std::atomic_exchange, std::atomic_exchange_explicit

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Atomic operations library

Types

atomic (C++11)

atomic_is_lock_free (C++11)

Functions

atomic_store atomic_store_explicit (C++11) (C++11)

atomic_load atomic_load_explicit (C++11) (C++11)

atomic_exchange atomic_exchange_explicit (C++11) (C++11)

atomic_compare_exchange_weak atomic_compare_exchange_weak_explicit atomic_compare_exchange_strong atomic_compare_exchange_strong_explicit (C++11) (C++11) (C++11) (C++11)

atomic_fetch_add atomic_fetch_add_explicit (C++11) (C++11)

atomic_fetch_sub atomic_fetch_sub_explicit (C++11) (C++11)

atomic_fetch_and atomic_fetch_and_explicit (C++11) (C++11)

atomic_fetch_or atomic_fetch_or_explicit (C++11) (C++11)

atomic_fetch_xor atomic_fetch_xor_explicit (C++11) (C++11)

Atomic flags

atomic_flag (C++11)

atomic_flag_test_and_set atomic_flag_test_and_set_explicit (C++11) (C++11)

atomic_flag_clear atomic_flag_clear_explicit (C++11) (C++11)

Initialization

atomic_init (C++11)

ATOMIC_VAR_INIT (C++11)

ATOMIC_FLAG_INIT (C++11)

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

  (since C++11)

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 (C++11)	atomically replaced the value of the atomic object and obtains the value held previously (public member function of std::atomic)
atomic_compare_exchange_weak atomic_compare_exchange_weak_explicit atomic_compare_exchange_strong atomic_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)