The generic function
read-char-no-hang. It returns either a character read from the stream, or
:eof if end-of-file is reached, or
nil if no input is available. The default method provided by
fundamental-character-input-stream simply calls
stream-read-char which is sufficient for file streams, but interactive streams should define their own method.
LispWorks User Guide and Reference Manual - 20 Sep 2017