Erlang 中gen_tcp 用于编写TCP程序，gen_udp用于编写UDP程序。一个简单的TCP服务器echo示例：
复制代码 代码如下:
Start_echo_server()->
         {ok,Listen}= gen_tcp:listen(1234,[binary,{packet,4},{reuseaddr,true},{active,true}]),
         {ok,socket}=get_tcp:accept(Listen),
         gen_tcp:close(Listen),
         loop(Socket).
 
loop(Socket) ->
         receive
                  {tcp,Socket,Bin} ->
                            io:format(“serverreceived binary = ~p~n”,[Bin])
                            Str= binary_to_term(Bin),
                            io:format(“server  (unpacked) ~p~n”,[Str]),
                            Reply= lib_misc:string2value(Str),
                            io:format(“serverreplying = ~p~n”,[Reply]),
                            gen_tcp:send(Socket,term_to_binary(Reply)),
                            loop(Socket);
                   {tcp_closed,Socket} ->
                            Io:format(“ServerSocket closed ~n”)
         end.

Tcp 的echo客户端示例：
复制代码 代码如下:
echo_client_eval(Str) ->
         {Ok,Socket} = gen_tcp:connect(“localhost”,2345,[binary,{packet,4}]),
         ok= gen_tcp:send(Socket, term_to_binary(Str)),
         receive
                   {tcp,Socket,Bin}->
                            Io:format(“Clientreceived binary  = ~p~n”,[Bin]),
                            Val=binary_to_term(Bin),
                            io:format(“Clientresult = ~p~n”,[Val]),
                            gen_tcp:close(Socket)
         end.

UDP server示例
复制代码 代码如下:
udp_demo_server(Port) ->
         {ok,Socket}= gen_udp:open(Open,[Binary]),
         loop(Socket).
Loop(Socket)->
         receive
                   {udp,Socket,Host,Port,Bin}->
                            BinReply= …,
                            gen_udp:send(Socket,Host,Port,BinReply),
                            loop(Socket)
         End.

UDP client 示例：
复制代码 代码如下:
udp_demo_client(Request) ->
         {ok,Socket}= gen_udp:open(0,[Binary]),
         ok= gen_udp:send(Socket,”localhost”,1234,Request),
         Value=    receive
                                     {udp,Socket,_,_,Bin}-> {ok,Bin}
                            after2000 -> error
                            end,
         gen_udp:close(Socket),
         Value

注意，因为UDP是不可靠的，一定要设一个超时时间，而且Reqeust最好小于500字节。
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