[erlang-questions] Erlang udp server can't receive accept packets

[Sergej Jurečko]

Server can't receive the first packet if it is set to {active,false}. Change gen_udp:open param to {active,once}

Sergej
> On 15 May 2017, at 11:27, yuzhu chen <billcyz@REDACTED> wrote:
> 
> I've posted the same question on stackoverflow, but haven't got answers yet. So I posted this question again in here hoping for a solution.
> 
> 
> 
> down vote
>  <>favorite
>  <http://stackoverflow.com/questions/43972714/erlang-udp-server-cant-receive-accept-packets#>	
> I have one simple udp server written in gen_server behaviour. When I run it, and try to send message by using gen_udp:send, the server replies nothing, seems like the udp server didn't accept packet successfully. Here is my code
> 
> gen_udp_server.erl:
> 
> -module(gen_udp_server).
> -behaviour(gen_server).
> -export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
> 
> -export([start_link/1]).
> 
> -define(SERVER, ?MODULE).
> 
> -record(state, {socket,
>                 port,
>                 local_ip,
>                 broad_ip}).
> 
> start_link(Port) ->
>     {ok, Socket} = gen_udp:open(Port, [binary, 
>                                        {active, false},
>                                        {reuseaddr, true}]),
>     gen_server:start_link(?MODULE, [Socket, Port], []).
> 
> init([Socket, Port]) ->
>     {ok, #state{socket = Socket, port = Port}}.
> 
> handle_cast(_Request, State) ->
>     {noreply, State}.
> 
> handle_call(_Request, _From, State) ->
>     {noreply, State}.
> 
> handle_info({udp, _Socket, _Addr, _Port, Data}, #state{socket = Socket} = State) ->
>     inet:setopts(Socket, [{active, once}]),
>     io:format("Server received data ~p from socket ~p~n", [Data, Socket]),
>     {ok, State}.
> 
> terminate(_Reason, {socket = LSocket}) ->
>     gen_udp:close(LSocket).
> 
> code_change(_OldVsn, State, _Extra) ->
>     {ok, State}.
> Starting server on server 192.168.146.129 <http://192.168.146.129/>: gen_udp_server:start_link(10000).
> 
> Sending message from 192.168.146.128 <http://192.168.146.128/>:
> 
> {ok, Socket} = gen_udp:open(4399, [binary, {active, false}]).
> gen_udp:send(Socket, {192,168,146,129}, 10000, "hello").
> The udp server should print some message when it receives packets, but my one failed. Can anyone help me?
> 
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