[erlang-questions] Exit signals are funny things

[Robert Virding]

Yes, and no. When a process dies with a reason then that reason is sent in
a signal to all the processes in the link set. So if the process dies with
the reason 'foo' then a 'foo' exit signal will be sent, and if I do exit/2
to send a 'foo' exit signal then a 'foo' exit signal will be sent. In both
cases, if the receiving process is not trapping it will die with the reason
'foo' [*] and if the receiving process is trapping then the exit signal
will be converted to a message with reason 'foo'. In both cases it IS a
'foo' exit signal.

However, if the exit reason is 'kill' then the 'kill' exit signal sent from
the process will be trapped while if it is sent with exit/2 it is not
trappable. THIS IS THE ONLY EXIT SIGNAL WHICH BEHAVES DIFFERENTLY! [**]

Again where's the logic in that? Why the inconsistency? We tried hard back
in the old days to be consistent.

Robert

* As 'foo' is not the value 'normal' it will kill the process.
**Sorry of raising my voice.


On 5 May 2017 at 14:08, Alex S. <alex0player@REDACTED> wrote:

> There is exit/1 and exit/2 which are very, very different things. exit/1
> is NOT an exit signal.
> There is no “kill and kill”, there is exit signal kill, which is special,
> and exit reason kill, which is not special.
> > 5 мая 2017 г., в 15:05, Robert Virding <rvirding@REDACTED> написал(а):
> >
> > There is more. So:
> >
> > 1> process_flag(trap_exit, true).
> > false
> > 2> Pid = spawn_link(fun() -> process_flag(trap_exit,true),
> timer:sleep(infinity) end).
> > <0.60.0>
> >
> > We trap exits and spawn_link which also traps exits and just hangs there
> waiting.
> >
> > 3> spawn(fun() -> link(Pid), exit(kill) end).
> > <0.62.0>
> > 4> process_info(Pid, messages).
> > {messages,[{'EXIT',<0.62.0>,kill}]}
> >
> > Now we spawn a new process which links to our hanger and exits with the
> reason 'kill'. We can then check our hanger and see that it received a
> 'kill' signal which it converted to a message because it was trapping.
> Finally:
> >
> > 5> spawn(fun() -> exit(Pid, kill) end).
> > <0.65.0>
> > 6> flush().
> > Shell got {'EXIT',<0.60.0>,killed}
> > ok
> >
> > we spawn another process which uses exit/2 to send a 'kill' signal to
> our hanger and in this case it cannot trap the signal and dies with the
> reason 'killed'. So there is 'kill' and 'kill' depending on how I send it.
> >
> > Where's the logic in that?
> >
> > Robert
> >
> > P.S. Yes, I know that getting a 'killed' from a process which has been
> killed with a 'kill' signal is the correct.
> >
> > _______________________________________________
> > erlang-questions mailing list
> > erlang-questions@REDACTED
> > http://erlang.org/mailman/listinfo/erlang-questions
>
>
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