erlang: to controlling_process(), or not to controlling_process()
erlang, gen-tcp, sockets
Solution
Erlang documentation says about gen_tcp:controlling_process/1:
Assigns a new controlling process Pid to Socket. The controlling process is the process which receives messages from the socket. If called by any other process than the current controlling process, {error, not_owner} is returned.
You created listen socket with option {active, false} and you read the socket synchronously with gen_tcp:recv/2 so your code will work even without calling of gen_tcp:controlling_process/1. However if you want to receive data asynchronously you must create listen socket with option {active, true}. In that case owner of accepted connection will receive messages about incoming data. So if you dont call gen_tcp:controlling_process/1 these messages will be sent to listener process instead of worker.
Problem
Consider the following erlang code of a simple echo server: Echo listener: ``` -module(echo_listener). -export([start_link/1]). -define(TCP_OPTIONS, [binary, {packet, 0}, {reuseaddr, true}, {keepalive, true}, {backlog, 30}, {active, false}]). start_link(ListenPort) -> {ok, ListenSocket} = gen_tcp:listen(ListenPort, ?TCP_OPTIONS), accept_loop(ListenSocket). accept_loop(ListenSocket) -> {ok, ClientSocket} = gen_tcp:accept(ListenSocket), Pid = spawn(echo_worker, usher, [ClientSocket]), gen_tcp:controlling_process(ClientSocket, Pid), accept_loop(ListenSocket). ``` Echo worker: ``` -module(echo_worker). -export([echo/1]). echo(ClientSocket) -> case gen_tcp:recv(ClientSocket, 0) of {ok, Data} -> gen_tcp:send(ClientSocket, Data), echo(ClientSocket); {error, closed} -> ok end. ``` Whenever a client socket is accepted, the echo server spawns a echo worker and pass the client socket directly as a function parameter. In the code there is controller_process(), but I have tried the code without calling controlling_process(), it also works. What is the real purpose of controlling_process()? When is it needed? Thanks in advance.