QThreadpool/QRunnable blocking

[John Howe]

Hello everyone,

Still a Qt rookie. I'm trying to implement the example of the HelloWorldTask on the QThreadpool page within my existing application. The HelloWorldTask is called within a slot, executes one time, and then blocks execution thereafter. My code:

class HelloWorldTask : public QRunnable
{
    void run() override;
};

void HelloWorldTask::run()
{
    qDebug() << "Hello world from thread" << QThread::currentThread();
}

void slotTask()
{
    ...do stuff...
    HelloWorldTask *hello = new HelloWorldTask();
    pool->tryStart(hello);
    ...resume doing stuff...
}

I want the HelloWorldTask to execute every time the slot triggers. My understanding was that this would hand off the task as a worker thread and the remaining code in my slot would continue to execute, but it seems to block all execution. Do I need to call a wait somewhere? Do I need to call releaseThread? Kinda lost.

[SGaist]

Hi,

What do you mean by block all execution ?
How did you determine that ?
Which version of Qt ?
On which platform ?

[John Howe]

What do you mean by block all execution ?

Whereas my program normally continues running and the slotTask will be triggered almost constantly, the program hangs after the 'hello from thread x' statement.

How did you determine that ?

Output to the console window stops. Execution (and console output) would normally continue until I terminate the program.

Which version of Qt ?

5.9.3 -- not chosen by me, so I'm stuck with it.

On which platform ?

Windows 10

[SGaist]

Does it block directly on the first tryStart call ?

[John Howe]

Yes, first tryStart call.

[SGaist]

How long does it block ?

You're runnable being pretty short and quick to do, you might just see it done sooner that what you expect.

[JonB]

This post is deleted!

[John Howe]

@SGaist Execution stops after that.

[SGaist]

What do you mean by stop ?
Does your application hang or crashes ?

[John Howe]

The application hangs.

[John Howe]

Made a couple of tweaks and turned on some print statements in a different part of the app (a different slot triggered by QTimer).

It almost looks like slotTask just stops printing to the console and log file.

[nagesh]

void HelloWorldTask::run()
{
    qDebug() << "Hello world from thread" << QThread::currentThread();
}

@John-Howe Does your implementation contains these statements only in run method? or any while loop ?

[kshegunov]

@John-Howe the qDebug stream is serialized internally, which is what @nagesh is alluding to, I imagine. That notwithstanding, you should just interrupt the debugger when the program hangs, and extract a complete stack trace. Upload it somewhere/post it here, so we can take a look.