Hi czralv,
Yes it is a circular buffer that forms a FIFO. So setting the Head to the Tail or vice versa will set the FIFO empty. Note your User Thread might get preempted (interrupted) at any time and if a new character came in then the Head (pRS232RecIn) would advance. Doing a WaitNextTimeSlice() beforehand will guarantee nothing unexpected happens.
HTH
Regards
TK