6.10 BUFW: allocate I/O buffer

      subroutine bufw(ifil,lenbuf,nbuf)

allocate a buffer of length abs(lenbuf) for file ifil ${\tt lenbuf} < 0$ sets istay to 1, which freezes the contents of buffer when it is full, except that finds may use the top portion of the buffer; otherwise i/o is unbuffered. ${\tt lenbuf} = 0$ does all i/o unbuffered. nbuf: dummy in present implementation