Do I have to use atomic for “exit” bool variable?

Question

I need to set a flag for another thread to exit. That other thread checks the exit flag from time to time. Do I have to use atomic for the flag or just a plain bool is enoug

Answer 1

Yes, you must have some synchronization. The easiest way is, as you say, with atomic.

Formally, as @AndyProwl says, the language definition says that not using an atomic here gives undefined behavior. There are good reasons for that.

First, a read or write of a variable can be interrupted halfway through by a thread switch; the other thread may see a partly-written value, or if it modifies the value, the original thread will see a mixed value. Second, when two threads run on different cores, they have separate caches; writing a value stores it in the cache, but doesn't update other caches, so a thread might not see a value written by another thread. Third, the compiler can reorganize code based on what it sees; in the example code, if nothing inside the loop changes the value of exit, the compiler doesn't have any reason to suspect that the value will change; it can turn the loop into while(1).

Atomics address all three of these problems.