Steel Bank Common Lisp

Hello!

I'm trying to use an external program interactively (that is, make it
run along with my Lisp application, get requests and send replies
etc.) this way:

(let ((process (sb-ext:run-program "/path/to/program" args
                                   :input :stream
                                   :output :stream
                                   :wait nil)))
  (write-line some-string (process-input process))
  ...
  (read-line (process-output process)))

But I can't read anything from (process-output process) until the
process is running, and get (the whole) output only when it
terminates. Making FD-STREAMs of Unix pipes and passing them as input
and output parameters to RUN-PROGRAM leads to the same behavior.

Could you tell what am I doing wrong?

Thanks in advance,
 -- Oleg.