Random function call from multiple threads in Qt/C++

Question

I have a multi-thread QT application that sometimes need a random alphanumeric string from one of its threads (some threads start at application startup, others start or die during lifetime), and I would like to obtain that by calling a function defined in a common header, to avoid code replication.

Here there's a code snippet:

QString generateRandomAlphanumericString(int length)
{
    qsrand(static_cast<uint>(QTime::currentTime().msec())); //bad
    QString randomAS = QString();

    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";

    for (int i = 0; i < length; ++i)
        randomAS[i] = alphanum[qrand() % (sizeof(alphanum) - 1)];

    return randomAS;
}

I initially did some mistakes.

At the beginning I called qsrand(static_cast<uint>(QTime::currentTime().msec())); in the main function, but I've learned that it should be done per-thread.

Then I put the qsrand call in the function above, but it's not correct.

Please consider that at program startup many threads start "together", so if I initialize the seed with current time in msec the seed is the same among them.

Is there a way to change that function accordingly without modify all points in my application where a thread starts its life? Any implementation done in pure C++ (without the use of QT) is fine. Could the new random C++11 library help in some way to achieve my task?

I would like to know why you think `qsrand` should be called on each thread? (since i'm not familiar to QT) — apple apple, Jan 25 '18 at 14:55
I suppose you could use `QThreadStorage` to have a per-thread flag indicating whether `qsrand` has been called on this thread yet. If in fact `qsrand` needs to be called on every thread. — Igor Tandetnik, Jan 25 '18 at 14:57
`so if I initialize the seed with current time in msec the seed is the same among them.` you don't have to give it the time; you can always give it a time + offset and adjust the offset for each thread... but I don't think this is really related to your issue — UKMonkey, Jan 25 '18 at 15:00
`Could the new random C++11 library help in some way to achieve my task?` yes; but you'd have to research and try it. — UKMonkey, Jan 25 '18 at 15:01
@appleapple: I've edit my question with a link. It's not related to QT — ABCplus, Jan 25 '18 at 15:07
@ABCplus your update contains link about `std::srand`, not `qsrand` as you mentioned. — apple apple, Jan 25 '18 at 15:09
if you really need/want to seed it, here is some hint: you can always seed a random with values other than current time. — apple apple, Jan 25 '18 at 15:13
@appleapple `qsrand` calls `srand` apart of Android environment. — ABCplus, Jan 25 '18 at 15:16
@IgorTandetnik: could help, but 2 threads started one after the other could have the same seed if it's generated using `QTime::currentTime().msec()`. Maybe I should add an offset as @UKMonkey suggested....thread ID?! — ABCplus, Jan 25 '18 at 15:20
@ABCplus as I mentioned in my previous comment, you can use any value as seed. And there is a more serious problem, you should not seed RNG every time you call `generateRandomAlphanumericString`, especially when you seed them with current time. — apple apple, Jan 25 '18 at 15:24
@appleapple: I understand, but 2 threads with the same seed generate the same random numbers. So, time in msec cannot be used since if they threads start "together" the seed is the same. I fully agree with seed generated every time in my function, in fact is marked as "bad" in inline comment — ABCplus, Jan 25 '18 at 15:35
1. seed RNG with something other than time. 2. seed them at start of thread. (if needed). — apple apple, Jan 25 '18 at 15:39

score 0 · Answer 1 · answered Jan 26 '18 at 06:49

void InitSeedForThread(uint globalSeed, int myThreadIndex)
{
    qsrand(globalSeed);
    for (int i = 0; i < myThreadIndex; ++i)
        qrand();
}

auto GetRandom(int numThreads)
{
    for (int i = 0; i < numThreads - 1)
        qrand();
    return qrand();
}

Given an ordered list of numbers A, B, C, D, E, F, G, H, ... splits it into n lists. If n was 4, you would get

1. A, E, I, ...
2. B, F, J, ...
3. C, G, K, ...
4. D, H, L, ...

Con: Doing RNG is somewhat expensive, and you're repeating a lot of work. However, since you're doing QT (UI-bound) I'm assuming that performance isn't an issue.

Alternatively, you could do a global random function with a mutex, but that ain't free either.

score 0 · Accepted Answer · answered Jan 26 '18 at 10:27

I finally found a good solution (thanks everybody who has contributed with comments):

enum ThreadData {TD_SEED};
static QThreadStorage<QHash<ThreadData, uint> *> cache;

inline void insertIntoCache(ThreadData data, uint value)
{
    if (!cache.hasLocalData())
        cache.setLocalData(new QHash<ThreadData, uint>);
    cache.localData()->insert(data, value);
}

inline void removeFromCache(ThreadData data)
{
    if (cache.hasLocalData())
        cache.localData()->remove(data);
}

inline bool hasInCache(ThreadData data)
{
    if (!cache.hasLocalData()) return false;
    return cache.localData()->contains(data);
}

inline uint getCachedData(ThreadData data)
{
    if (cache.hasLocalData() && cache.localData()->contains(data))
        return cache.localData()->value(data);
    return 0;
}

inline int getThRandom()
{
    uint seed = 0;
    if (!hasInCache(TD_SEED))
    {
        seed = QDateTime::currentMSecsSinceEpoch() % 100000000;
#ifdef Q_OS_WIN     
        seed += GetCurrentThreadId();
#else
        seed += QThread::currentThreadId();
#endif
        qsrand(static_cast<uint>(seed));
        insertIntoCache(TD_SEED, seed);
    }
    else {
        seed = getCachedData(TD_SEED);      
    }

    return qrand();
}

Basically, as suggested by Igor I've made use of QThreadStorage to store a seed for each thread. I've used an hash for future extensions. Then, I've made use of QDateTime::currentMSecsSinceEpoch() instead of QTime::currentTime().msec() to have a different number across multiple application starts (if for example the random generated value is stored in a file/db and should be different). Then, I've add an offset, as suggested by UKMonkey, using the thread ID.

So, my original function will be:

QString generateRandomAlphanumericString(int length)
{
    QString randomAS = QString();

    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";

    for (int i = 0; i < length; ++i)
        randomAS[i] = alphanum[getThRandom() % (sizeof(alphanum) - 1)];

    return randomAS;
}

I've run some tests, producing from different threads thousand of alphanumeric strings, storing them to multiple files and double checked for duplicates among them and between multiple application run.

Random function call from multiple threads in Qt/C++

2 Answers2