How to terminate long running Qt Thread - c++

Question

I have a threaded application in which main thread is the gui thread and the secondary thread is the one doing major computing inside nested for loops. Now the problem is how to terminate the secondary thread. When I press the stop button on the GUI thread it called a slot in the secondary thread which sets the stop variable to true. Problem with this technique is that, testing stop variable in for loops (as shown below) is not enough. Because if pressed the stop button just after thread has entered the third for loop, it will execute all lengthy functions before it will check the stop flag. Thats why we need to test it in each function before executing its contents, which I think is not not a very elegant solution.
I cannot call terminate on this thread as this thread allocates some decent amount of memory of memory and calling terminate will cause that memory to leak.
So for such a scenario what would be the best way to terminate the thread and any point?
Following code skeleton is just to give you and idea of what I/m talking about
void run( ) {
for(int i = 0; i < 4 && !stop; i++) {
for(int j = 0; j < 3 && !stop; j++) {
for(int k; k < 6 && !stop; k++) {
callA( );
callB( );
callC( );
callD( );
callE( );
callF( );
callF( );
callG( );
}
}
}
}
callA( ) { callL( ); }
callB( ) { callM( ); }
callC( ) { callN( ); }
callD( ) { callO( ); }
callE( ) { callP( ); }
callF( ) { callQ( ); }
callF( ) { callR( ); }
callG( ) { callS( ); }
callL( ) { sleep(20); }
callM( ) { sleep(60); }
callN( ) { QProcess( ); }
callO( ) { sleep(30);}
callP( ) { QProcess( ); }
callQ( ) { }
callR( ) { sleep(60); }
callS( ) { QProcess( ); }

Answer 1

You could call QApplication::processEvents() inside your work loop to allow the thread to receive events. An event could for instance be your main thread calling a slot setting the variable that you check to see if you should interupt your work.
//main thread calls this slot
void interupt_slot()
{
stop_was_called = true;
}
void do_work()
{
for(;;)
{
QApplication::processEvents();
if(stop_was_called)
break;
//Work
}
//Exectution is over and the thread can be stopped etc
emit its_over();
}
Edit:
It won't interupt the thread's sleep though. If the main thread invokes the interupt_slot() slot while the thread is sleeping the slot will be called when sleep(n) is over. But you could also fake sleeping by doing the work in slots that are called on a timer.

Answer 2

You could wrap your functions in classes and store the list of Objects of these classes in a list. So calling your functions would be in a loop iterating over this list and you will have only one code-place to check for stop condition.
By the way, in order to avoid stop-chacking in each nested loop you could use goto or throw.
In C++ leaving nested loops is the only "legal" use-case for goto.
Concerning throw, B.Stroustrup preferes it over goto and encourages to use it as a control structure.