Acquire-release
- C++ course
- Expert
- Lesson 17/47
- 5 min
Learn acquire-release semantics.
Acquire-release
The acquire-release model provides synchronization cheaper than seq_cst. It gives ordering only between paired release stores and acquire loads; it does not guarantee a total order.
One thread does flag.store(true, std::memory_order_release); the other, on seeing true via flag.load(std::memory_order_acquire), sees all writes the first thread did before the release. This is the common publish-then-observe pattern.
Publish-then-observe
A common use of acquire-release is the publish-then-observe pattern. Thread A prepares payload, then publishes flag with release. Thread B reads flag with acquire; on seeing true, payload is ready.
The difference from seq_cst: only this matched pair is ordered; other threads may see different order. But for most cases this is enough; performance is better.
Practice Write publish-observe with acquire-release
Define a global std::atomic<bool> flag{false}; and int payload = 0;. In main write payload = 99;, then flag.store(true, std::memory_order_release);, then bool v = flag.load(std::memory_order_acquire); and std::cout << v;.
Starter code
#include <iostream>
#include <atomic>
// Define the flag and payload variables here
int main() {
// Set payload, publish flag, load and print
return 0;
}Sample solution
#include <iostream>
#include <atomic>
std::atomic<bool> flag{false};
int payload = 0;
int main() {
payload = 99;
flag.store(true, std::memory_order_release);
bool v = flag.load(std::memory_order_acquire);
std::cout << v;
return 0;
}Expected output
1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the release store
flag.store(true, std::memory_order_____);flag.store(true, std::memory_order_release);Explanation: release marks the writing side.
2 Match each ordering to its role
| Term | Meaning |
|---|---|
store + release |
Write publishes |
load + acquire |
Read sees publishes |
relaxed |
No ordering guarantee |
seq_cst |
Total order guarantee |
3 What guarantee does the acquire-release model provide?
flag.store(true, release);
bool v = flag.load(acquire);- Ordering between paired write-read Correct answer
- Total order across all threads
- No ordering guarantee
- Only single-thread order
Explanation: Acquire-release gives ordering only between paired release-acquire operations.