gen_server:什么是monitor‘`start_monitor/4’？

Question

OTP文档表示，gen_server:start_monitor/4的签名是：

start_monitor(ServerName, Module, Args, Options) -> Result
OTP 23.0
Types
ServerName = {local,Name} | {global,GlobalName}
  | {via,Module,ViaName}
 Name = atom()
 GlobalName = ViaName = term()
Module = atom()
Args = term()
Options = [Option]
 Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}
  Dbgs = [Dbg]
   Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}
  SOpts = [term()]
Result = {ok,{Pid,Mon}} | ignore | {error,Error}
 Pid = pid()
 Error = {already_started,Pid} | term()

Mon是什么？医生们只是说：

"Mon是对为监视服务器而设置的监视器的引用“

现在我知道Mon是一个监视器，它监视它，但是它的类型是什么？

读到erlang.erl，我发现：

-type registered_name() :: atom().
-type registered_process_identifier() :: registered_name() | {registered_name(), node()}.
-type monitor_process_identifier() :: pid() | registered_process_identifier().

Mon pid() | atom() | {atom(), node()}的类型也是如此

Answer 1

Mon是监视器的引用：

monitor_return({{ok, Pid}, Mon}) when is_pid(Pid), is_reference(Mon) ->
    %% Successful start_monitor()...
    {ok, {Pid, Mon}};

(代码为这里)