Waiting for data in QQueue

[Clarck_D]

Hi everyone !

I want to wait on a QQueue to have data to treat and see if I get what I expect.
For the waiting part, I tought about using a QtConcurrent::run, calling a function where I could do a while loop to check data in my queue. Also, I saw some code on StackOverflow wich inspired me this :

  //    /* Wait for reception */
bool timeout = false;
_timeout = 100;
queueAckEmpty = queueAck.isEmpty();

while (!timeout && queueAckEmpty){
    qCritical() << "Ack empty : " << queueAckEmpty;
    queueAckEmpty = queueAck.isEmpty();
    timeout = elapsedTime.elapsed() > _timeout;
    QApplication::processEvents(QEventLoop::AllEvents);
}

The problem with my code is that I would expect it to go out of the while loop as soon I have data in the queue but I don't and it print hundreds "Ack empty : ".

I wanted to have your opinion on which is the best way to do this and why ?

Thanks :)

[JonB]

@Clarck_D
If you are going to do it this way (you would be better with signal & slot), one processEvents() in a loop without delay is awfully busy. You would want some delay in the loop, like 1/100th or 1/1000th of a second to avoid too many calls.

[Asperamanca]

If you write a little wrapper class around QQueue, you could use synchronization classes like QMutex:

• Your queue offers a mutex
• The mutex is locked while the queue is empty
• The code processing the items of your queue wait for the mutex to unlock
• When an item is added to the queue, the mutex is unlocked. As the last item is removed from the queue, the mutex is locked again.

[Clarck_D]

I think I'm gonna try to use QtConcurrent and QMutex then. Thanks guys :)

[Clarck_D]

Still I have a question, if I use QtConcurrent::run, the thread used will be released when function has finished ?

[SGaist]

Hi,

Looks like a case for QWaitCondition.

[Clarck_D]

@SGaist Ok so now I'm lost as for now I have no idea how to implement QtConcurrent::run, QMutex and QWaitCondition. The documentation is not really clear..

[SGaist]

You would use QtConcurrent once you have have data in your queue.

[kshegunov]

Queues and such:
https://forum.qt.io/topic/75880/proper-way-to-manage-information-between-threads

[Clarck_D]

@SGaist No I can't because I want to use it for waiting data

I came across this :

However, QtConcurrent can't be used when communication with the running thread is needed, and it shouldn't be used to handle blocking operations.

So I guess that get rid of Qtconcurrent for my purpose.
For now I went with the code I posted.

Still, thanks guys ;)

[Asperamanca]

There is mainly two use cases for multithreading:

1. Avoid blocking some other operation (often GUI)
2. Distribute work across multiple cores

QtConcurrent is good for 2) (especially if you need to perform an operation across multiple items in a container), but not really suited for 1)