atomic_thread_fence

atomic_thread_fence is a function in C++ that ensures atomicity of operations in a multi-threaded environment. It is used to prevent unwanted reordering of memory operations by the compiler, CPU or the cache, which may lead to race conditions or other synchronization issues.

The function has four possible memory order parameters:
- memory_order_acquire: it ensures that all memory operations before the fence are visible to the current thread.
- memory_order_release: it ensures that all memory operations after the fence are visible to other threads.
- memory_order_acq_rel: it combines the effects of memory_order_acquire and memory_order_release.
- memory_order_seq_cst: it ensures that all memory operations before and after the fence are visible to all threads in a sequentially consistent order.

Here is an example of a use case for atomic_thread_fence:

#include <atomic>
#include <thread>
#include <iostream>

std::atomic<int> x = {0};
std::atomic<int> y = {0};
bool flag = false;

void write_x_then_y() {
    x.store(1, std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_release);
    y.store(1, std::memory_order_relaxed);
}

void read_y_then_x() {
    while (!flag);
    std::atomic_thread_fence(std::memory_order_acquire);
    if (y.load(std::memory_order_relaxed) == 1 && x.load(std::memory_order_relaxed) == 0) {
        std::cout << "Race condition detected!\n";
    }
}

int main() {
    std::thread t1(write_x_then_y);
    std::thread t2(read_y_then_x);
    t1.join();
    flag = true;
    t2.join();
    return 0;
}

In this example, two threads are created: t1 writes a value to x, then y, while t2 reads y and x in that order. Without the atomic_thread_fence, t2 could read x before y, which would lead to a race condition. However, the use of the acquire and release memory orders ensures that the operations are performed atomically, and the fence prevents reordering.