C++ access to Int variable from two threads. Thread safe?

Question

There is an int variable that holds balance.
Many threads reference this int variable.
Each thread does their job and can access to int pointer at any time and add some value.

Is it thread safe to do this? Or I should use mutex lock?

Here is example (it works without any issues, just need to be sure if forever):

#include <QCoreApplication>
#include <QThread>
#include <QDebug>
#include <QTimer>

int someInt=0;

class MyThread : public QThread
{
public:
    MyThread(){start();}
    void run()
    {
        for(int n=0;n<1000;n++)someInt-=1;
        for(int n=0;n<1000;n++)someInt+=2;
    }
};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    qDebug()<<"someInt="<<someInt;      
    QTimer::singleShot(5000,&a,SLOT(quit()));

    MyThread thread1;
    MyThread thread2;

    a.exec();

    //There always 2000 output, and there is no issue, 
    //just need to be sure if forever.
    qDebug()<<"someInt="<<someInt;

    return 0;
}

Give a short code sample (not the complete code), that shows what you are trying to do. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented May 28, 2014 at 17:49
Read the info page for multithreading: stackoverflow.com/tags/multithreading/info — Zan Lynx
– Zan Lynx, Commented May 28, 2014 at 17:51
Thank you. I'll read it and try to make some code for example. — IGHOR
– IGHOR, Commented May 28, 2014 at 17:54
Thanks. I'll use std::atomic<quint64> and std::atomic<double>. Looks like it works good only for "+=", "-=" etc. operators, but it useles if use someInt=someInt+1. And std::atomic<double> does not have this operators. But I know direction. Thanks for answers! — IGHOR
– IGHOR, Commented May 28, 2014 at 22:54

πάντα ῥεῖ · Accepted Answer · 2014-05-28 17:47:45Z

10

No, it's not thread-safe and you should either use a lock or atomic operations.

edited May 28, 2014 at 17:47

πάντα ῥεῖ

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answered May 28, 2014 at 17:42

Ulrich Eckhardt

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9 Comments

IGHOR Over a year ago

Is lock or atomic ooperations faster?

MSalters Over a year ago

@IGHOR: No simple answer. It may be faster to sum up the totals only in the end, and keep per-thread partial sums.

IGHOR Over a year ago

It not possible to keep peer partial sums, because each thread should get access to this int realtime.

Ulrich Eckhardt Over a year ago

The variable doesn't influence in any way how the two threads behave, so there is no reason not to update it only once. Unless of course there is a requirement that you haven't mentioned yet.

LtWorf Over a year ago

Go with atomic. If you don't use c++11, gcc implements them anyway with some special functions.

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