Thread mutex lock and return value

[^NyAw^]

Hi,

I have this situation:
One thread class that needs to protect a QQueue but also needs to return dequeued value.

bool myThread::getResult()
{
  m_qMutex.lock();
  return m_qQueue.dequeue();
  m_qMutex.unlock();    //This will never be executed because of return statment, and will cause a deadlock
}

bool myThread::getResult()
{
  m_qMutex.lock();
  return m_qQueue.dequeue();
  m_qMutex.unlock();    //This will never be executed because of return statment, and will cause a deadlock
}

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So, how can I do this in a correct way. One solution is to get the mutex of the thread from the caller object and lock and unlock there but I want to know if there is a more elegant solution.

Thanks,

[high_flyer]

In general it is a VERY bad and dangerous practice to lock mutex and call other methods/functions, since you never know if another mutex will be locke there too.
You should only lock local data.

Like so:

reentrantFunction()
{
//good
 m_muext.lock()
 m_data = some_data;
 m_mutex.unlock()
 
//bad
m_mutex.lock();
someFunc();
m_mutex.unlock()
}

reentrantFunction()
{
//good
 m_muext.lock()
 m_data = some_data;
 m_mutex.unlock()

//bad
m_mutex.lock();
someFunc();
m_mutex.unlock()
}

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In the case you posted, m_qQueue.dequeue(); has to be re-entrant it self, which means there should the mutex be locked and released and not in getResult(). ( as you can see, another good reason not to lock mutex before function calls)

[pitonyak]

m_qMutex.lock();
rc = m_qQueue.dequeue();
m_qMutex.unlock();
return rc;

m_qMutex.lock();
rc = m_qQueue.dequeue();
m_qMutex.unlock();
return rc;

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[^NyAw^]

Hi,

Thanks,

pitonyak solution is good.

[high_flyer]

indeed, I over looked the fact you are using a "standard" class, basically as a type - sorry for that.
But be careful in any other cases with locking mutexes before function calls.

[franz]

Also possible:

QMutexLocker locker(&m_qMutex);
return m_qQueue.dequeue();

QMutexLocker locker(&m_qMutex);
return m_qQueue.dequeue();

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