C++ Multithread with global variables

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.

Answer 2

No, it's not. It may be, depending on the implementation, the compile-time options and even the phase of the moon.

But the standard doesn't mandate something is thread-safe, specifically because there's nothing about threading in the current standard.

See also here for a more detailed analysis of this sort of issue.

If you're using an environment where threads are supported, you can use a mutex: examine the pthread_mutex_* calls under POSIX threads, for example.

If you're coding for C++0x/C++11, use either a mutex or one of the atomic operations detailed in that standard.