Scieneer Common Lisp 1.3.9 online documentation

ext:stream-read-byte-no-hang stream[Generic Function]

Returns either a byte from the stream, nil if no input is currently available, or :eof if the stream is at the end-of-file. The default method calls ext:stream-read-byte which should be fine for a file stream, however unbuffered binary interactive and socket streams will probably want to defined a method for this function.