[erlang-questions] variable unbound

[Gamoto]

What is wrong in this function ? I receive the following error: variable 'S' is unbound !
I also receive two warnings: File is unused, S is unused.
This process can receive two messages: one file to open and one message to write

Pid ! {open,"filetoopen.txt"}

Pid ! {data, Message}

log_message()->
    receive
		{open,File} -> case file:open("File", [append]) of
							{ok,S} ->  log_message();
		                    {error,Reason} -> io:format( "~s~n",[Reason])
		       	            end;
		{data,Data} ->  io:format(S,"~s~n",[Data]),
	    					  log_message()
   end.