Public functions in a QThread class: safe?

[dmginc]

The question I am about to ask was actually asked in this thread: http://www.qtforum.org/article/32113/qthread-questions.html

But wasn't really properly answered...

Here is some code to explain the question:

Subclassed from QThread:

thread::run()
{
while(1)
{
if(someCondition)
privateMember++;
}
}


thread::getPrivateMember()
{
return privateMember;
}

Q: Is it necessary to protect privateMember with mutexes in the above case [as in a lock()/unlock() pair in run() and a QMutexLocker in getPrivateMember()] ?

OR, is privateMember automatically protected seeing as the "get" function is a member function of a QThread-derived class. In other words, run() and getPrivateMember() will never run at the same time and are thus protected?

Can someone shed some light on this?

Cheers

[helloworld]

1. It's not safe, since objects with different thread affinity can call getPrivateMember().

2. You should use a wait condition instead of busy looping.
QWaitCondition


3. I would recommend that you instead subclass QObject, create a QThread instance and use moveToThread() to set the thread affinity of your object, as mentioned in this article:

http://labs.qt.nokia.com/2010/06/17/youre-doing-it-wrong/

With this approach + using signal and slot connections you can avoid a lot of these issues.



Usually, this means simply changing your class to inherit from QObject instead of QThread and, possibly, changing the class name. QThread has a started() signal that you can connect to when you need to perform some initialization. To actually have your code run in the new thread context, you need to instantiate a QThread and assign your object to that thread using the moveToThread() function.

[high_flyer]

Q: Is it necessary to protect privateMember with mutexes in the above case [as in a lock()/unlock() pair in run() and a QMutexLocker in getPrivateMember()] ?
There is a simple rule to it:
If the variable can be accessed by at least two threads at the same time, it has to be guarded by a mutex, or semaphore or similar mutual exclusion mechanism.
This has nothing to do with variable access privileges (public/protected/private).

[helloworld]

On second thought, a wait condition is perhaps not what you need in this case. But busy looping is still a bad idea, and you can still use above mentioned approach taking advantage of Qts signal/slots mechanism instead.

[wysota]

If privateMember is of type QAtomicInt, it doesn't have to be protected. If it is a regular int, all what has already been said here applies.

[dmginc]

If privateMember is of type QAtomicInt, it doesn't have to be protected. If it is a regular int, all what has already been said here applies.

Thanks for that. This is the answer to the question I wanted to know :)


On second thought, a wait condition is perhaps not what you need in this case. But busy looping is still a bad idea, and you can still use above mentioned approach taking advantage of Qts signal/slots mechanism instead.

Perhaps the "if" condition in my example code wasn't the best choice. I merely wanted to show a private member of the class being altered in run().


1. It's not safe, since objects with different thread affinity can call getPrivateMember().


So basically, when I call the "get" public function from another thread, it is executed at the same time as run() (that's what I wasn't sure of..)? In other words, run() does not block until the "get" function returns?

Thanks again.

[wysota]

So basically, when I call the "get" public function from another thread, it is executed at the same time as run() (that's what I wasn't sure of..)?
Not necessarily but there is no guarantee that it doesn't.


In other words, run() does not block until the "get" function returns?
It's just a function call like any other. Why would anything block? Especially that the QThread object lives in the same thread as the method calling it.