Writes binary data to a gzipped file

readbgz.cc

//g++ -o readbgz readbgz.cc -lboost_iostreams -lz
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/device/file.hpp>
#include <boost/iostreams/filter/gzip.hpp>

#include <iostream>
#include <fstream>
#include <cstdlib>
#include <vector>

#include <zlib.h>

using namespace std;

void via_boost( const char * fn )
/*
  Uses boost's filtering_istream library.

  Pro: this library is super-convenient via C++ synyax;
  
  Con: streams are not seekable. Attempting in.seekg( pos ) 
  in the code below will put the stream into a bad state.
  These scripts also cannot be opened in an append mode, if I recall correctly
 */
{
  boost::iostreams::filtering_istream in;
  in.push(boost::iostreams::gzip_decompressor());
  in.push(boost::iostreams::file_source("test.gz"), ios_base::in | ios_base::binary );

  unsigned nrecs;
  in.read( reinterpret_cast<char *>(&nrecs), sizeof(unsigned) );
  cout << nrecs << '\n';
  vector<double> buffer2(nrecs);
  in.read( reinterpret_cast<char *>(&buffer2[0]), nrecs*sizeof(double) );

  for(unsigned i=0;i<nrecs;++i)
    {
      cout << buffer2[i] << '\n';
    }

  in.pop();
  in.pop();
}

/*
  Uses the C-language zlib library

  Pro: this library exists basically everywhere.  Files are seekable,
  although seeking is emulated and maybe very slow on some systems. 
  Files may be appended to.

  Cons: the C-language syntax may be less convenient
 */
void via_zlib( const char * fn )
{
  gzFile gzin = gzopen("test.gz","r");
 
  unsigned nrecs;
  int rv =  gzread( gzin, &nrecs, sizeof(unsigned) );

  cout << nrecs << '\n';
  vector<double> buffer2(nrecs);
  rv = gzread( gzin, &buffer2[0], nrecs*sizeof(double) );

  for(unsigned i=0;i<nrecs;++i)
    {
      cout << buffer2[i] << '\n';
    }

  gzclose(gzin);
}


int main( int argc, char ** argv )
{
  cout << "Using boost:\n";
  via_boost("test.gz");

  cout << "Now, using zlib:\n";
  via_zlib("test.gz");
}

writebgz.cc

//g++ -o writebgz writebgz.cc -lz
#include <zlib.h>
#include <sstream>
#include <iostream>

using namespace std;

int main(int argc, char ** argv)
{
  ostringstream o;
  unsigned nrecords=100;
  o.write( reinterpret_cast<char *>(&nrecords), sizeof(unsigned) );

  for(unsigned i = 0 ; i < nrecords ; ++i )
    {
      double x = 1./double(i);
      cerr << x << '\n';
      o.write( reinterpret_cast<char *>(&x), sizeof(double) );
    }
  
  gzFile gzo = gzopen("test.gz","w");

  gzwrite( gzo, o.str().c_str(), o.str().size() );

  gzclose( gzo );
}