receive specific amount of data from gen_tcp socket

Question

I have the following code using a gen_tcp socket, which should receive using {active, true} some binary commands that are always of size 5 (e.g. Command = <<"hello">>)

{ok, ListenSocket} = gen_tcp:listen(5555,  [{active, true}, {mode, binary}, 
                                            {packet, 0}]),
{ok, Socket} = gen_tcp:accept(ListenSocket),    
loop().

receive() ->
    {tcp, _Socket, Message:4/binary} ->
         io:format("Success~n", []),
         loop();
    {tcp, _Socket, Other} ->
         io:format("Received: ~p~n", [Other]),
         loop()
end.

But if I try to connect as a client like this:

1> {ok, S} = gen_tcp:connect("localhost", 5555, []).
   {ok,#Port<0.582>}
2> gen_tcp:send(S, <<"hello">>).
ok

I have in the receiving part:

Received: {tcp,#Port<0.585>,<<"hello">>}

so I suppose my error is in the pattern matching...but where?

Answer 1

Your receive clause doesn't look right, it should be:

{tcp, _Socket, <<Message:5/binary>>} ->
  ....

Answer 2

As far as I understood you need gen_tcp:recv function. From documentation:

recv(Socket, Length) -> {ok, Packet} | {error, Reason}

In your code it will be something like:

...
{ok, Socket} = gen_tcp:accept(ListenSocket),
loop(Socket).

loop(Socket) -> 
    case gen_tcp:recv(Socket, 5) of
        {ok, Data} ->
            io:format("Success~n", []),
            loop(Socket);
        {error, closed} -> 
            ok
end.