handle_info() 处理到达gen_server 邮箱的未知消息（即未处理的消息）。但是当进程以被动模式打开套接字时：{active, false}，发送到套接字的消息不会到达进程的邮箱。相反，该进程必须通过调用gen_udp:recv(Socket, Length) 从套接字手动读取消息。毕竟，创建被动套接字的全部目的是防止消息淹没进程的邮箱。因此，当客户端向被动套接字发送消息时，handle_info() 不会被调用。
此外，由于gen_server 是事件驱动的，您需要调用gen_udp:recv(Socket, Length) 来响应某些事件。例如，您可以定义服务器函数：
process_message() ->
gen_server:cast(?MODULE, process_msg).
handle_cast(process_msg, #state{socket=Socket} = State) ->
Data = gen_udp:recv(Socket, 0),
io:format("Server received data ~p from socket ~p~n", [Data, Socket]),
{noreply, State}.
那么你需要有人定期打电话给process_message()。以下似乎有效：
start() ->
io:format("start~n"),
{ok, Server} = gen_server:start_link({local,?MODULE}, ?MODULE, [], []),
Poller = spawn(?MODULE, poll, []), %%<***** HERE *****
io:format("Server: ~w~nPoller: ~w~n", [Server,Poller]).
...
...
handle_cast(process_msg, #state{socket=Socket} = State) ->
case gen_udp:recv(Socket, 10000, 500) of
{error, timeout} -> %%Timeout message.
ok;
{error, Error} ->
io:format("Error: ~p~n", [Error]);
Data ->
io:format("Server received data ~p from socket ~p~n", [Data, Socket])
end,
{noreply, State}.
poll() ->
timer:sleep(1000),
process_message(),
poll().
至于recv() 中的Length，我不确定您应该指定什么：我尝试了 0、2 和 10,000，但我看不出有什么区别。
这是我的客户：
client() ->
Port = 15000,
{ok, Socket} = gen_udp:open(0, [binary, {active, false}]),
gen_udp:send(Socket, "localhost", Port, "hello").
请注意，open(0, ....) 指示 erlang 打开任何空闲端口（如果在同一台计算机上运行，客户端和服务器无法打开相同的端口——这与您需要 gen_tcp 套接字相反）。但是，gen_udp:send() 必须指定服务器打开的相同端口。此外，原子localhost 和列表"localhost" 都对我有用。
完整的服务器代码：
-module(s2).
-behaviour(gen_server).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
terminate/2, code_change/3]).
-export([start/0, process_message/0, poll/0]).
-record(state, {socket,
port,
local_ip,
broad_ip}).
%%======== PASSIVE SOCKET: {active,false} ===========
%% External interface:
start() ->
io:format("start~n"),
{ok, Server} = gen_server:start_link({local,?MODULE}, ?MODULE, [], []),
Poller = spawn(?MODULE, poll, []),
io:format("Server: ~w~nPoller: ~w~n", [Server,Poller]).
process_message() ->
gen_server:cast(?MODULE, process_msg).
poll() ->
timer:sleep(1000),
process_message(),
poll().
%%Internal server methods:
init([]) ->
Port = 15000,
{ok, Socket} = gen_udp:open(Port, [binary,
{active, false},
{reuseaddr, true}]),
{ok, #state{socket = Socket, port = Port}}.
handle_cast(process_msg, #state{socket=Socket} = State) ->
case gen_udp:recv(Socket, 10000, 500) of
{error, timeout} -> %%Timeout message.
ok;
{error, Error} ->
io:format("Error: ~p~n", [Error]);
Data ->
io:format("Server received data ~p from socket ~p~n", [Data, Socket])
end,
{noreply, State}.
handle_call(_Request, _From, State) ->
{noreply, State}.
handle_info(Msg, State) ->
io:format("Msg: ~w, State:~w~n", [Msg, State]),
{noreply, State}.
terminate(_Reason, #state{socket = LSocket}) ->
gen_udp:close(LSocket).
code_change(_OldVsn, State, _Extra) ->
{ok, State}.
在外壳中：
外壳#1---
$ erl
Erlang/OTP 19 [erts-8.2] [source] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V8.2 (abort with ^G)
1> c(s2).
{ok,s2}
2> s2:start().
start
Server: <0.64.0>
Poller: <0.65.0>
ok
壳#2--
$ erl
Erlang/OTP 19 [erts-8.2] [source] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V8.2 (abort with ^G)
1> c(c2).
{ok,c2}
2> c2:client().
ok
外壳#1--
Server received data {ok,{{127,0,0,1},61841,<<"hello">>}} from socket #Port<0.2110>
3>
壳#2--
3> c2:client().
ok
4>
外壳#1--
Server received data {ok,{{127,0,0,1},63983,<<"hello">>}} from socket #Port<0.2110>
3>