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| ANSI Common Lisp 21 Streams 21.2 Dictionary of Streams
21.2.18 read-char-no-hang | Function
  Syntax: read-char-no-hang `&optional` input-stream eof-error-p eof-value recursive-p char Arguments and Values: input-stream -- an input stream designator. The default is standard input. eof-error-p - a generalized boolean. The default is true. eof-value - an object. The default is nil. recursive-p - a generalized boolean. The default is false. char - a character or nil or the eof-value. Description: read-char-no-hang returns a character from input-stream if such a character is available. If no character is available, read-char-no-hang returns nil. If recursive-p is true, this call is expected to be embedded in a higher-level call to read or a similar function used by the Lisp reader. If an end of file2 occurs and eof-error-p is false, eof-value is returned. Examples:
      ;; This code assumes an implementation in which a newline is not
      ;; required to terminate input from the console.
       (defun test-it ()
         (unread-char (read-char))
         (list (read-char-no-hang)
               (read-char-no-hang)
               (read-char-no-hang)))
        TEST-IT
      ;; Implementation A, where a Newline is not required to terminate
      ;; interactive input on the console.
       (test-it)
      a
       (#\a NIL NIL)
      ;; Implementation B, where a Newline is required to terminate
      ;; interactive input on the console, and where that Newline remains
      ;; on the input stream.
       (test-it)
      a[Return]
       (#\a #\Newline NIL)
Affected By: *standard-input*, *terminal-io*. Exceptional Situations: If an end of file2 occurs when eof-error-p is true, an error of type end-of-file is signaled . See Also: listen Notes: read-char-no-hang is exactly like read-char, except that if it would be necessary to wait in order to get a character (as from a keyboard), nil is immediately returned without waiting. Allegro CL Implementation Details: None.

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