QtConcurrent::run() and exit()

[pmitas]

Long story short: when I call exit() from a process started from QtConcurrent::run() it enters an infinite loop here:

0	pthread_cond_wait	pthread_cond_wait.S	162	0x00007ffff609459c	
1	wait	qwaitcondition_unix.cpp	87	0x00007ffff6a27a7b	
2	QWaitCondition::wait	qwaitcondition_unix.cpp	159	0x00007ffff6a27a7b	
3	QThreadPoolPrivate::waitForDone	qthreadpool.cpp	295	0x00007ffff6a1c808	
4	QThreadPool::~QThreadPool	qthreadpool.cpp	428	0x00007ffff6a1cfff	
5	QGlobalStaticDeleter<QThreadPool>::~QGlobalStaticDeleter	qglobal.h	1796	0x00007ffff6a1d695	
6	__run_exit_handlers	exit.c	78	0x00007ffff55ad045	
7	exit	exit.c	100	0x00007ffff55ad095	
8	fsElement::fsElement	fselement.cpp	41	0x000000000040b266	
9	fsElement::populateDirList	fselement.cpp	65	0x000000000040b70b	
10	fsElement::findFilesAndDirs	fselement.cpp	120	0x000000000040be77	
11	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
12	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
13	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
14	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
15	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
16	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
17	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
18	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
19	QtConcurrent::VoidStoredMemberFunctionPointerCall0<void, fsElement>::runFunctor	qtconcurrentstoredfunctioncall.h	216	0x000000000040adc4	
20	QtConcurrent::RunFunctionTask<void>::run	qtconcurrentrunbase.h	120	0x000000000040a396	
21	QThreadPoolThread::run	qthreadpool.cpp	106	0x00007ffff6a1ca4f	
22	QThreadPrivate::start	qthread_unix.cpp	248	0x00007ffff6a26a25	
23	start_thread	pthread_create.c	301	0x00007ffff608fc1a	
24	clone	clone.S	115	0x00007ffff5649a9d

0	pthread_cond_wait	pthread_cond_wait.S	162	0x00007ffff609459c	
1	wait	qwaitcondition_unix.cpp	87	0x00007ffff6a27a7b	
2	QWaitCondition::wait	qwaitcondition_unix.cpp	159	0x00007ffff6a27a7b	
3	QThreadPoolPrivate::waitForDone	qthreadpool.cpp	295	0x00007ffff6a1c808	
4	QThreadPool::~QThreadPool	qthreadpool.cpp	428	0x00007ffff6a1cfff	
5	QGlobalStaticDeleter<QThreadPool>::~QGlobalStaticDeleter	qglobal.h	1796	0x00007ffff6a1d695	
6	__run_exit_handlers	exit.c	78	0x00007ffff55ad045	
7	exit	exit.c	100	0x00007ffff55ad095	
8	fsElement::fsElement	fselement.cpp	41	0x000000000040b266	
9	fsElement::populateDirList	fselement.cpp	65	0x000000000040b70b	
10	fsElement::findFilesAndDirs	fselement.cpp	120	0x000000000040be77	
11	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
12	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
13	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
14	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
15	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
16	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
17	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
18	fsElement::findFilesAndDirs	fselement.cpp	122	0x000000000040be9c	
19	QtConcurrent::VoidStoredMemberFunctionPointerCall0<void, fsElement>::runFunctor	qtconcurrentstoredfunctioncall.h	216	0x000000000040adc4	
20	QtConcurrent::RunFunctionTask<void>::run	qtconcurrentrunbase.h	120	0x000000000040a396	
21	QThreadPoolThread::run	qthreadpool.cpp	106	0x00007ffff6a1ca4f	
22	QThreadPrivate::start	qthread_unix.cpp	248	0x00007ffff6a26a25	
23	start_thread	pthread_create.c	301	0x00007ffff608fc1a	
24	clone	clone.S	115	0x00007ffff5649a9d

To copy to clipboard, switch view to plain text mode 

So I guess I shouldn't use exit() in this fashion. Is there some other way to exit a thread in this situation?

Bonus, longer question: I just want a way to perform some heavy i/o work and still keep the GUI up to date and responsive (I'm writing a file searching app as an exercise). Doing it in a seperate thread worked quite well, but I'd also like to have the ability to pause and cancel the work. Is threading the best way to do it? What should I do to do it right? QThread? QRunnable? Or is QtConcurrent the best way?

[wysota]

[wiki]Keeping the GUI Responsive[/wiki]

[pmitas]

Great article, thanks. But what about my first question? Is there no way to exit a process in a similar way?

[wysota]

You want to exit the whole process or just the thread?

[pmitas]

Oops. Yeah, whenever I said "process" I meant "thread" :P

[wysota]

Just return from your concurrent function then. You cannot abort a running concurrent function from outside - it could render your application in an unpredictable state. If you want to be able to abort an inprogress operation, you have to use some other approach (i.e. the "solving the problem step by step" one from the article - either in an external thread or in the main thread).

[pmitas]

Just return from your concurrent function then.

It's a multi-branch recursive function, returning to the bottom wouldn't be easy.

You cannot abort a running concurrent function from outside - it could render your application in an unpredictable state.

I'm not - I'm trying to abort it from the inside at a safe point in response to a flag being set.

To sum things up, I just want to be able to call a function from a thread that would make the thread just drop everything and stop messing with my data structures Can I do that?

[wysota]

You can throw exceptions from your function and catch them at the upper-most level and then return from the thread function.

[pmitas]

Good suggestion. Thanks again.