Yes there is tons of surprising and weird UB in C, but this article doesn't do a great job of showcasing it. It barely scratches the surface.
Here's a way weirder example:
volatile int x = 5;
printf("%d in hex is 0x%x.\n", x, x);
This is totally fine if x is just an int, but the volatile makes it UB. Why? 5.1.2.4.1 says any volatile access - including just reading it - is a side effect. 6.5.1.2 says that unsequenced side effects on the same scalar object (in this case, x) are UB. 6.5.3.3.8 tells us that the evaluations of function arguments are indeterminately sequenced w.r.t. each other.
So in common parlance, a "data race" is any concurrent accesses to the same object from different threads, at least one of which is a write. In C, we can have a data race on a single thread and without any writes!
Here's a way weirder example:
This is totally fine if x is just an int, but the volatile makes it UB. Why? 5.1.2.4.1 says any volatile access - including just reading it - is a side effect. 6.5.1.2 says that unsequenced side effects on the same scalar object (in this case, x) are UB. 6.5.3.3.8 tells us that the evaluations of function arguments are indeterminately sequenced w.r.t. each other.So in common parlance, a "data race" is any concurrent accesses to the same object from different threads, at least one of which is a write. In C, we can have a data race on a single thread and without any writes!