std::atomic_exchange,std::atomic_exchange_explicit (3) Linux Manual Page

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

See also

                                               atomically replaces the value of the atomic object and obtains the value held previously
exchange (public member function of std::atomic<T>)
atomic_compare_exchange_weak
atomic_compare_exchange_weak_explicit
atomic_compare_exchange_strong
atomic_compare_exchange_strong_explicit atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not
                                               (function template)
(C++11)
(C++11)
(C++11)
(C++11)
                                               specializes atomic operations for std::shared_ptr
std::atomic_exchange(std::shared_ptr)_ (function template)
std::atomic_exchange_explicit(std::shared_ptr)