A character input stream can be created by defining a class that includes fundamental-character-input-stream and defining methods for the following generic functions.
stream-read-char [Generic Function]
Summary: Reads one character from stream , and returns either a character object or the symbol :eof if the stream is at end-of-file. There is no default method for this generic function, so every subclass of fundamental-character-input-stream must define a method.
stream-unread-char [Generic Function]
Summary: Undoes the last call to stream-read-char , as in unread-char , and returns nil . There is no default method for this, so every subclass of fundamental- character-input-stream must define a method.
stream-read-char-no-hang [Generic Function]
Summary: Returns either a character, or nil if no input is currently available, or :eof if end-of-file is reached. This is used to implement read-char-no-hang . The default method provided by fundamental-character-input-stream simply calls stream-read-char ; this is sufficient for file streams, but interactive streams should define their own method.
stream-peek-char [Generic Function]
Summary: Returns either a character or :eof without removing the character from the stream's input buffer. This is used to implement peek-char ; this corresponds to peek-type of nil . The default method calls stream-read-char and stream-unread-char.
stream-listen [Generic Function]
Summary: Returns t if there is any input pending on stream ; otherwise, it returns nil . This is used by listen . The default method uses stream-read-char-no-hang and stream-unread-char . Most streams should define their own method, as it will usually be trivial and will generally be more efficient than the default method.
stream-read-line [Generic Function]
Summary: Returns a string as the first value, and t as the second value if the string was terminated by end-of-file instead of the end of a line. This is used by read-line . The default method uses repeated calls to stream-read-char .
stream-clear-input [Generic Function]
Summary: Clears any buffered input associated with stream , and returns nil . This is used to implement clear-input . The default method does nothing.
Common Lisp Interface Manager 2.0 User's Guide - 20 Sep 2011