[M3devel] join twice?

[Tony Hosking]

The original threads spec for Modula-3 does not mention join can only  
run once.  The only mention is in Thread.i3.

I agree that it seems silly to join twice.  On the other hand, Java  
semantics does not prohibit multiple joins.

Antony Hosking | Associate Professor | Computer Science | Purdue  
University
305 N. University Street | West Lafayette | IN 47907 | USA
Office +1 765 494 6001 | Mobile +1 765 427 5484




On 3 Nov 2009, at 10:04, Jay K wrote:

>
> To me..with a Win32 background..join means "wait for thread to  
> finish".
>
> And there is a separate action "get thread result".
>
> You could consider these one merged operation "wait for thread to  
> finish and get its result".
>
> In either case, it is reasonable to allow it multiple times.
>
> Waiting for a thread to finish that has already finished is just fast.
>
> Getting a thread result that you already got is also easy albeit  
> usually unnecessary.
>
>
>
> Now, if the operation is "wait for thread to finish, get its result,  
> and lose track of its result",
>
> that isn't idempotent.
>
>
>
> I'm torn. The existing implementations all either support multiple  
> join or have to go out of
>
> their way to prevent it. Yet Thread.i3 has been commented so  
> presumably forever.
>
>
>
> - Jay
>
>
>
>
> Date: Tue, 3 Nov 2009 09:36:23 -0500
> From: rcoleburn at scires.com
> To: m3devel at elegosoft.com
> Subject: Re: [M3devel] join twice?
>
>
> I think the comment in the code is correct.
>
> Semantically, it doesn't make sense to join a thread more than  
> once.  Join is the compliment to Fork.  With fork, one thread  
> becomes two.  With Join, two threads become one.  After they've  
> become one, the 2nd thread is no more, so you can't join to it  
> anymore.
>
> IMO, the implementation should enforce this behavior.  What would be  
> the "benefit" of allowing more than one join?  What does it "mean"  
> to join a "dead thread"?
>
> Regards,
> Randy Coleburn
>
>>>> Tony Hosking <hosking at cs.purdue.edu> 11/2/2009 11:54 PM >>>
>
>
>
>
> OK, sure.  Fair enough.
>
>
> On 2 Nov 2009, at 16:43, Jay K wrote:
>
>> (Whether the implementation actually prohibits it or not is an  
>> implementation decision.)
>
> Tony, I'm sympathetic to the smaller version but I think it is wrong.
> "It is a checked runtime error to call this more than once for any t"
> is a much different/stronger statement than e.g.
> "it is implementation defined what happens if you call join more  
> than once for any t".
> The printed Reactor 4.1 docs have the same comment as current  
> Thread.i3.
>
> You know, ideally if I write:
> Thread.Join(t);
> Thread.Join(t);
>
> and it works today on any system, it will continue to work on all  
> systems

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