The relaxed semantics is the end of the scale. The relaxed semantic is the weakest of all memory models and guarantees that the operations on atomic variables are atomic.

Acquire and release fences guarantee similar synchronization and ordering constraints as atomics with acquire-release semantics. Similar because the differences are in the details.

The key idea of a std::atomic_thread_fence is to establish synchronization and ordering constraints between threads without an atomic operation.

A release operation synchronizes-with an acquire operation on the same atomic variable. So we can easily synchronise threads, if ... . Today's post is about the if.

std::memory_order_consume is the most legendary of the six memory models. That's for two reasons. On the one hand, std::memory_order_consume is extremely hard to get. On the other hand - that may change in the future - no compiler supports it.

A release operation synchronizes with an acquire operation on the same atomic variable and establishes, in addition, ordering constraints. These are the components to synchronize threads in a performant way in case they act on the same atomic. But how can that work, if two threads share no atomic variable? We want no sequential consistency because that is too heavy. We want the light acquire-release semantic.

With the acquire-release semantics, the memory model gets very thrilling. Because now, we do not have to reason about the synchronization of threads, we have to reason about the synchronization of the same atomic in different threads.

I have introduced In the post Sequential Consistency the default memory model. This model, in which all operations in all threads take place in a global time clock, has a big advantage but also a significant disadvantage.

In this post, our tour through the c++ memory model goes one step deeper. Until now, the posts were only about the atomicity of the atomic data types, but now we deal with the synchronization and ordering constraints of the operations.

In addition to booleans, there is atomics for pointers, integrals, and user-defined types. The rules for user-defined types are special.