Bounds checks have nothing to do with data races. GP is right, you can add bounds checks. Either using macros or (in C++) with templates and operator overloading.
Alas, in C or C++ you have mutable aliasing, so I'm afraid you do incur a potential data race because your bounds might alias. Be careful out there.
Also remember that in C++ you may get a reference in these cases and if you keep that reference rather than using it immediately now you also have a potential TOCTOU race because the reference was only valid when you did the bounds check.
With mutable aliasing the length might change even though the data you care about did not, and so adding the check means incurring a race which did not previously exist and which certainly the naive C programmer cannot see...
We can definitely mitigate this in the type system for most real world scenarios, but you don't mitigate problems you don't know about, so knowing is what's important.
