Question

After watching the Pragmatic Studio screen casts on Erlang, the last video on Supervisors mentioned that in order for a supervisor to get a notification about one of its children so it can properly restart it, the child should register with process_flag(trap_exit, true). Perhaps I just misunderstood the author (and chances are VERY high that I did misunderstand), but I thought supervisors automagically know when their children die (probably through spawn_link or something similar in the background). Is this really necessary? When should one use process_flag(trap_exit, true) in a real world case because the documentation explicitly states the following:

http://www.erlang.org/doc/man/erlang.html#process_flag-2

process_flag(trap_exit, Boolean)

When trap_exit is set to true, exit signals arriving to a process are converted to {‘EXIT’, From, Reason} messages, which can be received as ordinary messages. If trap_exit is set to false, the process exits if it receives an exit signal other than normal and the exit signal is propagated to its linked processes. Application processes should normally not trap exits.“

Answer 1

You have 3 idioms:

1/ I don’t care if my child process dies:

spawn(...)

2/ I want to crash if my child process crashes:

spawn_link(...)

3/ I want to receive a message if my child process terminates (normally or not):

process_flag(trap_exit, true),
spawn_link(...)

Please see this example and try different values (inverse with 2 or 0 to provoke an exception, and using trap_exit or not):

-module(play).
-compile(export_all).

start() ->
    process_flag(trap_exit, true),
    spawn_link(?MODULE, inverse, [2]),
    loop().

loop() ->
    receive
        Msg -> io:format("~p~n", [Msg])
    end,
    loop().

inverse(N) -> 1/N.