[Interest] QThread and sleep

[Thiago Macieira]

On terça-feira, 19 de março de 2013 10.37.56, Hugo Drumond Jacob wrote:
> Hi folks!
> 
> Today some doubt has ocurred on my office. Suppose that for some
> reason we need to sleep a thread (not the main thread) for some time

Suppose that you're wrong and you came to the wrong solution that sleeping is 
the way to go. Try to find a solution that makes you do what you need to do and 
not sleep.

> (eg. 100 ms). The QThread::wait(int) "provides similar functionality
> to the POSIX pthread_join()" and this don't will sleep the target
> thread, but the caller thread.
> 
> Call the QThread::wait(int) from the target thread is forbidden
> (according to line 651 of qthread_unix.cpp).

You were looking for QThread::sleep, not wait. The latter is used to wait for 
a thread to finish, with a given timeout. The former suspends and blocks 
execution of the calling thread for a given time.

> Furthermore, is "wrong" call QThread::wait(int) except for
> synchronously thread termination with QThread::terminate() ? ( I know,
> isn't so healthy use QThread::terminate() ) QThread::wait(int) is a
> busy wait, right?

No, it isn't. It's like a pthread_join: it will suspend the execution of the 
calling thread until the other thread exits or the timeout happens, whichever 
comes first. QThread::wait() is normal use and is, in fact, used even by the 
QThread destructor.

QThread::sleep() is, more often than not, a bad choice. Avoid it if you can. 
If you can't, redesign your code so you don't need to use it.

> The attached code illustrate the question.

Your example has the main thread waiting for 100 ms for the auxiliary thread 
to exit. But your auxiliary thread is busy-looping doing a qDebug and will 
never exit. Since it doesn't exit, QThread::wait() will always timeout.

> My immediate solution is: create some MyThread class inherited of
> QThread just for expose the QThread::*sleep(unsigned long) methods and
> create some slot or Q_INVOKABLE method on Worker to call these exposed
> methods. Anyone propose some more elegant solution?

Yes: don't sleep.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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