QMutex ,wait here if a thread is in function

Question

I just want to implement the code like below.

QString Class1::getNonce()
{
    //if some thread is getting nonce wait here until it finishes the its own job.
    mutex.lock();
    QString nonce=QString("%1").arg(QDateTime::currentDateTime().toTime_t());
    mutex.unlock();
    return nonce;    
}

even I write with mutex different threads get same nonce. How can I solve this problem? Thanks.

Is mutex a static? Or are you using only once instance of Class1? — drescherjm, Dec 20 '13 at 01:21
yes, it is in Class1 as QMutex mutex; and I just use mutex just this function. — Veysel Bekir Macit, Dec 20 '13 at 01:23
Maybe because `QDateTime::currentDateTime` has only millisecond resolution? — user703016, Dec 20 '13 at 01:31
that way,how can we ensure every threads that call this function to get different and always bigger nonce than older nonce? — Veysel Bekir Macit, Dec 20 '13 at 01:43
Do you have a c++11 compiler? c++11 has higher resolution time. — drescherjm, Dec 20 '13 at 01:46
@BMacit Different: possible if you append a random number to your string. Bigger: not possible without a shared counter or a clock with much higher resolution. — user703016, Dec 20 '13 at 02:12
@Purrfection: it's worse than `QDateTime::currentDateTime()` having only millisecond resolution because `QDateTime::toTime_t()` has a resolution of only **one second**. — Michael Burr, Dec 20 '13 at 02:25
@LaszloPapp: it's a value that will be used in only a single communication or session - it can help prevent replay attacks by ensuring that each separate communication is unique, even if the payload is the same. It's similar idea to a salt or initialization vector and used for similar reasons. See http://en.wikipedia.org/wiki/Cryptographic_nonce — Michael Burr, Dec 20 '13 at 08:42
When I was helping yesterday I thought that nonce was a word in some other language that meant timestamp or similar.. — drescherjm, Dec 20 '13 at 13:05

Casey · Answer 1 · 2013-12-20T17:00:17.353

2

Use an atomic counter for your nonce:

QString Class1::getNonce()
{
    static std::atomic<unsigned long long> counter;
    return QString::number(counter++);
}

edited Dec 20 '13 at 17:00

answered Dec 20 '13 at 04:48

Casey

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score 2 · Accepted Answer · answered Dec 20 '13 at 04:57

2

I prefer the use of a QMutexLocker.

Class1::Class1()
{
    m_mutex = new QMutex();

}

QString Class1::getNonce()
{
    static int counter = 0;
    QMutexLocker locker(m_mutex);
    counter++;
    return QString::number(counter);
}

Hope that helps.

answered Dec 20 '13 at 04:57

phyatt

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score 1 · Answer 3 · answered Dec 21 '13 at 21:00

Thank you for all messages I used a way like that

nonce=QDateTime::currentDateTime().toTime_t()+7500;

......

QString Class1::getNonce()
{
    QElapsedTimer timer;
    timer.start();

    mutex.lock();
    nonce+=timer.nsecsElapsed()/250;
    mutex.unlock();
    return QString("%1").arg(nonce);
}

QMutex ,wait here if a thread is in function

3 Answers3