From: David J. Steele
Subject: Use of LISTEN or READ-CHAR-NO-HANG for Testing Keyboard Input
Date: Tue, 06 Oct 1998 00:00:00 +0000
Message-ID: <6vcdn5$qck$1@mawar.singnet.com.sg>

Wonder if somebody can help me with the following problem:
I am working on a project for man-machine conversational
dialogue, and would like to be able to test for keyboard input
from the user such that, if the user has not responded within
x seconds, the machine would issue another prompt. It seems
to me that one ought to be able to use either LISTEN or READ-
CHAR-NO-HANG to test whether a character has been input
to the keyboard as *st andard-input*. I tried a test program
along the following lines:

(defun test-listen (&aux init sum next char input)                 ;Notes
  (setf init (car (multiple-value-list (get-decoded-time)))           ;01
        sum 0)
  (loop
    (setf next (car (multiple-value-list (get-decoded-time))))      ;02
    (cond
      ((setf char
         (read-char-no-hang *standard-input* nil nil))
;03
         (format t
;04
 "~%Character ~a read from console after ~a seconds" char sum)
         (unread-char char)
;05
         (setf input (read-line))
;06
         (format t
;07
 "~%Input = ~a" input)
         (return))
;08
      (t (when (not (eq next init))
;09
           (setf sum (1+ sum)
                 init next)
           (format t
 "~%Another second elapsed; sum = ~a" sum))))))                ;10

;Notes
;01  Initialise start of time measurement
;02  Get current time
;03  Is character available from the console?
;04  If so, print message as to time elapsed
;05  Push character back onto the front of the input stream
;06  Read input typed into the keyboard
;07  Confirm input read by (read-line)
;08  Return from loop and exit test function
;09  Otherwise, increment time at next even second elapsed
;10  Print message as to total time elapsed

but when I run it (or a similar version structured for LISTEN) keyboard
input is completely ignored and the program just keeps counting the
seconds. What am I doing wrong? I am using ACL Version 3.0.1.
Regards,
Dave Steele

From: Barry Margolin
Subject: Re: Use of LISTEN or READ-CHAR-NO-HANG for Testing Keyboard Input
Date: Tue, 06 Oct 1998 00:00:00 +0000
Message-ID: <8mrS1.37$8A3.788625@burlma1-snr1.gtei.net>

In article <············@mawar.singnet.com.sg>,
David J. Steele <········@singnet.com.sg> wrote:
>but when I run it (or a similar version structured for LISTEN) keyboard
>input is completely ignored and the program just keeps counting the
>seconds. What am I doing wrong? I am using ACL Version 3.0.1.

If this is on Unix, the terminal driver is normally in line-at-a-time
mode.  Unless you've used a system-specific function to change to character
mode, neither LISTEN nor READ-CHAR-NO-HANG will detect characters available
until you hit RETURN, since the tty driver doesn't make them available to
the application until then.

-- 
Barry Margolin, ······@bbnplanet.com
GTE Internetworking, Powered by BBN, Burlington, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.