Calling non-reentrant code from Multiple threads.
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My GUI thread creates two threads of type "MyThread" which is a QThread subclass. The run function of these threads uses a non-reentrant function call. I want to make sure that this function call is used in a serialized manner, i.e only one of the instances of MyThread makes this call, the other waits for the function call to return. I wrote the run function as below(sMutex is a static member of MyThread):
@void MyThread::run()
{
sMutex.lock();
NonReentrantFuncCall();
sMutex.unlock();emit done();
}@
This does not work for me. Is this the right approach? -
Hello,
take a look at "Thread-Safety":http://doc.qt.nokia.com/4.7/threads-reentrancy.html. I think it will help you :) -
Hi,
By "not work for me", which part doesn't work? The locking around the function looks ok, although I agree with @maxpayne the locking should be inside the function. You might have problems with the emit if you don't have an event loop running for the thread. I think you need to have an exec() in the run function somewhere.
Steve
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Side note: if you're using the Qt naming convention, "non reentrant" means that it's not safe to call from multiple threads at all, no matter of serializing accesses (to shared data) using mutexes.
http://developer.qt.nokia.com/wiki/Threads_Events_QObjects#ee68e5b99222bbc29a480fcb0d1d6ee2
http://doc.qt.nokia.com/latest/threads-reentrancy.html -
[quote author="Gerolf" date="1305893735"]emitting signals is no problem, but the connect statement must be correct (QueuedConnection!).
Please show us some more code (creation and conbnect of the thread) and describe your problem a bit more precisely...[/quote]@// GUI thread
MyThread *thread = new MyThread();connect(thread, SIGNAL(done()),
this, SLOT(Loaded()));thread->start();
@
@void MyThread::run()
{
sMutex.lock();
NonReentrantFuncCall();
sMutex.unlock();emit done();
this->exec();
}@Since the emit is in a different thread than the receiver and I have created an automatic connection, I need the event loop. But the above signal does not reach the slot. Is it due to the fact that the exec call is after the emit? But I can not move it above the emit as the emit will never be called. Also when do I call the exit().
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[quote author="spsingh" date="1305899696"]
Since the emit is in a different thread than the receiver and I have created an automatic connection, I need the event loop. But the above signal does not reach the slot. Is it due to the fact that the exec call is after the emit? But I can not move it above the emit as the emit will never be called. Also when do I call the exit().
[/quote]Wrong, I think. The emitting object has the same thread affinity as your receiving GUI thread, no matter if the emit was triggered from another thread. That is why the reimplement-QThread method is trickly when using signals and slots.
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[quote author="spsingh" date="1305899696"]
Since the emit is in a different thread than the receiver and I have created an automatic connection, I need the event loop. But the above signal does not reach the slot. Is it due to the fact that the exec call is after the emit? But I can not move it above the emit as the emit will never be called. Also when do I call the exit().
[/quote]The snippet is fine, but
you need an event loop in the receiver's thread, NOT in that thread. Calling exec() there seems wrong
what does NonReentrantFuncCall(); do exactly?
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[quote author="Andre" date="1305925948"]
Wrong, I think. The emitting object has the same thread affinity as your receiving GUI thread, no matter if the emit was triggered from another thread. That is why the reimplement-QThread method is trickly when using signals and slots. [/quote]Nope. It works that way. See http://developer.qt.nokia.com/wiki/Threads_Events_QObjects#913fb94dd61f1a62fc809f8d842c3afa the note about the wrong documentation.
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[quote author="peppe" date="1305941696"]
[quote author="Andre" date="1305925948"]
Wrong, I think. The emitting object has the same thread affinity as your receiving GUI thread, no matter if the emit was triggered from another thread. That is why the reimplement-QThread method is trickly when using signals and slots. [/quote]Nope. It works that way. See http://developer.qt.nokia.com/wiki/Threads_Events_QObjects#913fb94dd61f1a62fc809f8d842c3afa the note about the wrong documentation.
[/quote]
I stand corrected. Thanks.