Q: NULL values in ESQL/C

Hello out there, I've written my own database interface based on INFORMIX-ESQL/C. Now, I want to rewrite this interface, using ODBC. ESQL/C has a nice function called risnull() to determine if a C-variable contains a NULL value. Does anybody know, how these NULL values are encoded in these variables? I've an idea how to encode strings, but I wonder how to encode a variable with a datatype of long or float. Do you know that? What's Informix doing here? Any hints are welcome. Thanks in advance ... Dirk Scheer