MDAC 2.5 SDK - ODBC Programmer's Reference
Appendix F: ODBC Cursor Library
The cursor library creates a buffer in the cache for each length/indicator buffer bound to the result set with SQLBindCol. It uses the values in these buffers to construct a WHERE clause when it emulates positioned update or delete statements. It updates these buffers from the rowset buffers when it fetches data from the data source and when it executes positioned update statements.
If the C type of a data buffer is SQL_C_CHAR or SQL_C_BINARY, and the length/indicator value is SQL_NTS, the string length of the data is put into the length/indicator buffer.
Note The cursor library does not update its cache for a column if *StrLen_or_IndPtr in the corresponding rowset buffer is SQL_DATA_AT_EXEC or the result of the SQL_LEN_DATA_AT_EXEC macro.