Locking

I'm using informix online dynamic database server. On the client side, odbc met visual basic. Odbc driver 2.10 from Intersolv. Problem: I want to lock a row exclusive ! If I get a row with the select for update statement I want that every other connection gets an error message when the select the same row for update, not read only. The only way I get the desired behaviour is if I lock the table with the lock table statement, but then an other process can not even read the row. Please help....