Why is volatile keyword not needed for thread synchronisation?

Question

I am reading that the volatile keyword is not suitable for thread synchronisation and in fact it is not needed for these purposes at all.

While I unders

Answer 1

The keyword volatile tells the compiler to treat any write or read of the variable as an "observable side-effect." That is all it does. Observable side-effects of course must not be optimized away, and must appear to the outside world as occurring in the order the program indicates; The compiler may not re-order observable side-effects with regard to each other. The compiler is however free to reorder them with respect to non-observables. Therefore, volatile is only appropriate for accessing memory-mapped hardware, Unix-style signal handlers and the like. For inter-thread concurrence, use std::atomic or higher level synchronization objects like mutex, condition_variable, and promise/future.