[erlang-questions] Why do two processes appear after a spawn?

Dominic Williams <>
Tue Nov 6 22:53:58 CET 2007


Hello,

Can someone explain to me what is going on here?

Erlang (BEAM) emulator version 5.5.4 [source] [async-threads:0]  
[hipe] [kernel-poll:false]

Eshell V5.5.4  (abort with ^G)
1> F = fun() -> P=processes(), {length(P), lists:member(self(),P)} end.
#Fun<erl_eval.20.112921583>
2> G = fun() -> P=F(), timer:sleep(500), Q=F(), io:fwrite("~p~n", 
[{P,Q}]) end.
#Fun<erl_eval.20.112921583>
3> F().
{24,true}
4> spawn(G).
<0.35.0>
{{25,true},{26,true}}

So the newly spawned process is immediately listed by processes(),  
but a bit later another one appears. What is this 26th process?
Upon further investigation:

6> H = fun(P, Q) -> [process_info(Pid) || Pid <- Q -- P] end.
#Fun<erl_eval.12.24269688>
7> J = fun() -> P=processes(), timer:sleep(500), Q=processes(),  
io:fwrite("~p~n",[H(P,Q)]) end.
#Fun<erl_eval.20.112921583>
8> F().
{24,true}
9> spawn(J).
<0.42.0>
[[{current_function,{io,wait_io_mon_reply,2}},
   {initial_call,{erlang,apply,2}},
   {status,waiting},
   {message_queue_len,0},
   {messages,[]},
   {links,[<0.25.0>]},
   {dictionary,[]},
   {trap_exit,false},
   {error_handler,error_handler},
   {priority,normal},
   {group_leader,<0.24.0>},
   {heap_size,233},
   {stack_size,6},
   {reductions,15},
   {garbage_collection,[{fullsweep_after,65535}]}]]

Any explanation?

Thanks,

Dominic Williams
http://dominicwilliams.net

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