atomic flag
- C++ course
- Advanced
- Lesson 28/83
- 5 min
Learn spinlock with atomic_flag.
atomic_flag
std::atomic_flag is the simplest atomic type. It has only two states: set and clear. It is guaranteed to be lock-free on all hardware (is_lock_free is always true). It is initialized to clear with ATOMIC_FLAG_INIT.
f.test_and_set() returns the previous state and sets the flag. f.clear() clears the flag. These two operations make atomic_flag ideal for a spinlock that loops while busy. Unlike other atomic types, atomic_flag has no load or store members.
Spinlock pattern
A spinlock is a lock that loops while waiting for the lock. The pattern while (f.test_and_set()) { /* wait */ } spins until it acquires the flag. When done, f.clear(); releases it. Unlike a mutex, the thread does not sleep; it spins.
Spinlocks are effective for short critical sections because there is no sleep/wake cost. For long critical sections they waste CPU; a mutex is preferred there. A spinlock built with std::atomic_flag is the most basic pattern of lock-free synchronization in the standard library.
Practice atomic_flag test
Initialize std::atomic_flag f = ATOMIC_FLAG_INIT;. Get the first state and set it with bool was_set = f.test_and_set();. Write std::cout << was_set << " " << f.test_and_set();: the previous state and the second call's result (now 1 since the flag is set).
Starter code
#include <iostream>
#include <atomic>
int main() {
// Initialize atomic_flag, print via test_and_set
return 0;
}Sample solution
#include <iostream>
#include <atomic>
int main() {
std::atomic_flag f = ATOMIC_FLAG_INIT;
bool was_set = f.test_and_set();
std::cout << was_set << " " << f.test_and_set();
return 0;
}Expected output
0 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each expression to its role
| Term | Meaning |
|---|---|
std::atomic_flag |
Simplest atomic type |
f.test_and_set() |
Return old state, set it |
f.clear() |
Clear the flag |
ATOMIC_FLAG_INIT |
Clear initialization |
2 Complete the test_and_set line
std::atomic_flag f = ATOMIC_FLAG_INIT;
bool was_set = f.____();std::atomic_flag f = ATOMIC_FLAG_INIT;
bool was_set = f.test_and_set();Explanation: f.test_and_set() returns the old state and sets the flag.
3 Which lock is built with atomic_flag?
- Mutex
- Recursive lock
- Semaphore
- Spinlock Correct answer
Explanation: atomic_flag is ideal for building a spinlock.