unread-char character &optional input-stream nil

    Arguments and Values

    character — a character; must be the last character that was read from input-stream.

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


    unread-char places character back onto the front of input-stream so that it will again be the next character in input-stream.

    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 
    Affected By

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

    See Also

    peek-char, read-char, Section 21.1 (Stream Concepts)


    unread-char is intended to be an efficient mechanism for allowing the Lisp reader and other parsers to perform one-character lookahead in input-stream.