【原创】Erlang 的 gen_tcp:send 为什么没有 timeout 选项-白红宇

在 gen_tcp 文档中有如下说明：

The fact that the send call does not accept a timeout option, is because timeouts on send is handled through the socket optionsend_timeout. The behavior of a send operation with no receiver is in a very high degree defined by the underlying TCP stack, as well as the network infrastructure. If one wants to write code that handles a hanging receiver that might eventually cause the sender to hang on a send call, one writes code like the following.

大致意思为，sender 发送时没有 receiver 的情况，已经通过底层 TCP 协议栈很好的解决了，如果 sender 想使用带 timeout 功能的 send ，则需要在 connect 的时候指定 {send_timeout, xxx} 。

假定的使用场景（取自

）：

business process <--> client process(connect) <--> server process(accept)

                         
...
gen_tcp:connect(HostAddress, Port, [{active,
false
}, {send_timeout, 5000}, {packet,2}]),
loop(Sock),
...
In the loop where requests are handled, we can now detect send timeouts:
loop(Sock) ->
    
receive
        
%% 这里是来自 business 的消息发送命令
        
{Client, send_data, Binary} ->
            
case
gen_tcp:send(Sock,[Binary]) of
                
%% 这里完成针对 send 的超时处理
                
{error, timeout} ->
                    
io:
format
(
"Send timeout, closing!~n"
, []),
                    
%% 可以针对超时做业务处理
                    
handle_send_timeout(), % Not implemented here
                    
Client ! {self(),{error_sending, timeout}},
                    
%% Usually, it's a good idea to give up 
in
case
of a 
                    
%% send timeout, as you never know how much actually 
                    
%% reached the server, maybe only a packet header?!
                    
%% 这里建议直接关闭
                    
gen_tcp:close(Sock);
                
{error, OtherSendError} ->
                    
io:
format
(
"Some other error on socket (~p), closing"
,
                              
[OtherSendError]),
                    
Client ! {self(),{error_sending, OtherSendError}},
                    
gen_tcp:close(Sock);
                
ok ->
                    
Client ! {self(), data_sent},
                    
loop(Sock)
            
end
    
end.
        
      

通常情况下，上述代码足以检测 receive 端的超时问题。大多数协议都会存在某种来自 server 端的确认机制，若存在，则不需要上述代码实现的 send 超时检测。需要使用的唯一场景是应用层协议是单向、无确认机制的情况。