| Defined in header <atomic> | ||
|---|---|---|
| (1) | (since C++11) | |
template< class T > T atomic_exchange( std::atomic<T>* obj, T desr ); | ||
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 ); | ||
template< class T >
T atomic_exchange_explicit( volatile std::atomic<T>* obj, T desr,
std::memory_order order ); |
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.
Exceptions
noexcept specification: noexceptExample
A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to std::atomic_flag_test_and_set:
#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>...
See also
| (C++11) | atomically replaces the value of the atomic object and obtains the value held previously (public member function of std::atomic) |
| 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) | |
specializes atomic operations for std::shared_ptr (function template) | |
C documentation for atomic_exchange, atomic_exchange_explicit | |
Please login to continue.