Question

If I don’t care whether Thread1 changes Flag1 at the same time Thread2 changes Flag1, is there anything else to worry about besides logic errors? Will it cause a crash etc if:

1. Thread1 and Thread2 read Flag1 at the exact same time?
2. Thread1 is writing to Flag1 at the same time as Thread2 is reading Flag1?

In these examples, Flag1 is a bool.

Answer 1

According to the rules of the C++11 memory model:

1. Thread1 and Thread2 read Flag1 at the exact same time? This is always safe.
2. Thread1 is writing to Flag1 at the same times as Thread2 is reading flag1? This is a data race.

A data race is undefined behaviour. Although it’s unlikely to crash on any sane hardware, it’s undefined behaviour, so anything could happen.