21.2.34 listen [Function]

listen &optional input-stream ⇒ generalized-boolean

Arguments and Values::

input-stream—an input stream designator. The default is standard input.

generalized-boolean—a generalized boolean.

Description::

Returns true if there is a character immediately available from input-stream; otherwise, returns false. On a non-interactive input-stream, listen returns true except when at end of file_1. If an end of file is encountered, listen returns false. listen is intended to be used when input-stream obtains characters from an interactive device such as a keyboard.

Examples::

 (progn (unread-char (read-char)) (list (listen) (read-char)))
 |>  |>>1<<|
⇒  (T #\1)
 (progn (clear-input) (listen))
⇒  NIL ;Unless you're a very fast typist!

Affected By::

*standard-input*

See Also::

interactive-stream-p , read-char-no-hang