21.2.18 read-char-no-hang [Function]

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 file_2 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[<–~]<<|
⇒  (#\a #\Newline NIL)

Affected By::

*standard-input*, *terminal-io*.

Exceptional Situations::

If an end of file_2 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.