    # File lib/ffi/tools/struct_generator.rb, line 58
        def calculate(options = {})
          binary = File.join Dir.tmpdir, "rb_struct_gen_bin_#{Process.pid}"

          raise "struct name not set" if @struct_name.nil?

          Tempfile.open("#{@name}.struct_generator") do |f|
            f.puts "#include <stdio.h>"

            @includes.each do |inc|
              f.puts "#include <#{inc}>"
            end

            f.puts "#include <stddef.h>\n\n"
            f.puts "int main(int argc, char **argv)\n{"
            f.puts "  #{@struct_name} s;"
            f.puts %[  printf("sizeof(#{@struct_name}) %u\\n", (unsigned int) sizeof(#{@struct_name}));]

            @fields.each do |field|
              f.puts "printf(\"\#{field.name} %u %u\\\\n\", (unsigned int) offsetof(\#{@struct_name}, \#{field.name}),\n(unsigned int) sizeof(s.\#{field.name}));\n"
            end

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

            output = `gcc #{options[:cppflags]} #{options[:cflags]} -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
              @found = false
              output = output.split("\n").map { |l| "\t#{l}" }.join "\n"
              raise "Compilation error generating struct #{@name} (#{@struct_name}):\n#{output}"
            end
          end

          output = `#{binary}`.split "\n"
          File.unlink(binary + (FFI::Platform.windows? ? ".exe" : ""))
          sizeof = output.shift
          unless @size
            m = /\s*sizeof\([^)]+\) (\d+)/.match sizeof
            @size = m[1]
          end

          line_no = 0
          output.each do |line|
            md = line.match(/.+ (\d+) (\d+)/)
            @fields[line_no].offset = md[1].to_i
            @fields[line_no].size   = md[2].to_i

            line_no += 1
          end

          @found = true
        end