Question

Anyone know whether primitive global variable is thread safe or not?

// global variable
int count = 0;

void thread1()
{
   count++;
}

void thread2()
{
   count--;
   if (count == 0) print("Stuff thing");
}

Can I do it this way without any lock protection for count?

Thank you.

Answer 1

This is not threadsafe. You have a race-condition here. The reason for that is, that count++ is not necessarily atomic (means not a single processor operation). The value is first loaded, then incremented, and then written back. Between each of these steps, the other thread can also modify the value.