temporary tables and informix hang

Our users recently reported that their informix system (7.?) was having problems. A transaction had hung. They reported messages to the effect that /tmp was full. This was where they were creating temp tables. /tmp itself still appeared to be 40 % free. They reckoned that an onstat -u showed a 'b' flag in position 3. (This doesn't appear in the manual as a valid flag in that position) Now I'm being asked where should temp tables go - /tmp or a dbspace - any ideas ? Also we seem to have a session variable of DBTEMP in use - is this the same as DBTEMPSPACE as documented ? Any advice on the hang or placement of temp tables and why would be useful. Des