在Erlang進程環初始過程不會退出

Question

我已經寫以下代碼：

-module(ring). 

-export([start/3]). 

start(1, 1, Message) -> %special case, no new processes to be launched 
    io:format(" Start/3, branch 1~n~n"), 
    loop(self(), 1, 1), 
    self() ! Message, 
    self() ! {self(), quit}; 
start(1, MsgNum, Message) when MsgNum > 1 -> %first process, register self, launch next, send message, loop 
    io:format(" Start/3 branch 2, ProcNum: ~b, MsgNum: ~b, process: ~p~n~n", [1, MsgNum, self()]), 
    register(process_1, self()), 
    self() ! {self(), Message}, 
    NextPid = spawn(ring, start, [2, MsgNum, Message]), 
    NextPid ! {self(), Message}, 
    loop(NextPid, 1, MsgNum); 
start(MsgNum, MsgNum, _) when MsgNum >= 1 -> %last process, don't start new process, quit master, run loop 
    io:format(" Start/3 branch 3, MsgNum = ProcNum = ~b, process: ~p~n~n", [MsgNum, self()]), 
    process_1 ! {self(), quit}, 
    loop(process_1, MsgNum, MsgNum); 
start(ProcNum, MsgNum, Message) when ProcNum >= 1 -> %everything else, start next process, send it a message, start loop 
    io:format(" Start/3 branch 4, ProcNum: ~b, MsgNum: ~b, process: ~p~n~n", [ProcNum, MsgNum, self()]), 
    NextPid = spawn(ring, start, [ProcNum + 1, MsgNum, Message]), 
    NextPid ! {self(), Message}, 
    loop(NextPid, ProcNum, MsgNum). 

loop(NextPid, ProcNum, MsgNum) -> 
    receive 
     {Sender, quit} -> % send quit to next process in queue, then terminate with 'ok' 
     io:format(" Sender: ~p Quitting process ~p Next: ~p~n~n", [Sender, self(), NextPid]), 
     NextPid ! {self(), quit}, 
     ok; 
    {Sender, Msg} -> % print message and wait for next message 
     io:format(" Message: ~w~n procNum: ~b msgNum: ~b sender: ~p self: ~p next: ~p~n~n", [Msg, ProcNum, MsgNum, Sender, self(), NextPid]), 
     loop(NextPid, ProcNum, MsgNum); 
    _ -> %flush garbage, should never get here... 
     io:format(" Oops. Received someting unexpected") 
    end. 

我期望此代碼以啓動一系列的處理，應該每個打印一個消息，然後禮貌退出。例如，如果我運行ring：start（1,3，hola），我希望看到三個進程，三個消息和三個退出。它幾乎工作，除了初始進程不會終止，因爲在這個shell會話看出：

Eshell V5.10.1 (abort with ^G) 
1> c(ring). 
{ok,ring} 
2> processes(). 
[<0.0.0>,<0.3.0>,<0.6.0>,<0.7.0>,<0.9.0>,<0.10.0>,<0.11.0>, 
<0.12.0>,<0.13.0>,<0.14.0>,<0.15.0>,<0.16.0>,<0.17.0>, 
<0.18.0>,<0.19.0>,<0.20.0>,<0.21.0>,<0.22.0>,<0.23.0>, 
<0.24.0>,<0.25.0>,<0.26.0>,<0.27.0>,<0.28.0>,<0.32.0>] 
3> ring:start(1,3,hola). 
Start/3 branch 2, ProcNum: 1, MsgNum: 3, process: <0.32.0> 

Message: hola 
    procNum: 1 msgNum: 3 sender: <0.32.0> self: <0.32.0> next: <0.41.0> 

Start/3 branch 4, ProcNum: 2, MsgNum: 3, process: <0.41.0> 

Message: hola 
    procNum: 2 msgNum: 3 sender: <0.32.0> self: <0.41.0> next: <0.42.0> 

Start/3 branch 3, MsgNum = ProcNum = 3, process: <0.42.0> 

Message: hola 
    procNum: 3 msgNum: 3 sender: <0.41.0> self: <0.42.0> next: process_1 

Sender: <0.42.0> Quitting process <0.32.0> Next: <0.41.0> 

Sender: <0.32.0> Quitting process <0.41.0> Next: <0.42.0> 

Sender: <0.41.0> Quitting process <0.42.0> Next: process_1 

ok 
4> processes(). 
[<0.0.0>,<0.3.0>,<0.6.0>,<0.7.0>,<0.9.0>,<0.10.0>,<0.11.0>, 
<0.12.0>,<0.13.0>,<0.14.0>,<0.15.0>,<0.16.0>,<0.17.0>, 
<0.18.0>,<0.19.0>,<0.20.0>,<0.21.0>,<0.22.0>,<0.23.0>, 
<0.24.0>,<0.25.0>,<0.26.0>,<0.27.0>,<0.28.0>,<0.32.0>] 

我的長碼道歉/日誌塊，但我認爲這使得它很清楚，啓動/ 3導致啓動三個進程（< 0.32.0>，< 0.41.0>和< 0.42.0>），並且每個進程打印一條消息，並且每個進程都會收到其退出消息。但是，全部完成後，初始進程（< 0.32.0>）仍在運行。我的理解是，一旦沒有更多的代碼可以運行，流程應該終止 - 第一個流程有什麼特別的地方可以阻止它的終止？

在此先感謝。

Answer 1

您的函數start/3不會啓動您稱爲'initial'的進程。而是該函數在已存在的進程中運行。這是erlang shell的過程，您正在使用它。它有pid < 0.32.0>。在函數過程（）的第一個調用的輸出仔細一看：

2> processes(). 
[<0.0.0>,<0.3.0>,<0.6.0>,<0.7.0>,<0.9.0>,<0.10.0>,<0.11.0>, 
<0.12.0>,<0.13.0>,<0.14.0>,<0.15.0>,<0.16.0>,<0.17.0>, 
<0.18.0>,<0.19.0>,<0.20.0>,<0.21.0>,<0.22.0>,<0.23.0>, 
<0.24.0>,<0.25.0>,<0.26.0>,<0.27.0>,<0.28.0>,<0.32.0>] 

最近PID列表是< 0.32.0>。因此，當你調用start/3時，它會在該進程中運行，當它退出循環時，進程< 0.32.0>將不會被完成 - 它只會返回到erlang shell並等待下一個命令。