When (3 > 120000) resolves to TRUE (with unsigned longs, millis() and dark magic)

[rickkas7]

Using C++ std::atomic is a good way to prevent accessing a variable in the middle of its modification. There's a brief example here:

Volatile Structures and EEPROM

The limitation with volatile is how the FRAM library works. You can’t pass a volatile variable to it, because the FRAM library needs to obtain a pointer to it, which cause the variable to lose its volatile attribute. However, there’s a more fundamental reason why you can’t do that. Even though the variable is declared volatile, that only prevents the compiler from making assumptions about the variable, mostly with regards to optimization. It always assumes a side-effect has changed the value a…

And since it's a standard C++ library feature, there's a lot written about it online. It's a little confusing at first, but very powerful.