[erlang-questions] Print a message when a process dies

[Michael Loftis]

You could also do a spawn_monitor (or possibly spawn_link depending on
how you want to handle things) to avoid racing a potentially crashing
process.

On Mon, Jan 12, 2015 at 7:52 PM, Harit Himanshu
<harit.subscriptions@REDACTED> wrote:
> Thank you very much Felix. really appreciate your time and effort
>
> On Mon, Jan 12, 2015 at 3:50 PM, Felix Gallo <felixgallo@REDACTED> wrote:
>>
>> a) spawn() never fails.  You'll get a Pid back, or the BEAM will crash in
>> which case mere erlang programs are the least of your problems.  The Pid
>> will then either apply M:F to A, if M:F exists and is of arity befitting A,
>> or apply error_handler:undefined_function to [M,F,A]), which in turns prints
>> out that message you saw unless you've redefined the error handler, which
>> you should definitely not do.
>>
>> b) monitor() also will never fail.  If monitor() is called on a
>> non-existent Pid, you will get a DOWN message immediately.  monitor() will
>> still return a Ref.  (note: this applies to erlang released within the last
>> few years; before, you would get a badarg exception).
>>
>> os:timestamp() will also never fail.
>>
>> In general your approach should be to assume that everything will work out
>> fine and only program, at first, the 'happy path' for your code.  Then fix
>> them up as you find valid inputs which cause them to crash.
>>
>> http://erldocs.com/ and http://www.erlang.org/doc/ are really good, and
>> can help you answer a lot of these questions; the functions you list are
>> well documented, for example.  If you're using a Mac, I can recommend
>> getting 'Dash' (a documentation-at-your-hotkey app with a search bar) as
>> well.
>>
>> F.
>>
>> On Mon, Jan 12, 2015 at 3:14 PM, Harit Himanshu
>> <harit.subscriptions@REDACTED> wrote:
>>>
>>> I need to make my own function my_spawn(M,F,A) which work similar to
>>> spawn(M,F,A) but if a spawned process dies, I need to print a message
>>> telling how long a process lived.
>>>
>>>
>>> My attempt looks like
>>>
>>> -export([my_spawn/3]).
>>>
>>> my_spawn(Mod, Func, Args) ->
>>>   Pid = spawn(Mod, Func, Args),
>>>   Ref = monitor(process, Pid),
>>>   Start = os:timestamp(),
>>>   receive
>>>     {'DOWN', Ref, process, Pid, _Why} ->
>>>       io:format("process died after ~p microseconds.~n",
>>> [timer:now_diff(os:timestamp(), Start)])
>>>   end,
>>>   Pid.
>>>
>>>
>>> so when I run this, I see
>>>
>>> 1> Pid = error_handling:my_spawn(area_server1, area, []).
>>> process died after 363 microseconds.
>>>
>>> =ERROR REPORT==== 12-Jan-2015::15:06:53 ===
>>> Error in process <0.33.0> with exit value:
>>> {undef,[{area_server1,area,[],[]}]}
>>>
>>> <0.33.0>
>>> 2>
>>>
>>> Problems?
>>> - I am learning both Erlang and concurrency for the first time, so I see
>>> following potential problems with this approach
>>>
>>> a.) What if spawned process failed in the first place, so I get no Pid
>>> b.) What if command fails executing either "Ref = .." or "Start = .."?
>>>
>>> In such scenarios the program will not work effectively, what are the
>>> recommended ways to solve such thing?
>>>
>>> Also, do you see any potential race conditions here? I take a guess
>>> saying no because even if multiple process trying to execute the same code,
>>> they will return different Pid anyway, is that understanding correct?
>>>
>>> Thanks a lot
>>> + Harit
>>>
>>> _______________________________________________
>>> erlang-questions mailing list
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>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>
>>
>
>
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