The “problem” is when you fill the buffer with the whole buffersize, than the end-Pointer has the same position as the begin pointer, which is an ambiguous state, this can mean that the buffer is full, or empty.
So, thats why you have add 1 to your buffersize. (This is something you learn when you write your own fifo-class ( http://www.rawmaterialsoftware.com/viewtopic.php?f=2&t=10118 )