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Thread: Thread mutex lock and return value

  1. #1
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    Default Thread mutex lock and return value

    Hi,

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

    Qt Code:
    1. bool myThread::getResult()
    2. {
    3. m_qMutex.lock();
    4. return m_qQueue.dequeue();
    5. m_qMutex.unlock(); //This will never be executed because of return statment, and will cause a deadlock
    6. }
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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,
    Òscar Llarch i Galán

  2. #2
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    Default Re: Thread mutex lock and return value

    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:
    Qt Code:
    1. reentrantFunction()
    2. {
    3. //good
    4. m_muext.lock()
    5. m_data = some_data;
    6. m_mutex.unlock()
    7.  
    8. //bad
    9. m_mutex.lock();
    10. someFunc();
    11. m_mutex.unlock()
    12. }
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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)
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  3. #3
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    Default Re: Thread mutex lock and return value

    Qt Code:
    1. m_qMutex.lock();
    2. rc = m_qQueue.dequeue();
    3. m_qMutex.unlock();
    4. return rc;
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  4. The following user says thank you to pitonyak for this useful post:

    ^NyAw^ (12th February 2010)

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    Default Re: Thread mutex lock and return value

    Hi,

    Thanks,

    pitonyak solution is good.
    Òscar Llarch i Galán

  6. #5
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    Default Re: Thread mutex lock and return value

    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.
    ==========================signature=============== ==================
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    Do you write clean code? - if you are TDD'ing then maybe, if not, your not writing clean code.

  7. The following user says thank you to high_flyer for this useful post:

    ^NyAw^ (12th February 2010)

  8. #6
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    Default Re: Thread mutex lock and return value

    Also possible:
    Qt Code:
    1. QMutexLocker locker(&m_qMutex);
    2. return m_qQueue.dequeue();
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