Binary streams can be created by defining a class that includes either fundamental-binary-input-stream or fundamental-binary-output-stream (or both) and defining a method for stream-element-type and for one or both of the following generic functions.
stream-read-byte [Generic Function]
Summary: Returns either an integer or the symbol :eof if stream is at end-of-file. This is used by read-byte .
stream-write-byte [Generic Function]
Arguments: stream integer
Summary: Writes integer to stream and returns integer as the result. This is used by write-byte .
Common Lisp Interface Manager 2.0 User's Guide - 3 Mar 2015