[Qt-interest] Multithreading - yield function

[Thiago Macieira]

Em Segunda-feira 31. Agosto 2009, às 21.46.20, você escreveu:
> I will consider that, I don't have much experience with threads.
> 
> One thing I don't understand, why there would be a function like yield that
> don't always work. What is the reason such a function to exist if it's not
> good to use it.
> Sorry if my questions are obvious or something like that. :)

The function works.

It just doesn't do what you expect it to do. You're expecting that another 
thread gets woken up and run, while your thread is suspended for a long time.

That's not what happens. That only causes the current thread to give up its 
processor time. Other threads may wake up, if they are ready for execution. 
But the current thread will wake up too, very soon.

If you don't insert a synchronisation primitive, like a QWaitCondition or a 
QSemaphore, there's no guarantee of how long your thread will get suspended 
for. It may get suspended but then woken up again before the other thread 
finishes execution.

If you want to program with threads, I recommend you read some books on the 
subject first. It is *very* tricky to get it right. Programming threads when, 
like you said, you don't have much experience, is a very daunting task.

-- 
Thiago Macieira - thiago.macieira (AT) nokia.com
  Senior Product Manager - Nokia, Qt Development Frameworks
     Sandakerveien 116, NO-0402 Oslo, Norway

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