How 'undefined' a race condition can be?

Question

Let\'s say I define a following C++ object:

class AClass
{
public:
    AClass() : foo(0) {}
    uint32_t getFoo() { return foo; }
    void changeFoo() { foo          


        

Answer 1

In practice, you will not see anything else than 0 or 5 as far as I know (maybe some weird 16 bits architecture with 32 bits int where this is not the case).

However whether you actually see 5 at all is not guaranteed.

Suppose I am the compiler.

I see:

while (aObject.getFoo() == 0) {
    printf("Sleeping");
    sleep(1);
}

I know that:

• printf cannot change aObject
• sleep cannot change aObject
• getFoo does not change aObject (thanks inline definition)

And therefore I can safely transform the code:

while (true) {
    printf("Sleeping");
    sleep(1);
}

Because there is no-one else accessing aObject during this loop, according to the C++ Standard.

That is what undefined behavior means: blown up expectations.