unread-char

(unread-char character &optional stream)

Puts character -- which must be the one just read -- back so the next read returns it again, and answers nil. On a Gray stream instance it dispatches to rontolisp:stream-unread-char, whose default method parks the character in the protocol's one-slot pushback; a class that can rewind its own source defines that generic instead. On a stream HANDLE -- a file, a string input stream, a socket -- the character goes into a handle-side pushback of its own, which read-char, peek-char, read-char-no-hang and read-line drain.

One character for one stream is all either cell holds, which is what CL promises: a second unread-char with the cell still full signals. read-byte, read-sequence and read do not consult the handle-side cell.