[erlang-questions] Questions about PID of remote process

[Ulf Wiger]

Your test program does not keep a reference to the Pid, so each
process you create will perform its very short job, then die, making
that Pid available for reuse.

If you would keep the pids in e.g. a list, there would be no duplication.

BR,
Ulf W

On 05/13/2010 08:26 AM, B. Nicholson wrote:
> I thought that the PID of a process was made up of three pieces.
> 
> <Node,Lowbits of local processid, high bits of local process_id>.
> 
> The low bits and the high bits combine to make up a 28 bit number. So, I
> think it is possible to create the same process id by creating 268435456
> processes. I wrote a quick program to test this.
> 
> -module(proctest).
> -export([loop/1]).
> 
> loop(N) ->
> testspawn(N),
> loop(N+1).
> 
> testspawn(N) ->
> Pid = spawn(fun() -> donothing() end),
> PidStr = pid_to_list(Pid),
> case PidStr of
> "<0.1000.0>" -> io:format("Process ~p found @ ~p (iteration
> ~p)~n",[Pid,calendar:local_time(),N]);
> _ -> ok
> end.
> 
> donothing() ->
> _A = 1 + 2.
> 
> The output follows:
> 
> Erlang R13B03 (erts-5.7.4) [source] [64-bit] [smp:8:8] [rq:8]
> [async-threads:0] [hipe] [kernel-poll:false]
> 
> Eshell V5.7.4 (abort with ^G)
> 1> c(proctest).
> {ok,proctest}
> 2> proctest:loop().
> Process <0.1000.0> found @ {{2010,5,13},{1,14,45}} (iteration 959)
> Process <0.1000.0> found @ {{2010,5,13},{1,24,5}} (iteration 268223423)
> 
> I think you have to account for 'duplicate' local pids if your Erlang
> node is up for a considerable amount of time or goes
> through large numbers of processes for some reason.
> 
> Hope this helps.
> 
> Barry Nicholson
> 
> On 05/13/2010 12:17 AM, Bernard Duggan wrote:
>> Hi 钱晓明,
>>
>> On 13/05/10 15:09, 钱晓明 wrote:
>>   
>>> 1. How do I know if the remot process identified by one PID is alive? I
>>> know I can send a message to it, and receive response with timeout. But I
>>> hope there is some way more conveniently.
>>>   
>>>     
>> One option (the one we use) is erlang:monitor(process, Pid). If he Pid
>> dies or the node goes down, or is down when you make the monitor() call
>> you'll get a message notifying you of the fact.
>>   
>>> 2. Can the PID be reused in one node? If a process identified by one PID
>>> died, then this PID can be used to identify other process created later?
>>>   
>>>     
>> Not as far as I know - the fact that you can call node(Pid) to get the
>> node on which a PID resides, even after that PID has terminated, implies
>> that the PID embeds (or at least links to) unique node information.
>>
>> Cheers,
>>
>> Bernard
>>
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>>   
> 
> 
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