scsh-0.6/scsh/newports.scm

;;; A Unix file port system to completely replace S48 file ports.
;;; We use S48 extensible ports.
;;; Copyright (c) 1993 by Olin Shivers.

(define-record fdport-data
  channel
  revealed)

; This stuff is _weak_.
; Vector of weak pointers mapping fd -> fdport.

(define fdports (make-integer-table))

(define (install-fdport fdport)
  (let* ((fdport* (fdport-data fdport))
	 (ch (fdport-data:channel fdport*))
	 (ch-number  (channel-os-index ch)))
    (if (not (= (fdport-data:revealed fdport*) 0))
	(table-set! fdports ch-number fdport)
	(weak-table-set! fdports ch-number fdport))))

(define (maybe-fdes->port fdes)
  (weak-table-ref fdports fdes))

;Hmm... these shouldn't be necessary.  But still.
;Fake defrec routines for backwards compatibility.
(define (fdport-data:fd fdport*)
  (channel-os-index  (fdport-data:channel fdport*)))

(define (fdport-data:closed? fdport*)
  (eq? (channel-status (fdport-data:channel fdport*)) 
       (enum channel-status-option closed)))

;;; Support for channel-ready?
;;; This applies to input- and output-ports

(define (fdport-channel-ready? fdport)
  (channel-ready? (fdport-data:channel (port-data fdport))))

;Arbitrary, for now.
(define buffer-size 255)

(define open-fdchannel open-channel)

(define (make-input-fdchannel fd)
  (open-fdchannel fd (enum channel-status-option input)))

(define (make-output-fdchannel fd)
  (open-fdchannel fd (enum channel-status-option output)))

;The two following routines are to build ports from stdin and stdout channels.
(define (channel-port->input-fdport channel-port)
  (let ((p (make-buffered-input-port input-fdport-handler
			    (make-fdport-data
			     (channel-cell-ref (port-data channel-port)) 1)
			    (make-byte-vector buffer-size 0) 0 0)))
    (obtain-port-lock channel-port)
    (set-port-lock! p (port-lock channel-port))
    (set-port-locked?! p (port-locked? channel-port))
    (install-fdport p)
    (release-port-lock channel-port)
    p))

(define (channel-port->output-fdport channel-port)
  (let ((p (make-buffered-output-port 
	    output-fdport-handler
	    (make-fdport-data  (channel-cell-ref(port-data channel-port)) 1)
	    (make-byte-vector buffer-size 0) 0 buffer-size)))
    (obtain-port-lock channel-port)
    (set-port-lock! p (port-lock channel-port))
    (set-port-locked?! p (port-locked? channel-port))
    (install-fdport p)
    (periodically-force-output! p)
    (release-port-lock channel-port)
    p))

(define (channel-port->unbuffered-output-fdport channel-port)
  (let ((p (make-unbuffered-output-port unbuffered-output-fdport-handler
			     (make-fdport-data 
			      (channel-cell-ref (port-data channel-port)) 1))))
    (obtain-port-lock channel-port)
    (set-port-lock! p (port-lock channel-port))
    (set-port-locked?! p (port-locked? channel-port))
    (install-fdport p)
    (periodically-force-output! p)
    (release-port-lock channel-port)
    p))

(define (alloc-input-fdport fd revealed)
  (make-buffered-input-port input-fdport-handler
		   (make-fdport-data (make-input-fdchannel fd) revealed)
		   (make-byte-vector buffer-size 0) 0 0))

(define (alloc-output-fdport fd revealed)
  (make-buffered-output-port output-fdport-handler
		    (make-fdport-data (make-output-fdchannel fd) revealed)
		    (make-byte-vector buffer-size 0) 0 buffer-size))

(define (make-input-fdport fd revealed)
  (let ((p (alloc-input-fdport fd revealed)))
    (install-fdport p)
    p))

(define (make-output-fdport fd revealed)
  (let ((p (alloc-output-fdport fd revealed)))
    (periodically-force-output! p)
    (install-fdport p)
    p))

(define (fdport? x)
  (cond ((or (and (input-port? x) (port-data x))
	     (and (output-port? x) (port-data x)))
	 => (lambda (d) (fdport-data? d)))
	(else #f)))
	
(define fdport-null-method (lambda (x) x #f))

(define null-func (lambda args #t))

(define (close-fdport* fdport*)
  (table-set! fdports (channel-os-index (fdport-data:channel fdport*)) #f)
  (close-channel (fdport-data:channel fdport*)))

;The handlers drop straight through to the convenient channel routines.
(define (make-input-fdport-handler bufferproc)
  (make-buffered-input-port-handler
   (lambda (fdport*)
     (list 'input-fdport (fdport-data:channel fdport*)))
   close-fdport*
   bufferproc
   fdport-channel-ready?
   (lambda (fdport* owner)
     (steal-channel! (fdport-data:channel fdport*) owner))))

(define input-fdport-handler
  (make-input-fdport-handler    
   (lambda (fdport* buffer start needed)
     (channel-read buffer start needed (fdport-data:channel fdport*)))))

(define (make-output-fdport-handler bufferproc)
   (make-buffered-output-port-handler
   (lambda (fdport*)
     (list 'output-fdport (fdport-data:channel fdport*)))
   close-fdport*
   bufferproc
   fdport-channel-ready?
   (lambda (fdport* owner)
     (steal-channel! (fdport-data:channel fdport*) owner))))

(define output-fdport-handler
  (make-output-fdport-handler
   (lambda (fdport* buffer start count)
     (channel-write buffer start count (fdport-data:channel fdport*)))))
 
(define unbuffered-output-fdport-handler
  (let ((buffer (make-byte-vector 1 0)))
    (make-output-fdport-handler
     (lambda (fdport* char)
       (byte-vector-set! buffer 0 (char->ascii char))
       (channel-write buffer 0 1 (fdport-data:channel fdport*))))))

(define fdport-data port-data)
; That was easy.

(define (guess-output-policy port)
  (if (= 0 (port-limit port)) 
      bufpol/none
      bufpol/block))

(define (set-port-buffering port policy . maybe-size) 
  (cond ((and (fdport? port) (open-input-port? port))
	 (let ((size (if (pair? maybe-size) (car maybe-size) 255)))
	   (set-input-port-buffering port policy size)))
	((and (fdport? port) (open-output-port? port))
	 (let ((size (if (pair? maybe-size) (car maybe-size) 255)))
	    (if (<= size 0) (error "size must be at least 1"))
	   (set-output-port-buffering port policy size)))
	(else
            (warn "port-type not supported" port))))

(define (set-output-port-buffering port policy size) 
  (cond ((eq? policy bufpol/none)
	 (install-nullbuffer port unbuffered-output-fdport-handler))
	((eq? policy bufpol/block)
	 (let ((old-size (byte-vector-length (port-buffer port)))
	       (new-buffer (make-byte-vector size 0)))
	   (if (< size old-size)
	       (begin
		 (really-force-output port)
		 (obtain-port-lock port)
 		 (set-port-index! port 0))
	       (begin 
		 (obtain-port-lock port)
		 (copy-bytes! (port-buffer port) 0 new-buffer 0 old-size)))
	   (install-buffer port new-buffer size)
	   (release-port-lock port)))
	((eq? policy bufpol/line)
	 (install-nullbuffer port (make-line-output-proc size)))
	(else (warn "policy not supported " policy))))

(define (install-nullbuffer port handler)
 (really-force-output port)
 (obtain-port-lock port)
 (set-port-limit! port 0)
 (set-port-index! port 0)
 (set-port-buffer! port (make-byte-vector 0 0))
 (set-port-handler! port handler)
 (release-port-lock port))

(define (install-buffer port new-buffer size)
  (if (eq? bufpol/none (guess-output-policy port))
      (set-port-handler! port output-fdport-handler))
  (set-port-limit! port size)
  (set-port-buffer! port new-buffer))

; TODO flush on stdinput is required but probably impossible since current-input-port is a fluid and may change without notice. One possibility would be to override (current-input-port)

;;; This port can ONLY be flushed with a newline or a close-output
;;; flush-output  won't help
(define (make-line-output-proc size)
  (let ((proc-buffer (make-byte-vector size 0))
	(proc-buffer-index 0))
    (make-buffered-output-port-handler
     (lambda (fdport*)
       (list 'output-fdport (fdport-data:channel fdport*)))
     (lambda (fdport*)
       (channel-write proc-buffer 
		      0 
		      proc-buffer-index 
		      (fdport-data:channel fdport*))
       (close-fdport* fdport*))
     (lambda (fdport* char)
       (byte-vector-set! proc-buffer proc-buffer-index (char->ascii char))
       (set! proc-buffer-index (+ proc-buffer-index 1))
       (cond ((or (eq? char #\newline) (= proc-buffer-index size))
	      (channel-write proc-buffer 
			     0 
			     proc-buffer-index 
			     (fdport-data:channel fdport*))
	      (set! proc-buffer-index 0))))
     fdport-channel-ready?
     (lambda (fdport* owner)
       (steal-channel! (fdport-data:channel fdport*) owner)))))

	
(define (set-input-port-buffering port policy size)
  (cond ((eq? policy bufpol/none)
	 (set-input-port-buffering port bufpol/block 1))
	((eq? policy bufpol/block)
	 (if (<= size 0) (error "size must be at least 1"))
	 (install-input-handler port input-fdport-handler size #t))
	((eq? policy bufpol/line)
	 (error "bufpol/line not allowed on input"))
	(else (warn "policy not supported " policy))))

(define (install-input-handler port new-handler size gentle?)
  	 (obtain-port-lock port)
	 (let* ((old-limit (port-limit port))
		(old-index (port-index port))
		(old-buffer (port-buffer port))
		(old-unread (- old-limit old-index))
		(new-unread (min old-unread size))
		(throw-away (max 0 (- old-unread new-unread)))
		(new-buffer (make-byte-vector size 0)))
	   (if (not gentle?)
	       (let ((ret (if (> throw-away 0)
			      (let ((return-buffer 
				     (make-byte-vector throw-away 0)))
				(copy-bytes! old-buffer old-index 
					     return-buffer 0 
					     throw-away) return-buffer)
			      #f)))
		   (copy-bytes! old-buffer (+ old-index throw-away) 
				new-buffer 0 
				new-unread)
		   (set-port-buffer! port new-buffer)
		   (set-port-index! port 0)
		   (set-port-limit! port new-unread)
		   (set-port-handler! port new-handler)
		   (release-port-lock port)
		 ret)
	        (begin 
		  (install-drain-port-handler 
		   old-buffer old-index old-limit port new-handler)
		  (set-port-buffer! port new-buffer)
		  (set-port-index! port 0)
		  (set-port-limit! port 0)
		  (release-port-lock port)
		  #t))))

(define (install-drain-port-handler 
	 old-buffer old-start old-limit port new-handler)
   (if (< 0 (- old-limit old-start))
       (set-port-handler! port 
			  (make-drain-port-handler 
			   old-buffer old-start old-limit port new-handler))
       (set-port-handler! port new-handler)))


;;; TODO: This reference to port will prevent gc !!!
(define (make-drain-port-handler 
	 very-old-buffer old-start old-limit port new-handler)
  (let ((old-buffer (make-byte-vector old-limit 0)))
    (copy-bytes! very-old-buffer 0 old-buffer 0 old-limit)
    (make-input-fdport-handler
     (lambda (data buffer start needed)
       (let ((old-left (- (byte-vector-length old-buffer) old-start)))
	 (let ((size (cond ((or (eq? needed 'any) (eq? needed 'immediate))
			    (min old-left
				 (byte-vector-length buffer)))
			   (else (min needed old-left)))))
	   (copy-bytes! old-buffer old-start buffer start size)
	   (set! old-start (+ size old-start))
	   
	   (if (= old-start (byte-vector-length old-buffer))  ;buffer drained ?
	       (begin 
		 (set-port-handler! port new-handler)
		 (if (and (integer? needed) (> needed size))
		     (+ size ((port-handler-buffer-proc new-handler) 
			      data buffer (+ start size) (- needed size)))
		     size))
	       size)))))))


;;; Open & Close
;;; ------------

;;; replace rts/channel-port.scm begin
(define (open-file fname flags . maybe-mode)
  (with-cwd-aligned
   (with-umask-aligned
    (let ((fd (apply open-fdes fname flags maybe-mode))
	  (access (bitwise-and flags open/access-mask)))
      ((if (or (= access open/read) (= access open/read+write))
	   make-input-fdport
	   make-output-fdport)
       fd 0)))))

(define (open-input-file fname . maybe-flags)
  (let ((flags (:optional maybe-flags 0)))
    (open-file fname (deposit-bit-field flags open/access-mask open/read))))

(define (open-output-file fname . rest)
  (let* ((flags (if (pair? rest) (car rest)
		    (bitwise-ior open/create open/truncate))) ; default
	 (maybe-mode (if (null? rest) '() (cdr rest)))
	 (flags (deposit-bit-field flags open/access-mask open/write)))
    (apply open-file fname flags maybe-mode)))

;;; replace rts/channel-port.scm end

;;; All these revealed-count-hacking procs have atomicity problems.
;;; They need to run uninterrupted.
;;; (port-locks should do the trick -df)
;;; (what else has atomicity problems? -df)

(define (increment-revealed-count port delta)
  (obtain-port-lock port)
  (let* ((data (fdport-data port))
	 (count (fdport-data:revealed data))
	 (newcount (+ count delta)))
    (set-fdport-data:revealed data newcount)
    (if (and (zero? count) (> newcount 0))          ; We just became revealed,
	(begin
	  (strengthen-weak-table-ref fdports (fdport-data:fd data))
	  (%set-cloexec (fdport-data:fd data) #f)))); so don't close on exec().
  (release-port-lock port))

(define (release-port-handle port)
  (check-arg fdport? port port->fdes)
  (obtain-port-lock port)
  (let* ((data (fdport-data port))
	 (rev (fdport-data:revealed data)))
    (if (not (zero? rev))
;	(set-fdport-data:old-revealed data
;				      (- (fdport-data:old-revealed data) 1))
	(let ((new-rev (- rev 1)))
	  (set-fdport-data:revealed data new-rev)
	  (if (zero? new-rev)			; We just became unrevealed, so
	      (begin                            ; the fd can be closed on exec.
		(weaken-weak-table-ref fdports (fdport-data:fd data))
		(%set-cloexec (fdport-data:fd data) #t))))))
  (release-port-lock port))

(define (port-revealed port)
  (let ((count (fdport-data:revealed
		(fdport-data
		 (check-arg fdport? port port-revealed)))))
    (and (not (zero? count)) count)))

(define (fdes->port fd port-maker) ; local proc.
  (cond  ((maybe-fdes->port fd) =>
	  (lambda (p)
	    (increment-revealed-count p 1)
	    p))
	 (else (port-maker fd 1))))

(define (fdes->inport fd)  (fdes->port fd make-input-fdport))

(define (fdes->outport fd) (fdes->port fd make-output-fdport))

(define (port->fdes port)
  (check-arg open-fdport? port port->fdes)
  (let ((data (fdport-data port)))
    (increment-revealed-count port 1)
    (fdport-data:fd data)))

(define (call/fdes fd/port proc)
  (cond ((integer? fd/port)
	 (proc fd/port))

	((fdport? fd/port)
	 (let ((port fd/port))
	   (dynamic-wind
	    (lambda ()
	      (if (not port) (error "Can't throw back into call/fdes.")))
	    (lambda () (proc (port->fdes port)))
	    (lambda ()
	      (release-port-handle port)
	      (set! port #f)))))

	(else (error "Not a file descriptor or fdport." fd/port))))

;;; Don't mess with the revealed count in the port case
;;; -- just sneakily grab the fdes and run.

(define (sleazy-call/fdes fd/port proc)
  (proc (cond ((integer? fd/port) fd/port)
	      ((fdport? fd/port) (fdport-data:fd (fdport-data fd/port)))
	      (else (error "Not a file descriptor or fdport." fd/port)))))


;;; Random predicates and arg checkers
;;; ----------------------------------

(define (open-fdport-data? x)
  (and (fdport-data? x)
       (not (fdport-data:closed? x))))

(define (open-fdport? x)
  (and (fdport? x) (or (open-output-port? x) (open-input-port? x))))

(define (fdport-open? port)
  (check-arg fdport? port fdport-open?)
  (not (fdport-data:closed? (port-data port))))


;;; Initialise the system
;;; ---------------------

;;; JMG: should be deprecated-proc
(define error-output-port
  current-error-port)


(define old-inport #f)	; Just because.
(define old-outport #f)
(define old-errport #f)

(define (init-fdports!)
  (if (not (fdport? (current-input-port)))
      (set! old-inport (current-input-port)))
  (if (not (fdport? (current-output-port)))
      (set! old-outport (current-output-port)))
  (if (not (fdport? (current-error-port)))
      (set! old-errport (current-error-port)))
  (set-fluid! $current-input-port  
	      (channel-port->input-fdport (current-input-port)))
  (set-fluid! $current-output-port 
	      (channel-port->output-fdport (current-output-port)))
  
  (set-fluid! $current-error-port  
	      (channel-port->unbuffered-output-fdport (current-error-port)))
  (set-fluid! $current-noise-port  (make-null-output-port)))

;;; Generic port operations
;;; -----------------------

;;; (close-after port f)
;;; 	Apply F to PORT. When F returns, close PORT, then return F's result.
;;;     Does nothing special if you throw out or throw in.

(define (close-after port f)
  (receive vals (f port)
    (close port)
    (apply values vals)))

(define (close port/fd)
  ((cond ((integer? port/fd) 	 close-fdes)
	 ((output-port? port/fd) close-output-port)
	 ((input-port?  port/fd) close-input-port)
	 (else (error "Not file-descriptor or port" port/fd)))	port/fd))

;;; If this fd has an associated input or output port,
;;; move it to a new fd, freeing this one up.

(define (evict-ports fd)
  (cond ((maybe-fdes->port fd) =>	; Shouldn't bump the revealed count.
         (lambda (port) 
	   (%move-fdport (%dup fd) port 0)))))

(define (%move-fdport fd port new-revealed)
  (obtain-port-lock port)
  (let* ((fdport* (fdport-data port))
	 (ch (fdport-data:channel fdport*))
	 (old-fd (channel-os-index ch))
	 (old-vector-ref (table-ref fdports old-fd)))
    (set-fdport-data:revealed fdport* new-revealed)
    (table-set! fdports old-fd #f)
    (close-channel ch)
    (set-fdport-data:channel 
     fdport*
     (make-fd-channel port fd))
    (table-set! fdports fd old-vector-ref)
    (%set-cloexec fd (not new-revealed)))
  (release-port-lock port)
  #f)  ; JMG: It used to return #f on succes in 0.5.1, so we do the same

(define (make-fd-channel port fd)
  ((if (input-port? port) make-input-fdchannel make-output-fdchannel) fd))

(define (close-fdes fd)
  (evict-ports fd)
  (%close-fdes fd))

(define (flush-fdport fdport)
  (check-arg fdport? fdport flush-fdport)
  (force-output fdport))

(define (flush-all-ports)
  (weak-table-walk
   (lambda (i fdport)
     (if (and fdport (output-port? fdport)) (flush-fdport fdport)))
   fdports))

;;; Extend R4RS i/o ops to handle file descriptors.
;;; -----------------------------------------------

(define s48-char-ready? (structure-ref scheme char-ready?))
(define s48-read-char   (structure-ref scheme read-char))

(define-simple-syntax
  (define-r4rs-input (name arg ...) stream s48name body ...)
  (define (name arg ... . maybe-i/o)
    (let ((stream (:optional maybe-i/o (current-input-port))))
      (cond ((input-port? stream) (s48name arg ... stream))
	    ((integer? stream) body ...)
	    (else (error "Not a port or file descriptor" stream))))))

(define-r4rs-input (char-ready?) input s48-char-ready?
  (%char-ready-fdes? input))

(define-r4rs-input (read-char) input s48-read-char
  (read-fdes-char input))

;structure refs changed to get reference from scheme -dalbertz
(define s48-display    (structure-ref scheme display))
(define s48-newline    (structure-ref scheme newline))
(define s48-write      (structure-ref scheme write))
(define s48-write-char (structure-ref scheme write-char))
(define s48-format     (structure-ref formats format))
(define s48-force-output (structure-ref i/o force-output))

(define-simple-syntax
  (define-r4rs-output (name arg ...) stream s48name body ...)
  (define (name arg ... . maybe-i/o)
    (let ((stream (:optional maybe-i/o (current-output-port))))
      (cond ((output-port? stream) (s48name arg ... stream))
	    ((integer? stream) body ...)
	    (else (error "Not a port or file descriptor" stream))))))

;;; This one depends upon S48's string ports.
(define-r4rs-output (display object) output s48-display
  (let ((sp (make-string-output-port)))
    (display object sp)
    (write-string (string-output-port-output sp) output)))

(define-r4rs-output (newline) output s48-newline
  (write-fdes-char #\newline output))

(define-r4rs-output (write object) output s48-write
  (let ((sp (make-string-output-port)))
    (write object sp)
    (write-string (string-output-port-output sp) output)))

(define-r4rs-output (write-char char) output s48-write-char
  (write-fdes-char char output))

;;; S48's force-output doesn't default to forcing (current-output-port). 
(define-r4rs-output (force-output) output s48-force-output
  (values)) ; Do nothing if applied to a file descriptor.


(define (format dest cstring . args)
  (if (integer? dest)
      (write-string (apply s48-format #f cstring args) dest)
      (apply s48-format dest cstring args)))

;;; with-current-foo-port procs
;;; ---------------------------

(define (with-current-input-port* port thunk)
  (let-fluid $current-input-port port thunk))

(define (with-current-output-port* port thunk)
  (let-fluid $current-output-port port thunk))

(define (with-current-error-port* port thunk)
  (let-fluid $current-error-port port thunk))

(define-simple-syntax (with-current-input-port port body ...)
  (with-current-input-port* port (lambda () body ...)))

(define-simple-syntax (with-current-output-port port body ...)
  (with-current-output-port* port (lambda () body ...)))

(define-simple-syntax (with-current-error-port port body ...)
  (with-current-error-port* port (lambda () body ...)))


;;; set-foo-port! procs
;;; -------------------
;;; Side-effecting variants of with-current-input-port* and friends.

(define (set-current-input-port!  port) (set-fluid! $current-input-port  port))
(define (set-current-output-port! port) (set-fluid! $current-output-port port))
(define (set-current-error-port!  port) (set-fluid! $current-error-port  port))


;;; call-with-foo-file with-foo-to-file
;;; -----------------------------------

;;; Copied straight from rts/port.scm, but re-defined in this module,
;;; closed over my versions of open-input-file and open-output-file.

(define (call-with-mumble-file open close)
  (lambda (string proc)
    (with-cwd-aligned
     (with-umask-aligned
      (let ((port #f))
	(dynamic-wind (lambda ()
			(if port
			    (warn "throwing back into a call-with-...put-file"
				  string)
			    (set! port (open string))))
		      (lambda () (proc port))
		      (lambda ()
			(if port
			    (close port)))))))))

;;; replace rts/channel-port.scm begin
(define call-with-input-file
  (call-with-mumble-file open-input-file close-input-port))

(define call-with-output-file
  (call-with-mumble-file open-output-file close-output-port))

(define (with-input-from-file string thunk)
  (call-with-input-file string
    (lambda (port)
      (let-fluid $current-input-port port thunk))))

(define (with-output-to-file string thunk)
  (call-with-output-file string
    (lambda (port)
      (let-fluid $current-output-port port thunk))))

;;; replace rts/channel-port.scm end



(define (nselect rvec wvec evec timeout)
  (let ((rlist (vector->list rvec))
	(wlist (vector->list wvec)))
    (let ((imm-r (filter char-ready? rlist))
	  (imm-w (filter output-port-ready? wlist)))
      (if (and (null? imm-r)
	       (null? imm-w))
	  (select-threaded rlist wlist timeout)
	  (values (list->vector imm-r)
		  (list->vector imm-w)
		  '#())))))

(define (timeout-thread result-lock timeout)
  (lambda ()
    ((structure-ref threads sleep) timeout)
    (release-lock result-lock)))

(define (select-threaded rlist wlist timeout)
  (let ((result-lock (make-lock))
	(ready-lock (make-lock))
	(read-ready (cons 'cell '()))
	(write-ready (cons 'cell '()))
	(are-we-ready? #f))
    (let* ((port-waiter
	    (lambda (ready? ready-list)
	      (lambda (port)
		(lambda ()
;		  ((structure-ref interrupts disable-interrupts!))
;		  (if (ready? port)
;		      ((structure-ref interrupts enable-interrupts!))
;		      (wait-for-channel ; enables interrupts
;		       (fdport-data:channel 
;			(fdport-data port))))
		  (let lp ()
		    (if (ready? port)
			(begin 
			  (obtain-lock ready-lock)
			  (set-cdr! ready-list (cons port (cdr ready-list)))
			  (release-lock ready-lock)
			  (release-lock result-lock))
			(if (not are-we-ready?)
			    (begin ((structure-ref threads sleep) 20)
				   (lp)))))))))
	   (read-waiter (port-waiter char-ready? read-ready))
	   (write-waiter (port-waiter output-port-ready? write-ready)))
      (obtain-lock result-lock)
      (for-each spawn (map read-waiter rlist))
      (for-each spawn (map write-waiter wlist))
      (if timeout (spawn (timeout-thread result-lock timeout)))
      (obtain-lock result-lock)
      (set! are-we-ready? #t)
;      (relinquish-timeslice) 
      (values (cdr read-ready)
	      (cdr write-ready)
	      '#()))))