std::atomic_exchange, std::atomic_exchange_explicit

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Atomic operations library

Types

atomic (C++11)

atomic_ref (C++20)

Functions

atomic_storeatomic_store_explicit (C++11)(C++11)

atomic_loadatomic_load_explicit (C++11)(C++11)

atomic_exchangeatomic_exchange_explicit (C++11)(C++11)

atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11)

atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11)

atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11)

atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11)

atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11)

atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11)

atomic_is_lock_free (C++11)

atomic_waitatomic_wait_explicit (C++20)(C++20)

atomic_notify_one (C++20)

atomic_notify_all (C++20)

Atomic flags

atomic_flag (C++11)

atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11)

atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11)

atomic_flag_testatomic_flag_test_explicit (C++20)(C++20)

atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20)

atomic_flag_notify_one (C++20)

atomic_flag_notify_all (C++20)

Initialization

atomic_init (C++11)(deprecated in C++20)

ATOMIC_VAR_INIT (C++11)(deprecated in C++20)

ATOMIC_FLAG_INIT (C++11)(deprecated in C++20)

Memory ordering

memory_order (C++11)

kill_dependency (C++11)

atomic_thread_fence (C++11)

atomic_signal_fence (C++11)

 

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 );	(2)	(since C++11)
template< class T > T atomic_exchange_explicit( std::atomic<T>* obj, T desr,                             std::memory_order order );	(3)	(since C++11)
template< class T > T atomic_exchange_explicit( volatile std::atomic<T>* obj, T desr,                             std::memory_order order );	(4)	(since C++11)

1-2) 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)

3-4) 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)

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

Run this code

#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

exchange	atomically replaces the value of the atomic object and obtains the value held previously (public member function of std::atomic<T>)
atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11)	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)
std::atomic_exchange(std::shared_ptr) std::atomic_exchange_explicit(std::shared_ptr)	specializes atomic operations for std::shared_ptr (function template)
C documentation for atomic_exchange, atomic_exchange_explicit