Sequential consistency
- C++ course
- Expert
- Lesson 15/47
- 5 min
Learn sequential consistency and other memory orders.
Sequential consistency
Sequential consistency (seq_cst) is the strongest memory ordering in C++ and the default for std::atomic operations. All threads see all operations in a single consistent order.
Example: x.store(1) uses std::memory_order_seq_cst by default. This strong guarantee carries a performance cost; but the program behavior is the easiest to reason about. On high-latency or many-core systems, weaker orderings may be preferred.
Total order
seq_cst operations form a single total order across all threads. Every thread sees the same order; no contradictory observations occur. This makes the program easy to reason about.
The cost is performance: the processor must wait for all previous writes to become visible. So weaker orderings (release-acquire or relaxed) are used only where they suffice. Wrong use can lead to races or inconsistent results.
Practice Write an atomic store with seq_cst
Define a global std::atomic<int> x{0};. In main write x.store(5); (default seq_cst) and std::cout << x.load();.
Starter code
#include <iostream>
#include <atomic>
// Define the x atomic here
int main() {
// Store a value and load it
return 0;
}Sample solution
#include <iostream>
#include <atomic>
std::atomic<int> x{0};
int main() {
x.store(5);
std::cout << x.load();
return 0;
}Expected output
5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each ordering to its strength
| Term | Meaning |
|---|---|
memory_order_seq_cst |
Default, strongest |
memory_order_relaxed |
Weakest |
memory_order_acquire |
Read side |
memory_order_release |
Write side |
2 Complete the seq_cst store
x.store(1, std::memory_order_____);x.store(1, std::memory_order_seq_cst);Explanation: seq_cst is the default and strongest ordering.
3 What is the default memory order for std::atomic operations?
x.store(1); // default- memory_order_acquire
- memory_order_relaxed
- memory_order_seq_cst Correct answer
- memory_order_release
Explanation: seq_cst is the default ordering; it gives the strongest guarantee.