format  void STM_SetCdbufFunc (StmGrpHn grp, StmHn stm, Sint32 nsct,
(  StmCdbufFunc func, void * obj)
 Input  grp: Stream group handle
 st  stm: Stream handle (STM_CDBUF_REST when checking free sectors)
  nsct: Number of sectors
:  func: Buffer size monitoring function (STM_CDBUF_NULLFUNC to cancel)
  obj: Registered object
 Output  None
 Function value  None
 Function  Registers a buffer size monitoring function for the specified stream group.
  The registration function is called under the following conditions. 

stm value	Monitor function func is called
Normal stream handle	Stream data in CD buffer> nsct
STM_CDBUF_REST	Free space in CD buffer ≦ nsct

  Remarks  (a) The registration function has the following format.
  void (* StmCdbufFunc) (void * obj)
  (b) The registration object is passed to the first argument of the registration function.