CODEWITHSUNDEEP
tcpelixir
Kshitij Mittal
Kshitij Mittal

Reputation: 2776

TCP Listener on Elixir using Erlang's gen_tcp module

I am using the following code to create a TCP listener on elixir :

defmodule KVServer do
use Application

@doc false
def start(_type, _args) do
  import Supervisor.Spec

  children = [
    supervisor(Task.Supervisor, [[name: KVServer.TaskSupervisor]]),
    worker(Task, [KVServer, :accept, [4040]])
  ]

  opts = [strategy: :one_for_one, name: KVServer.Supervisor]
  Supervisor.start_link(children, opts)
end

@doc """
  Starts accepting connections on the given `port`.
"""
def accept(port) do
  {:ok, socket} = :gen_tcp.listen(port,
                  [:binary, packet: :line, active: false, reuseaddr:     true])
  IO.puts "Accepting connections on port #{port}"
  loop_acceptor(socket)
end

defp loop_acceptor(socket) do
  {:ok, client} = :gen_tcp.accept(socket)
  {:ok, pid} = Task.Supervisor.start_child(KVServer.TaskSupervisor, fn -> serve(client) end)
  :ok = :gen_tcp.controlling_process(client, pid)
  loop_acceptor(socket)
end

defp serve(socket) do
  socket
  |> read_line()
  |> write_line(socket)

  serve(socket)
end

defp read_line(socket) do
  {:ok, data} = :gen_tcp.recv(socket, 0)
  data
end

defp write_line(line, socket) do
  :gen_tcp.send(socket, line)
end
end

It is taken from the following link: http://elixir-lang.org/getting-started/mix-otp/task-and-gen-tcp.html

When I try to get data from my gps deveice (for which I am writing this piece of code) using :gen_tcp.recv(socket,0), I get an error: {:error, reason} = :gen_tcp.recv(socket, 0) and the reason it showed is just "closed".

However, the device is sending data, which I confirmed using a tcp packet sniffer (tcpflow).

Also, when I try to send data using telnet as described in the tutorial above, it works fine.

Any help would be highly appreciated.

Upvotes: 4

Views: 1703

Answers (1)

Kshitij Mittal
Kshitij Mittal

Reputation: 2776

I was finally able to figure it out. Actually the device was sending raw stream of data not lines of data. So I had to change "packet: :line" parameter in :gen_tcp.listen function to "packet: :raw".

It was working on telnet because telnet sends lines of data (with line breaks).

Upvotes: 8

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