What is the purpose of trap_exit

[Ludovic Dem]

Hi,

I know that this process flag transforms the exits signals into messages that can be received. But what if we do not set the flag ? I guess a supervisor will still be able to kill/restart the process if needed, so what is the reason behind ?

Thank you.

[Ludovic Dem]

If I read the docs correctly, that would be to implement a supervisor …

[Ulf Wiger]

Nowadays, monitoring is best done using monitors, but process linking can be very useful for cascading exits. Conversely, enabling trap_exit may be needed if yi don't want to die immediately when a linked process dies. As far as the supervisor is concerned, the behaviors recognize an {'EXIT', Parent, shutdown} and self-terminate. Otherwise, the supervisor will send an exit(Child, kill)

[Ludovic Dem]

Thank you :)

Looking at the supervisor module code, it seems that whenever a process (even gen_server) does not traps exits, the supervisor will report an error, because the process will exit with reason killed which it neither shutdown nor normal.

So I think I need to trap exit in my processes in order to avoid this.

plain_fsm handles any exit message (and call terminate if exported), even when the signal does not come from the parent process, is that true ?

[Ulf Wiger]

Actually, no. If you use the pseudo-function plain_fsm:extended_receive(Expr), receive clauses that specifically match EXIT signals from the parent will be generated; other EXIT signals are ignored. Otherwise, you have to take care of your own messages, and plain_fsm can help you e.g. find the parent pid easily. See the documentation here for examples:

https://github.com/uwiger/plain_fsm/blob/master/doc/plain_fsm.md#a-insert-the-system-messages-in-the-receive-clause

[Ludovic Dem]

Oh yes, I missed that bit :

get_parent_expr() ->
    {match,0,
     {var,0,'__FSM_Parent'},
     {call,0,{remote,0,
              {atom,0,?PLAIN_FSM},
              {atom,0,info}},
      [{atom,0,parent}]}}.

Thank you very much for the explainations.