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std::atomic_exchange, std::atomic_exchange_explicit

From cppreference.com
< cpp‎ | atomic
 
 
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Initialization of atomic types
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Memory ordering
Free functions for atomic operations
atomic_exchangeatomic_exchange_explicit
(C++11)(C++11)
Free functions for atomic flags
 
Defined in header <atomic>
template< class T >

T atomic_exchange( std::atomic<T>* obj,

                   typename std::atomic<T>::value_type desired ) noexcept;
(1) (since C++11)
template< class T >

T atomic_exchange( volatile std::atomic<T>* obj,

                   typename std::atomic<T>::value_type desired ) noexcept;
(2) (since C++11)
template< class T >

T atomic_exchange_explicit( std::atomic<T>* obj,
                            typename std::atomic<T>::value_type desired,

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

T atomic_exchange_explicit( volatile std::atomic<T>* obj,
                            typename std::atomic<T>::value_type desired,

                            std::memory_order order ) noexcept;
(4) (since C++11)
1,2) Atomically replaces the value pointed to by obj with the value of desired and returns the value obj held previously, as if by obj->exchange(desired).
3,4) Atomically replaces the value pointed to by obj with the value of desired and returns the value obj held previously, as if by obj->exchange(desired, order).

Contents

[edit] Parameters

obj - pointer to the atomic object to modify
desired - the value to store in the atomic object
order - the memory synchronization ordering

[edit] Return value

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

[edit] Example

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

#include <atomic>
#include <iostream>
#include <thread>
#include <vector>
 
std::atomic<bool> lock(false); // holds true when locked
                               // holds false when unlocked
 
int new_line{1}; // the access is synchronized via atomic lock variable
 
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 << n << (new_line++ % 80 ? "" : "\n");
        std::atomic_store_explicit(&lock, false, std::memory_order_release);
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < 8; ++n)
        v.emplace_back(f, n);
    for (auto& t : v)
        t.join();
}

Possible output:

02222222222222222222222002222222222222222222222222222222222222222222222222222222
22222222200022222222202222211111111111110000011111111100000000000000110001111111
00011111000001111110000011111100000111000000001111111111111110000010000001001111
11011111111011111011000000000000111100000000000001111000011133333333333333333333
33333333333333333333333333333333333333333333333333333333333333333333333333333333
44444444444444444444444444444444444444444444444444444444444444444444444444444444
44444444444444444444555555555555555555555555555555555555555555555555555555555555
55555555555555555555555555555555555555556666666666666666666666666666666666666666
66666666666666666666666666666666666666666666666666666666666677777777777777777777
77777777777777777777777777777777777777777777777777777777777777777777777777777777

[edit] 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 was required because
T was deduced from multiple arguments
T is only deduced
from obj

[edit] See also

atomically replaces the value of the atomic object and obtains the value held previously
(public member function of std::atomic<T>) [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