Concurrency: Atomic and volatile in C++11 memory model

Question

A global variable is shared across 2 concurrently running threads on 2 different cores. The threads writes to and read from the variables. For the atomic variable can one th

Answer 1

Here's a basic synopsis of what the 2 things are:

1) Volatile keyword:
Tells the compiler that this value could alter at any moment and therefore it should not EVER cache it in a register. Look up the old "register" keyword in C. "Volatile" is basically the "-" operator to "register"'s "+". Modern compilers now do the optimization that "register" used to explicitly request by default, so you only see 'volatile' anymore. Using the volatile qualifier will guarantee that your processing never uses a stale value, but nothing more.

2) Atomic:
Atomic operations modify data in a single clock tick, so that it is impossible for ANY other thread to access the data in the middle of such an update. They're usually limited to whatever single-clock assembly instructions the hardware supports; things like ++,--, and swapping 2 pointers. Note that this says nothing about the ORDER the different threads will RUN the atomic instructions, only that they will never run in parallel. That's why you have all those additional options for forcing an ordering.