams/filter/gzip.hpp>

Synopsis

namespace boost { namespace iostreams {

namespace gzip {
                    
using namespace boost::iostreams::zlib;

    // Error codes used by gzip_error.

const int zlib_error;
const int bad_crc;
const int bad_length;
const int bad_header;
const int bad_footer;

}   // End namespace boost::iostreams::gzip

struct gzip_params;

template<typename Alloc = std::allocator<char> >
struct basic_gzip_compressor;

template<typename Alloc = std::allocator<char> >
struct basic_gzip_decompressor;

typedef basic_gzip_compressor<>   gzip_compressor;
typedef basic_gzip_decompressor<> gzip_decompressor;

class gzip_error;

} } // End namespace boost::io

Reference

Namespace boost::iostreams::gzip

The namespace boost::iostreams::gzip contains integral constants used to configure gzip Filters and to report errors. The constants have the following interpretations. (See [Deutsch3] for additional details.)

Class gzip_params

Description

Encapsulates the parameters used to configure basic_gzip_compressor and basic_gzip_decompressor.

Synopsis

struct gzip_params : zlib_params {

    // Non-explicit constructor.
    gzip_params( int level              = default value,
                 int method             = default value,
                 int window_bits        = default value, 
                 int mem_level          = default value, 
                 int strategy           = default value,
                 std::string file_name  = "",
                 std::string comment    = "",
                 std::time_t mtime      = 0 );

    std::string  file_name;
    std::string  comment;
    std::time_t  mtime;
};

gzip_params::gzip_params

    gzip_params( int level              = default value,
                 int method             = default value,
                 int window_bits        = default value, 
                 int mem_level          = default value, 
                 int strategy           = default value,
                 std::string file_name  = "",
                 std::string comment    = "",
                 std::time_t mtime      = 0 );

Constructs a gzip_params object, where the parameters have the following interpretations:

level	-	Compression level. Must be equal to zlib::default_compression or a value in the range 0-9. The value 0 yields no compression, while 9 yields the best compression ratio. Affects compression only.
method	-	Compression method. Must equal zlib::deflated. Affects compression only.
window_bits	-	The base two logarithm of the window size. Must be in the range 8-15; defaults to 15.
mem_level	-	Specifies the amount of memory to be used. Must be in the range 1-9; defaults to 8. Affects compression only.
strategy	-	Must be zlib::default_strategy, zlib::filtered or zlib::huffman_only. Affects compression only.
file_name	-	Represents the optional section of the GZIP header containing a file name.
comment	-	Represents the optional section of the GZIP header containing a comment.
mtime	-	Represents the optional section of the GZIP header containing a file modification time.

See [Gailly] and [Deutsch1] for additional details.

Class template basic_gzip_compressor

Description

Model of DualUseFilter which compresses data in the GZIP format ([Deutsch3]).

Synopsis

template<typename Alloc = std::allocator<char> >
struct basic_gzip_compressor {
    typedef char                    char_type;
    typedef implementation-defined  category;

    basic_gzip_compressor( const gzip_params& = zlib::default_compression,
                           std::streamsize buffer_size = default value );

    // DualUseFilter members.
};

typedef basic_gzip_compressor<> gzip_compressor;

Template Parameters

basic_gzip_compressor::basic_gzip_compressor

    basic_gzip_compressor( const gzip_params& = zlib::default_compression,
                           std::streamsize buffer_size = default value );

Constructs an instance of basic_gzip_compressor with the given parameters and buffer size. Since a gzip_params object is implicitly constructible from an int representing a compression level, an int may be passed as the first constructor argument.

Class template basic_gzip_decompressor

Description

Model of DualUseFilter which decompresses data in the GZIP format ([Deutsch3]).

Synopsis

template<typename Alloc = std::allocator<char> >
struct basic_gzip_decompressor {
    typedef char                    char_type;
    typedef implementation-defined  category;

    basic_gzip_decompressor( int window_bits = default value, 
                             std::streamsize buffer_size = 
                                 default value );

    // DualUseFilter members.
};

typedef basic_gzip_decompressor<> gzip_decompressor;

Template Parameters

basic_gzip_decompressor::basic_gzip_decompressor

    basic_gzip_decompressor( int window_bits = default value, 
                             std::streamsize buffer_size = 
                                 default value );

Constructs an instance of basic_gzip_decompressor with the given window bits value and buffer size. Other parameters affecting decompression are set to default values.

Class gzip_error

Description

Synopsis

class gzip_error : public std::ios_base::failure {
public:
    gzip_error(int error);
    gzip_error(const zlib_error& e);
    int error() const;
    int zlib_error_code() const;
};

gzip_error::gzip_error

    gzip_error(int error);
    gzip_error(const zlib_error& e);

The first member constructs an instance of gzip_error with the given error code from the namespace boost::iostreams::gzip. The second constructs an instance of gzip_error based on an instance of zlib_error.

gzip_error::error

    void error() const;

Returns an error code from the namespace boost::iostreams::gzip.

    void zlib_error_code() const;

Returns an error code from the namespace boost::iostreams::zlib. Meaningful only if error() returns the constant boost::iostreams::gzip::zlib_error.

Examples

#include <fstream>
#include <iostream>
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/gzip.hpp>

int main() 
{
    using namespace std;

    ifstream file("hello.gz", ios_base::in | ios_base::binary);
    boost::iostreams::filtering_streambuf<input> in;
    in.push(gzip_decompressor());
    in.push(file);
    boost::iostreams::copy(in, cout);
}

Installation

The gzip Filters depend on the third-party zlib library, which is not included in the Boost distribution. Prebuilt zlib binaries are available on most UNIX and UNIX-like systems, and will be found automatically by the Boost build system. Windows users can obtain prebuilt binaries at the zlib homepage. Users can also configure the Boost Iostream library to build zlib from the source code, which is available at the zlib homepage. For details on configuring the build system to find your zlib installation, please see Installation.

© Copyright 2008 CodeRage, LLC
© Copyright 2004-2007 Jonathan Turkanis

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)