underflow

The underflow function is called when characters are needed for fetching and none are available in the get area. Its general outline is similar to overflow(), but it deals with the get area rather than the put area.

   int fctbuf::underflow() {
   	// Check that input is allowed
   	if ( !(mode&ios::in) ) return EOF ;
   

   	// Make sure there is a holding area.
   	if (allocate()==EOF) return EOF ;
   

   	// If there are characters waiting for output
   	// send them ;
   	if ( pptr() && pptr() > pbase() ) overflow(EOF) ;
   

   	// Reset put area
   	setp(0,0) ;
   

   	// Setup get area ;
   	if ( blen() > 1 ) setg(base(),base()+1,ebuf()) ;
   	else              setg(base(),base(),ebuf()) ;
   

   	// Produce characters
   	int ok = (*fct)(base(),blen(),ios::in) ;
   

   	if ( ok ) {
   		return zapeof(*base()) ;
   		}
   	else {
   		setg(0,0,0) ;
   		return EOF ;
   		}
   	}

• EOF is returned immediately if we aren't supposed to do input or if a holding area cannot be allocated.

• allocate() is called to make sure that there is a holding area.

• setg() is used to establish the get area where fct will be asked to store characters. Its first argument sets up a pointer, eback, that marks the limit to which putback can move gptr. The second argument becomes gptr, and the last becomes egptr, pointing at the char after the last char containing values stored by the producer.

• blen() returns the size of the holding area. It may be as small as 1.

• If the action function indicated success underflow() returns the first character. It is left in the get area and may be extracted again. zapeof() is used to make sure that the returned result is not EOF. If zapeof() were omitted this might occur on a machine in which chars are signed and EOF is -1.