Re: [Boost-users] Boost Thread: condition.wait()

From: Vladimir Prus (ghost_at_[hidden])
Date: 2008-01-30 01:34:32

Kesseli Pascal wrote:

> Hi everyone
> 
> Recently I was trying to synchronize one of my functions using the
> following code:
> 
>     // write own data to shared vector...
>     // ...
>     if (++numberOfReadyThreads < numberOfInvolvedThreads) {
>         allChargingDataReady.wait(lock);
>         return;
>     }
>     numberOfReadyThreads = 0; // Only the last thread enters this section.
>     // write data to file...
>     allChargingDataReady.singal_all();
> 
> The idea is that all threads submit their data using this function and
> then wait before reading in the next set of data until all threads have
> submitted their data. The last thread to submit his data is the one who
> has to write the collected data to a file.
> 
> Using this structure, I find it very simple to realize this behavior.
> However, I know from programming with Java that the VM there sometimes
> awakens one's threads without any reason, which means that I would have to
> realize my condition like this instead:
> 
> 
>     // ...
>     while(numberOfReadyThreads < numberOfInvolvedThreads) {
>         allChargingDataReady.wait(lock);
>     }
>     // ...
> 
> Implementing the same behavior using this snippet is much more complicated
> though and I would rather avoid it if possible.

Why is it more complicated?

> And that leads us right 
> away to my question:
> 
> Is the while()-workaround using Boost & C++ needed as well as in Java? Or
> can I use the much simpler if()-version?

As docs say, you need to test your condition in a loop in Boost.Threads. 
This is not a particular property of Java or Boost.Threads -- that's how
POSIX threads work. (Windows don't have native condition, and I have much
clue how conditions are simulated on windows and whether window Event thing
has similar behaviour).

- Volodya