The Bottom Layer Flush Function
     int <flush function>(qpstream, bufptr, sizeptr)
            QP_stream         *qpstream;
            unsigned char    **bufptr;
            size_t            *sizeptr;
     Return Values:   QP_SUCCESS

The parameters and the return values of the flush function have the same syntax and the same meaning as the write function. The write function may buffer the output record without writing the record out. The flush function should write out the output record immediately when it is called. The middle layer function will not call the write function with an empty record (*sizeptr is zero), but the flush function may be called with an empty record passed in. In general, the flush function can be the same as write function unless the write function buffers output records. An output stream needs a bottom layer flush function only if flush_type of the stream is not FLUSH_ERROR.

In our example, the bottom layer write function does not buffer output record and it can also handle writing an empty record, so the bottom layer flush function is the same as the write function.