# File lib/ffi/tools/const_generator.rb, line 106
    def calculate(options = {})
      binary = File.join Dir.tmpdir, "rb_const_gen_bin_#{Process.pid}"

      Tempfile.open("#{@prefix}.const_generator") do |f|
        @includes.each do |inc|
          f.puts "#include <#{inc}>"
        end
        f.puts "\nint main(int argc, char **argv)\n{"

        @constants.each_value do |const|
          f.puts "#ifdef \#{const.name}\nprintf(\"\#{const.name} \#{const.format}\\\\n\", \#{const.cast}\#{const.name});\n#endif\n"
        end

        f.puts "\n\treturn 0;\n}"
        f.flush

        output = `gcc #{options[:cppflags]} -D_DARWIN_USE_64_BIT_INODE -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -x c -Wall -Werror #{f.path} -o #{binary} 2>&1`

        unless $?.success? then
          output = output.split("\n").map { |l| "\t#{l}" }.join "\n"
          raise "Compilation error generating constants #{@prefix}:\n#{output}"
        end
      end

      output = `#{binary}`
      File.unlink(binary + (FFI::Platform.windows? ? ".exe" : ""))
      output.each_line do |line|
        line =~ /^(\S+)\s(.*)$/
        const = @constants[$1]
        const.value = $2
      end

      missing_constants = @constants.select do |name, constant|
        constant.value.nil?
      end.map { |name,| name }

      if @required and not missing_constants.empty? then
        raise "Missing required constants for #{@prefix}: #{missing_constants.join ', '}"
      end
    end