Why does the program exit when I try to take input?

def handle_client(client) do
        msg = IO.gets("> ") |> String.trim
        :gen_tcp.send(client, buffering(msg))
        :gen_tcp.send(client, msg)

When I replace the function that takes input(I tried IO.gets/1 and IO.read/2) with a normal string, the message is sent normally.

Here is the function that listens and accepts connections(irrelevant, I think ?)

def listen_and_accept(ip_addr) do
        case :gen_tcp.listen(8000, [:binary, active: false, reuseaddr: true, ip: ip_addr]) do
            {:ok, server} ->
                IO.puts "Listening..."
                case :gen_tcp.accept(server) do
                    {:ok, client} ->
                        IO.puts "Client accepted"
                        spawn fn -> handle_client(client) end
                    {:error, reason} ->
                        IO.puts "Error in :gen_server.accept: #{reason}"
            {:error, reason} -> IO.puts "Error in :gen_tcp.listen: #{reason}"

Thanks in advance guys.

IO.gets/2 takes a device and a prompt, default to stdio, which is a shortcut to Process.group_leader/0 according to the docs … the spawn changes the process group leader and so it probably just sits there forever waiting for I/O from a process that never comes.

If you wish to send user input from the console to the connection, you’re going to probably want to send msgs to the connection handlers from a process accepting input from a console.

1 Like

I am pretty new to elixir, so please let me know if something is wrong.

So when a function is spawned, its IO will channelled through the parent process, as a result it will not be able to take input, so I should take input from a process that is outside of this family and then pass the input to the wanted process, right ?

And thanks for replying.

That’s about right, yes. You’ll want to take input from console, and then use message passing to get it into your client handler process.

However, that is a bit of an odd pattern in your code. Essentially, what it is doing is waiting for a client connection, then spawning an asynchronous process to handle that connection while the next connections may come in. So you can easily have N of these handlers running concurrently. Which one gets which user input?

If you really want to handle one connection at a time with user interaction, then don’t spawn the client connection handler (avoiding creating a separate process) and just handle it as you have now all in the main process (which presumably you are running from the REPL?).

But yes, that’s why what you have isn’t working as you expected, though the solution may involve changing how your program works a bit to actually fit your goals :slight_smile:

Welcome to Elixir, btw… it’s certain to be a fun journey!


Thanks for your help man, and yeah elixir is fun indeed.