[Qt-interest] ASSERT failure in QMutex::unlock(): "A mutex must be unlocked in the same thread that locked it."

[Sarvesh Saran]

Hi,

Sorry for the poor indentation. Let me rephrase:

I am simply having a tough time understanding how signals and slots work across threads.

Assume that I have a MyThread:QThread object called A running and it contains a public slot setValue().

//this code is in the main window
MyThread *a = new MyThread; //create the thread
connect(this,sendValue(int),a,setValue(int)));

later in the code I emit a signal:
emit sendValue(10); 

I would like to know how this emit is different from:
a->setValue(10) ?

in either case, who executes the slot setValue? Is it the MainWindow thread  or MyThread?

Thanks,
Sarvesh




-----Original Message-----
From: qt-interest-bounces at trolltech.com [mailto:qt-interest-bounces at trolltech.com] On Behalf Of John McClurkin
Sent: Friday, July 16, 2010 6:20 PM
To: qt-interest at trolltech.com
Subject: Re: [Qt-interest] ASSERT failure in QMutex::unlock(): "A mutex must be unlocked in the same thread that locked it."

Sarvesh Saran wrote:
> Hi All,
> 
>  
> 
> As a multi threading newbie I have been struggling with this particular 
> error message for some time now:
> 
>  
> 
> ASSERT failure in QMutex::unlock(): "A mutex must be unlocked in the 
> same thread that locked it."
> 
>  
> 
> It pops up when I exit my application._How do I begin to debug this? _
> 
>  
> 
> I also have a question about queued connections between objects in QT.
> 
> Assume I have a MY Thread object  :
> 
>  
> 
> MYClass::startAthread()                   
>                                                             
>                                                                 
>                 MyThread::setValue(int val)
> 
> {                                    
> 
> {                                                                                                                                                                                                              
>                 mutex.lock();
> 
> MyThread a = new MyThread(this); //this inherits 
> QThread                                                                                                         
> _val = val;
> 
>  
> 
> Connect(this,SIGNAL(sendValue(int)),a,SLOT(setValue(a)));                                                                                       
>                 mutex.unlock();  //I suspect the error is somewhere here....
> 
>                                                                                                                                                                                                                 
> }
> 
> a->start();
> 
> }

This is all very hard to understand since the indentation is all over 
the map.

> 
>  
> 
> And then later in the code I emit the signal:

Later in which code? What class is the sendValue signal a member of?
Only the class that owns a signal can emit the signal.

> 
>  
> 
> emit sendValue(10);  //who executes setValue slot? Is it the MyCLass 
> object or is it the MyThread object a ?)
> 
>  
> 
> if I instead directly call :
> 
>  
> 
> sendValue(10); //no emit, is this different from the above?
> 
>  
> 
> Kindly help me out here..
> 
>  
> 
> Thanks,
> 
> Sarvesh
> 
>  
> 
>  
> 
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