std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit (3) - Linux Manuals

std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit: std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit


std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit - std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit


Defined in header <atomic>
bool atomic_flag_test_and_set( volatile std::atomic_flag* p ) noexcept;
bool atomic_flag_test_and_set( std::atomic_flag* p ) noexcept;
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p, (1) (since C++11)
std::memory_order order ) noexcept; (2) (since C++11)
bool atomic_flag_test_and_set_explicit( std::atomic_flag* p,
std::memory_order order ) noexcept;

Atomically changes the state of a std::atomic_flag pointed to by p to set (true) and returns the value it held before.


p - pointer to std::atomic_flag to access
order - the memory synchronization order for this operation

Return value

The value previously held by the flag pointed to by p

Possible implementation

First version

  bool atomic_flag_test_and_set(volatile std::atomic_flag* p)
      return p->test_and_set();

Second version

  bool atomic_flag_test_and_set(std::atomic_flag* p)
      return p->test_and_set();

Third version

  bool atomic_flag_test_and_set_explicit(volatile std::atomic_flag* p,
                                         std::memory_order order)
      return p->test_and_set(order);

Fourth version

  bool atomic_flag_test_and_set_explicit(std::atomic_flag* p,
                                         std::memory_order order)
      return p->test_and_set(order);


A spinlock mutex can be implemented in userspace using an atomic_flag
// Run this code

  #include <thread>
  #include <vector>
  #include <iostream>
  #include <atomic>

  std::atomic_flag lock = ATOMIC_FLAG_INIT;

  void f(int n)
      for (int cnt = 0; cnt < 100; ++cnt) {
          while(std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
               ; // spin until the lock is acquired
          std::cout << "Output from thread " << n << '\n';
          std::atomic_flag_clear_explicit(&lock, std::memory_order_release);

  int main()
      std::vector<std::thread> v;
      for (int n = 0; n < 10; ++n) {
          v.emplace_back(f, n);
      for (auto& t : v) {


  Output from thread 2
  Output from thread 6
  Output from thread 7
  ...<exactly 1000 lines>...

See also

atomic_flag the lock-free boolean atomic type

atomic_flag_clear_explicit atomically sets the value of the flag to false

memory_order defines memory ordering constraints for the given atomic operation