A C++ function to check that a file both exists and is in .gz format.

isgz.cpp

/*
  A function to check that a file both exists and is in .gz format.
  
  It is done the easy way, using only C++ constructs instead of any lower-level calls
  
  Requires the boost iostreams library
  
  Programs compiled using this function need -lboost_iostreams
*/

//Required headers
#include <iostream>

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

bool isgz(const char * filename)
{
  //first, check that it exists
  std::ifstream in(filename);
  if(! in) 
  {
    return false;
  }
  in.close();
  
  /*
    If the file is not .gz and we try
    to read from it using boost's filtering_istream
    classes, an exception will be thrown.
    
    Therefore, catching the exception tells
    you the file is not .gz.
  */
  boost::iostreams::filtering_istream gzin;
  gzin.push(boost::iostreams::gzip_compressor());
  gzin.push(boost::iostreams::file_source(filename),std::ios_base::in | std::ios_base::binary);

  char c;
  try
    {
      gzin >> c;
    }
  catch ( boost::iostreams::gzip_error & e )
    {
      gzin.pop();
      return false;
    }
  return true;
}