Std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit (3) Linux Manual Page

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

Synopsis

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.

Parameters

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);
  }

Example

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) {
        t.join();
    }
}

Output:

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

std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit (3) Linux Manual Page