Getting attributes of XML from XSD...

HI,
I have one XML file which is confirming to one XSD. In XML, attributes can be optional. But in XSD, attributes are FIXED type and I want to extract the values of attributes from XSD if they are not present in XML file. I will be parsing the XML file using SAX. How can I get the attributes values from XSD? Please help me as I am stuck in one mission critical project. Thanks in advance.
Regards,
Ashish