How do I suspend a Qt Thread

Subbu

Hi, I have a thread say B. I want to stop this thread B from another thread say A. I tried the following ways. But the thread B keeps running. The program control doesnt come out of the ThreadB event loop. Is there a way to suspend a thread safely and immediately?

Here is my example code:
File: main.cpp

threadA->Start();
threadB->Start();

threadA: event loop: in fileA.cpp

void threadA::run()
{
    while(!m_quit)
    {
        if (threadB->isRunning())
        {
            threadB->quit();
            threadB->wait();            
        }
}

I also tried it this way to stop the thread B.

void threadA::run()
{
    while(!m_quit)
    {
        if (threadB->isRunning())
        {
            threadB->wait();            
            threadB->quit();
        }
}

Here is the thread B event loop in fileB.cpp

void threadB::run()
{
    while(!m_quit)
    {
        //check if USB device is connected
        //Do XYZ
     }
}

JoeCFD

use worker class for the threads and send a signal from worker A to worker B to stop thread B
https://wiki.qt.io/QThreads_general_usage

Chris Kawa

@Subbu The default implementation of QThread::run calls exec() that starts and runs an event loop. That loop can be stopped by calling quit(). You have overridden run and there's no Qt event loop running in it, so quit() does nothing. You have your own loop and stop condition using a member variable, so you need to use that i.e.

void threadA::run()
{
    while(!m_quit)
    {
        if (threadB->isRunning())
        {
            threadB->m_quit = true; //or using a setter method if you have one
        }
    }
}

Is there a way to suspend a thread safely and immediately

Immediately - no. Threads run independently, possibly on different cores or CPUs. ASAP is the closest thing you can get, but not immediately as in the next CPU instruction.
Safely - yes, but it takes a bit of effort. Your code is not safe. If thread B is stopped and destroyed just after you call if (threadB->isRunning()) but before threadB->quit(); then your code will crash from calling a method on destroyed object. You need proper synchronization - make sure that both thread objects are alive when they call methods on each other.

JonB

@Subbu
Everything that @Chris-Kawa said.

If you are in charge of the code/behaviour of thread B, see void QThread::requestInterruption() and bool QThread::isInterruptionRequested() const for one way of A stopping B. But this may not be your case.

Subbu

Thanks Fellas for the quick response. You have given me many solutions.