[erlang-questions] How to clean up before termination...

Ulf Wiger <>
Tue Mar 20 17:57:50 CET 2012


Hi Dawid,

The gen_server loop will always initiate a shutdown if it receives an {'EXIT', Parent, shutdown}. As part of this process, it calls Mod:terminate(shutdown, ModState). If it doesn't trap exits, the Erlang VM will simply terminate the process, and the EXIT signal will never reach the erlang code level. Thus, without the trap_exit flag set, there is no way the terminate/2 function can be called.

To further clarify, when the supervisor wants to terminate a child, it doesn't ask nicely; it simply calls exit(Child, shutdown).

BR,
Ulf W

On 20 Mar 2012, at 17:44, Dawid Figiel wrote:

> Heh..
> 
> I read it view times, then I call someone to read this for me... then
> that someone becomes confused too ;)
> 
> (1) We jump to terminate/1 when supervisor wants to shut down the gen_server.
> (2) Trap_exit lets you handle 'EXIT' signals from linked processes.
> 
> What is the relation between (1) and (2).
> In my opinion function terminate has nothing to do with trap_exit.
> 
> Regards,
> 
> Dawid
> 
> 
> On 3/20/12, tom kelly <> wrote:
>> Hi Dawid,
>> 
>> You may have mis-read the documentation slightly, or else I misunderstand
>> your question.
>> 
>> If trap_exit is set to true then the terminate function is called when the
>> gen_server is shutting down, if it is false then the terminate function is
>> never called. All cleanup should be done in terminate, nowhere else.
>> 
>> Is this what you're asking?
>> 
>> //TTom.
>> 
>> 
>> 
>> On Tue, Mar 20, 2012 at 4:17 PM, Dawid Figiel <>wrote:
>> 
>>> Hi,
>>> 
>>> There is a recommendation in Erlang doc, which is telling to set up
>>> the trap_exit to true in the gen_server:init, when want to do clean up
>>> before supervisor will terminate the gen_server.
>>> 
>>> 
>>> http://www.erlang.org/doc/design_principles/gen_server_concepts.html#id66567
>>> 
>>> My question is:
>>> What for trap_exit has to be set to true when the clean up is needed?
>>> 
>>> The place for doing some clean ups is under terminate function.
>>> We want to have trape_exit set to true when we need to handle 'EXIT'
>>> signals coming from linked processes to (for example) not crash
>>> gen_server.
>>> 
>>> The only explanation for me is that the supervisor may crash too and
>>> send 'Exit' to gen_server as well... but this doesn't convince even
>>> myself.
>>> 
>>> Has anyone some explanation for this advice ?
>>> 
>>> 
>>> Best,
>>> 
>>> Dawid
>>> _______________________________________________
>>> erlang-questions mailing list
>>> 
>>> http://erlang.org/mailman/listinfo/erlang-questions
>>> 
>> 
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