std::atomic_exchange,std::atomic_exchange_explicit (3) - Linux Manuals

std::atomic_exchange,std::atomic_exchange_explicit: std::atomic_exchange,std::atomic_exchange_explicit

NAME

std::atomic_exchange,std::atomic_exchange_explicit - std::atomic_exchange,std::atomic_exchange_explicit

Synopsis

Defined in header <atomic>
template< class T >
T atomic_exchange( std::atomic<T>* obj,
typename std::atomic<T>::value_type desr ) noexcept;
template< class T >
T atomic_exchange( volatile std::atomic<T>* obj,
typename std::atomic<T>::value_type desr ) noexcept;
template< class T > (1) (since C++11)
T atomic_exchange_explicit( std::atomic<T>* obj,
typename std::atomic<T>::value_type desr,
std::memory_order order ) noexcept; (2) (since C++11)
template< class T >
T atomic_exchange_explicit( volatile std::atomic<T>* obj,
typename std::atomic<T>::value_type desr,
std::memory_order order ) noexcept;

1) Atomically replaces the value pointed to by obj with the value of desr and returns the value obj held previously, as if by obj->exchange(desr)
2) Atomically replaces the value pointed to by obj with the value of desr and returns the value obj held previously, as if by obj->exchange(desr, order)

Parameters

obj - pointer to the atomic object to modify
desr - the value to store in the atomic object
order - the memory synchronization ordering for this operation: all values are permitted.

Return value

The value held previously by the atomic object pointed to by obj

Example

A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to std::atomic_flag_test_and_set:
// Run this code

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

  std::atomic<bool> lock(false); // holds true when locked
                                 // holds false when unlocked

  void f(int n)
  {
      for (int cnt = 0; cnt < 100; ++cnt) {
          while(std::atomic_exchange_explicit(&lock, true, std::memory_order_acquire))
               ; // spin until acquired
          std::cout << "Output from thread " << n << '\n';
          std::atomic_store_explicit(&lock, false, 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>...

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P0558R1 C++11 exact type match required because T is deduced from multiple arguments T is deduced from the atomic argument only