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

When input-stream is an echo stream, no attempt is made to undo any echoing of the character that might already have been done on input-stream. However, characters placed on input-stream by unread-char are marked in such a way as to inhibit later re-echo by read-char.

It is an error to invoke unread-char twice consecutively on the same stream without an intervening call to read-char (or some other input operation which implicitly reads characters) on that stream.

Invoking peek-char or read-char commits all previous characters. The consequences of invoking unread-char on any character preceding that which is returned by peek-char (including those passed over by peek-char that has a non-nil peek-type) are unspecified. In particular, the consequences of invoking unread-char after peek-char are unspecified.

 (with-input-from-string (is "0123")
    (dotimes (i 6)
      (let ((c (read-char is)))
        (if (evenp i) (format t "~&~S ~S~%" i c) (unread-char c is)))))
0 #\0
2 #\1
4 #\2
  NIL