C++
atomic flag
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.
Exercise 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).
Show example 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;
}
Test yourself
-
Match each expression to its role
- 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
-
Complete the test_and_set line
std::atomic_flag f = ATOMIC_FLAG_INIT;
bool was_set = f.____();
Show answer
std::atomic_flag f = ATOMIC_FLAG_INIT;
bool was_set = f.test_and_set();
-
Which lock is built with atomic_flag?
- Mutex
- Recursive lock
- Semaphore
- Spinlock
Show answer
Spinlock — atomic_flag is ideal for building a spinlock.
Next lesson: Fetch and add
