```erlang server() ->     LSock = listen(),     {ok, Sock} = gen_tcp:accept(LSock),     {ok, Bin} = do_recv(Sock, []),     ok = gen_tcp:close(Sock),     Bin. listen() ->     {ok, LSock} = gen_tcp:listen(5678, [binary, {packet, 4},                                         {active, false}]),     LSock. client() ->     SomeHostInNet = "localhost",     {ok, Sock} = gen_tcp:connect(SomeHostInNet, 5678,                                 [binary, {packet, 0}]),     ok = gen_tcp:send(Sock, "Some Data"),     ok = gen_tcp:close(Sock). do_recv(Sock, Bs) ->     case gen_tcp:recv(Sock, 0) of         {ok, B} ->             do_recv(Sock, [Bs, B]);         {error, closed} ->             {ok, list_to_binary(Bs)}     end. gen_tcp:listen中使用的options [binary, {packet, 4}, {active, true}, {reuseaddr,true}, {delay_send,true}] %% {active,true} 创建一个主动套字节(非阻塞) %% {active,false} 创建一个被动套字节(阻塞) %% {active,once} 创建一个主动套字节仅接收一条消息,如想接收下一条必须再次激活(半阻塞) %%{packet, 4} 表示每一个消息都是从一个4字节长的头部开始 意味着每个包有4个字节的包头，代表长度 ```