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std::atomic_exchange, std::atomic_exchange_explicit

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Defined in header <atomic>
template< class T >
T atomic_exchange( std::atomic<T>* obj, T desr );
(1) (since C++11)
template< class T >
T atomic_exchange( volatile std::atomic<T>* obj, T desr );
(1) (since C++11)
template< class T >

T atomic_exchange_explicit( std::atomic<T>* obj, T desr,

                            std::memory_order order );
(2) (since C++11)
template< class T >

T atomic_exchange_explicit( volatile std::atomic<T>* obj, T desr,

                            std::memory_order order );
(2) (since C++11)
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->exhange(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->exhange(desr, order)

Contents

Parameters

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

Return value

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

Exceptions

noexcept specification:  
noexcept
  

Example

A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to atomic_flag_test_and_set

#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
 
std::atomic<bool> lock = ATOMIC_VAR_INIT(false);
 
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>...

See also

(C++11)
atomically replaces the value of the atomic object and obtains the value held previously
(public member function of std::atomic) [edit]
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) [edit]
specializes atomic operations for std::shared_ptr
(function template)
C documentation for atomic_exchange, atomic_exchange_explicit