[erlang-questions] erlang:trace/3

Emilio De Camargo Francesquini <>
Thu Apr 12 19:39:19 CEST 2012

Hello again,

I was playing around with dbg:p/2 and I realized that it does not
accept the flag 'exiting'. I browsed the code a little bit and found
out that it accepts only the flags declared in dbg:all/0. Here's the

all() ->

I took a brief look at the documentation, and found that "The list can
also include any of the flags allowed in erlang:trace/3".

Is this a bug or a deliberate decision?


Le 10 avril 2012 14:46, Björn-Egil Dahlberg <> a écrit :
> On 2012-04-09 19:49, Emilio De Camargo Francesquini wrote:
>> Hello,
>> I'm having some trouble understanding the exact meaning of two of the
>> flags for the erlang:trace/3 function.
>> Flag:
>> - running
>>     Trace scheduling of processes.
>>     Message tags: in, and out.
>> I imagine that a message tagged "in" means that the process was chosen
>> by a scheduler and will begin/resume execution and a tag "out" means
>> that the processes has been preempted. Is that correct?
> Correct but, to be clear, not only preempted. The tag "out" will be sent if
> the process is scheduled out for any reason.
>> - exiting
>>     Trace scheduling of an exiting processes.
>>     Message tags: in_exiting, out_exiting, and out_exited.
>> I guess that these messages are sent when the proccess is about to
>> exit or has already exited. Am I right? What is the exact difference
>> between each one of them?
> With "in_exiting" and "out_exiting" the process is scheduled as normal but
> it is in limbo. This means the process is dead but has to be scheduled in
> order to terminate and cleanup internally in the runtime. In other words, it
> will not execute beam-code any more just terminate. Why? It simplifies
> things internally.
>> Moreover, what is the difference between "exiting" flag with the tag
>> "out_exited" and "procs" flag with the tag "exit"?
> The tag "exit" from procs is sent when the process is deemed dead, i.e. it
> is exiting. The last chunk of beam code the process executed was some form
> of exit.  At this moment in time process has the state EXITING but will
> potentially be scheduled again.
> The trace will after this be out_exited or, if more has to be done to
> terminate it, out_exiting and in_exiting. Ex:
> Eshell V5.9  (abort with ^G)
> 1> P = spawn(fun() -> receive Ok -> ok end end).
> <0.34.0>
> 2> erlang:trace(P, true, [running, procs, exiting]).
> 1
> 3> P ! ok.
> ok
> 4> flush().
> Shell got {trace,<0.34.0>,in,{erl_eval,receive_clauses,6}}
> Shell got {trace,<0.34.0>,exit,normal}
> Shell got {trace,<0.34.0>,out_exited,0}
> ok
> I hope this clarifies things.
> Regards,
> Björn-Egil
> Erlang/OTP
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