[erlang-questions] initializing library applications without processes

Ulf Wiger <>
Sun Apr 4 10:45:27 CEST 2010


Garrett Smith wrote:
> 
> The docs say you can return 'ignore' here as well. I've never tried
> that, but it might give this hack even more panache, assuming it
> doesn't implicitly terminate the process :)
> 
> Garrett

Actually, and unfortunately(?), that's exactly what happens.

Here's the relevant code from gen_server:init_it/6:

init_it(Starter, Parent, Name0, Mod, Args, Options) ->
     Name = name(Name0),
     Debug = debug_options(Name, Options),
     case catch Mod:init(Args) of
         {ok, State} ->
             proc_lib:init_ack(Starter, {ok, self()}),
             loop(Parent, Name, State, Mod, infinity, Debug);
         ...;
         ignore ->
             unregister_name(Name0),
             proc_lib:init_ack(Starter, ignore),
             exit(normal);

(supervisor.erl is implemented as a gen_server, and simply
passes on the 'ignore' return value from the Mod:init/1
function.)

Furthermore, in the specific case of application start,
the Mod:start/2 function is required to return a pid:

(from kernel/src/application_master.erl)

start_it_old(Tag, From, Type, ApplData) ->
     {M,A} = ApplData#appl_data.mod,
     case catch M:start(Type, A) of
         {ok, Pid} ->
             link(Pid),
             From ! {Tag, {ok, self()}},
             loop_it(From, Pid, M, []);
         {ok, Pid, AppState} ->
             link(Pid),
             From ! {Tag, {ok, self()}},
             loop_it(From, Pid, M, AppState);
         {'EXIT', normal} ->
             From ! {Tag, {error, {{'EXIT',normal},{M,start,[Type,A]}}}};
         {error, Reason} ->
             From ! {Tag, {error, {Reason, {M,start,[Type,A]}}}};
         Other ->
             From ! {Tag, {error,
                     {bad_return,{{M,start,[Type,A]},Other}}}}
     end.

BR,
Ulf W

-- 
Ulf Wiger
CTO, Erlang Solutions Ltd, formerly Erlang Training & Consulting Ltd
http://www.erlang-solutions.com
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