[erlang-questions] Update Pid State on a Timer

zxq9 <>
Mon Dec 5 06:30:05 CET 2016


On 2016年12月4日 日曜日 20:43:08 qp wrote:
> I am trying to update my process's state on a 3 second timer.
> 
> -define(INTERVAL, 3000).
> 
> start_link() ->
>     gen_server:start_link(?MODULE, [], []).
> 
> action(Pid, Action) ->
>   gen_server:call(Pid, Action).
> 
> init([]) ->
>   erlang:send_after(?INTERVAL, self(), trigger),
>   {ok, temple:new()}.
> 
> what I want to do is call this
> 
> handle_call({fight}, _From, Temple) ->
>   NewTemple = temple:fight(Temple),
>   {reply, NewTemple, NewTemple};
> 
> So I try
> 
> handle_info(trigger, _State) ->
>    land:action(self(), {fight}),
>    erlang:send_after(?INTERVAL, self(), trigger);
> 
> but I get
> 
> =ERROR REPORT==== 4-Dec-2016::19:00:35 ===
> ** Generic server <0.400.0> terminating
> ** Last message in was trigger
> ** When Server state == {{dict,0,16,16,8,80,48,
>                                {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],
>                                 []},
>                                {{[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],
>                                  []}}},
>                          []}
> ** Reason for termination ==
> ** {function_clause,[{land,terminate,
>                            [{timeout,{gen_server,call,[<0.400.0>,{fight}]}},
>                             {{dict,0,16,16,8,80,48,
>                                    {[],[],[],[],[],[],[],[],[],[],[],[],[],[],
>                                     [],[]},
>                                    {{[],[],[],[],[],[],[],[],[],[],[],[],[],
>                                      [],[],[]}}},
>                              []}],
>                            [{file,"src/land.erl"},{line,47}]}
> 
> 
> What am I missing here?

I assume the module name is `land`.

If that is the case the handler for 'trigger' will call

land:action(self(), {fight})

which puts us inside action/2 above, which tries to make a call to self().

Why would making a call to self() always fail with a timeout? Think about
this carefully -- it is the most basic form of deadlock you can create.

-Craig


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