How to decompress gzipstream with zlib

Question

Can someone tell me which function I need to use in order to decompress a byte array that has been compressed with vb.net's gzipstream. I would like to use zlib.

I've included the zlib.h but I haven't been able to figure out what function(s) I should use.

Answer 1

You can take a look at The Boost Iostreams Library:

#include <fstream>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/filter/gzip.hpp>

std::ifstream file;
file.exceptions(std::ios::failbit | std::ios::badbit);
file.open(filename, std::ios_base::in | std::ios_base::binary);

boost::iostreams::filtering_stream<boost::iostreams::input> decompressor;
decompressor.push(boost::iostreams::gzip_decompressor());
decompressor.push(file);

And then to decompress line by line:

for(std::string line; getline(decompressor, line);) {
    // decompressed a line
}

Or entire file into an array:

std::vector<char> data(
      std::istreambuf_iterator<char>(decompressor)
    , std::istreambuf_iterator<char>()
    );

Answer 2

You need to use inflateInit2() to request gzip decoding. Read the documentation in zlib.h.

There is a lot of sample code in the zlib distribution. Also take a look at this heavily documented example of zlib usage. You can modify that one to use inflateInit2() instead of inflateInit().