Atomics

Some embedded and systems bugs are hard to reproduce: adding a log makes them disappear, disabling optimization hides them, changing CPU architecture exposes them, or multicore load makes them fail occasionally.

The first reaction is often to add volatile.

But volatile is not a thread synchronization primitive. It is not a memory barrier, and it is not a lock. It can prevent the compiler from optimizing away certain accesses, but it does not guarantee multicore visibility order, and it does not turn x++ into an atomic operation.