recv message and write to file - erlang

Question

I have made a server here which receives messages from the terminal. What I want to do is when the server receives the messages from the terminal it should write it into a file. But I dont know how I should do this. I thought of writing the file methods file:open, file:write in the receive functions but it didnt work out as I thought.
-module(tcp).
-behaviour(gen_server).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3, start/0, stop/0, connect/0, send/1, recv/0]).
-export([start_link/0]).
%% =============================================================================
%% EXPORTED GEN_SERVER CALLBACKS
%% =============================================================================
init([]) -> {ok, {}}.
handle_call({send, Packet}, _From, State) ->
gen_tcp:send(State, Packet),
io:format("Send Working\n"),
{reply, ok, State};
handle_call(recv, _From, State) ->
Message = gen_tcp:recv(State, 0),
io:format("~w~n", [Message]),
{reply, Message, State}.
handle_cast(connect, _) ->
case gen_tcp:listen(6888, [binary]) of
{ok, LSocket}->
io:format("~w~n", [LSocket]),
case gen_tcp:accept(LSocket) of
{ok, Socket} ->
inet:setopts(Socket, [{active, false}]),
io:format("accepted\n"),
{noreply, Socket};
Other ->
error_logger:error_report(["An error occurred which is",Other,"in line",?LINE,"of module",?MODULE])
end;
{error, Reason} ->
error_logger:error_report("An error occurred", Reason, [?LINE,?MODULE])
end;
handle_cast(stop, State) -> {stop, normal, State}.
handle_info(_Info, State) -> {noreply, State}.
terminate(_Reason, _State) -> ok.
code_change(_OldVsn, State, _Extra) -> {ok, State}.
%% =============================================================================
%% EXPORTED CONVENIENCE FUNCTIONS TO START AND STOP THE SERVER
%% =============================================================================
start() ->
case gen_server:start({local, ?MODULE}, ?MODULE, [], []) of
{ok, Pid} ->
Pid;
Reason ->
error_logger:error_report("An error occurred", Reason, [?LINE,?MODULE])
end.
stop() ->
case gen_server:cast(?MODULE, stop) of
ok ->
ok;
_ ->
{error, stop_error}
end.
%% =============================================================================
%% PUBLIC API FUNCTIONS
%% =============================================================================
connect() -> gen_server:cast(?MODULE, connect).
send(Packet) -> gen_server:call(?MODULE, {send, Packet}).
recv() -> gen_server:call(?MODULE, recv).
write_file(Filename) ->
{ok, IoDevice} = file:open(test.txt, [write, append]),
ok.
start_link() -> gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
%% =============================================================================
%% LOCAL FUNCTIONS
%% =============================================================================
I did as you suggested and implemented it as a handle_call but when i run i get an error,
function handle_call/3 already defined error
I quite dont understand why I get this error since i have 3 arguments for the handle_call and it should work.
handle_call(write_file, Filename, S) ->
{ok, File} = file:open(Filename, [append]),
ok = file:write(File, S),
ok = file:close(File).
My API Function
write_file() -> gen_server:call(?MODULE, write_file).

Answer 1

Your handle call doesn't work because you have two definitions of it. You have written:
handle_call(...) ->
do_something.
handle_call(M, F, S) ->
do_some_other_thing.
You should make them into the same function, by changing:
do_something.
into:
do_something;
handle_call(M, F, S) ->
do_some_other_thing.

Answer 2

write_file should do something like:
write_file(Fname, S) ->
ok = file:write_file(Fname, S, [append]).
or
write_file(Fname, S) ->
{ok, File} = file:open(Fname, [append]),
ok = file:write(File, S),
ok = file:close(File).