How to receive a message in genserver?

Questions / Help
1

I have a gen server that receives a message On a TCP connection. To demonstrate what I want I have an erlang process running where I’m listening on a port and receive a message from the other erlang process.

Elrang part receiver
2> {ok, LSocket} = gen_tcp:listen(1200, [binary, {active, true}]).
{ok,#Port<0.5>}
3> {ok, ASocket} = gen_tcp:accept(LSocket)
3> .
{ok,#Port<0.6>}
4> flush().
Shell got {tcp,#Port<0.6>,<<"Heleleeeeee">>}
ok

I want to wrap this in a gen server process. So I have written a code something like this

  def start(), do: GenServer.start_link(__MODULE__, [], name: __MODULE__)

  def get(), do: GenServer.cast(__MODULE__, {:get})

  #callbacks

  def init([]) do
    {:ok, socket} = GenTCP.listen(@port, @tcp_options)
    :ok = Inet.setopts(socket, active: :once)
    {:ok, %State{socket: socket}}
  end

  def handle_cast({:get}, %State{socket: socket} = state) do
    {:ok, conn} = GenTCP.accept(socket)
    # case GenTCP.recv(conn, 0) do
    #   {}
    # end
    {:noreply, state}
  end

  def gen_tcp_accept(socket) do
    GenTCP.accept(socket)
  end

how can I do something similar here and receive a message and use the message to respond back to what kind of msg I have got?

2

Arbitrary messages are delivered to genservers using the handle_info callback: GenServer — Elixir v1.13.4

3

This sounds like you handle a call not a cast. You can return {:noreply, state} from handle_call and respond at a later time using GenServer.reply if you keep arount the from (second parameter) received in handle_call.

4

can we inspect the message that we’ve received in handle info?

suppose if I want to pattern match on this Shell got {tcp,#Port<0.6>,<<"Heleleeeeee">>} and extract the message from it.

5

Yes, just like in handle_cast and handle_call. handle_info receives the message as the first argument and the state as the second.

6

I added this

  def handle_info(msg, %State{socket: socket} = state) do
    Logger.info(msg, "sdasds")
    {:noreply, state}
  end

But I’m not getting any msg. I tried Logger also

7

msg is not necessarily a string (rather unlikely I’d even say) so are your sure it’s not just crashing?

8

msg will be this
{tcp,#Port<0.6>,<<“Heleleeeeee”>>}

so it should be something like this

 def handle_info({:tcp, socket, data}, %State{socket: socket} = state) do
   IO.inspect(data)
    {:noreply, state}
  end

But inspect is not returning any value. It does not reach handle_info

9

I’d use active: false for a simple gen_tcp server and ranch for the more complex cases.