cl-user> (let* ((port   9004)
                (server  (usocket:socket-listen  "localhost" port :element-type '(unsigned-byte 8)))
                (connect (usocket:socket-connect "localhost" port :element-type '(unsigned-byte 8)))
                (accept  (usocket:socket-accept server  :element-type '(unsigned-byte 8)))
                (output  (make-array 5 :element-type '(unsigned-byte 8))))
           (unwind-protect
                (progn
                  (write-sequence (map '(vector (unsigned-byte 8)) 'char-code "asdf")  (usocket:socket-stream connect))
                  (when (listen (usocket:socket-stream accept))
                    (read-sequence  output (usocket:socket-stream accept)))
                  output)
             (usocket:socket-close accept)
             (usocket:socket-close connect)
             (usocket:socket-close server)))
#(0 0 0 0 0)
cl-user> (let* ((port   9005)
                (server  (usocket:socket-listen  "localhost" port :element-type '(unsigned-byte 8)))
                (connect (usocket:socket-connect "localhost" port :element-type '(unsigned-byte 8)))
                (accept  (usocket:socket-accept server  :element-type '(unsigned-byte 8)))
                (output  (make-array 5 :element-type '(unsigned-byte 8))))
           (unwind-protect
                (progn
                  (write-sequence (map '(vector (unsigned-byte 8)) 'char-code "asdf")  (usocket:socket-stream connect))
                  (read-sequence  output (usocket:socket-stream accept))
                  output)
             (usocket:socket-close accept)
             (usocket:socket-close connect)
             (usocket:socket-close server)))
; Evaluation aborted on #<swank::invoke-default-debugger #x30200243726D>.
cl-user>