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

[Harit Himanshu]

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
>>
>> _______________________________________________
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>>
>
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