Using fscanf to read message from FIFO would lost message

In libcurl example HIPERFIFO.C, function FIFO_CB uses fscanf (FIFO InputStream,...) to read in a string.
RV=FSCANF (G->input, "%1023s%n", S, &n);
When the FIFO content producer sends-continuously with no interval, the FIFO_CB would lost the second URL.
But if the FIFO content producer sends the second URL 3ms later, the FIFO_CB would not lost it.
With using read (FD,...) instead of fscanf,
Read_res = Read (fd, &dwmsgptr, sizeof (DWMSGPTR));
The FIFO_CB would not lost the second URL even if the FIFO content producer sends-the-URLs continuously with no interval.

Using fscanf to read message from FIFO would lost message