Re: [Boost-users] Thread events

From: Thomas Costa (oohrah_at_[hidden])
Date: 2005-09-08 14:17:39

The mutex is used to keep the "state" of what you are waiting to  
happen from being changed in the window of time between your test of  
it and your thread actually being placed on the wait queue of the  
condition primitive.

Otherwise you could get:

Thread A tests the "state" and gets a false result.
Thread B changes the "state" to true and then wakes all the waiters  
on the queue.
Thread A is added to the wait queue.

Which would potentially leave Thread A waiting forever for something  
that has already occurred.

On 8 Sep 2005, at 12:06 PM, Johan 't Hart wrote:

> Thanks for your replies.
>
> To me boost::condition looks very much like the old event variable,  
> except
> that it works together with a mutex. I am now wondering why you  
> should lock
> a mutex before waiting for the condition? Is there a rationale  
> about this?
>
> Johan
>
>
> ----- Original Message -----
> From: "Oleg Smolsky" <oleg.smolsky_at_[hidden]>
> Newsgroups: gmane.comp.lib.boost.user
> Sent: Thursday, September 08, 2005 6:24 AM
> Subject: Re: Thread events
>
>
>
>> Hello Johan,
>>
>> Johan 't Hart wrote on 8/09/2005 at 11:04 a.m.:
>>
>>> I'm quite a newbie in the boost world, and I read the documentation
>>> about boost::threads. Inthere I read that events were not included
>>> in boost because they were too error-phrone. That made me wonder how
>>> to do multithreading without events, for example, how do you make a
>>> thread wait for something to do without events?
>>>
>> Well, I don't know if it's at all possible to implement a powerful  
>> (or
>> flexible) multi-threaded project using just conditions.
>>
>> I use sockets and select() instead of WaitForMultipleObjects(). IE
>> select() is the cross-platform multiplexor that you can use for
>> threading and network events. That's the "worker thread" side.
>>
>> As for the "user code" side, you can implement a simple event that  
>> can
>> be signalled and waited upon using conditions. IE it's common to have
>> event.Wait() and event.Signal(), which are very very basic but
>> cross-platform:
>> - you can use boost conditions
>> - or you can use WaitForSingleObject() on win32 and pthread condition
>>  on unix. AFAIK that's what boost does.
>>
>> Best regards,
>> Oleg.
>>
>
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