std::atomic_fetch_add, std::atomic_fetch_add_explicit

template< class T >
T atomic_fetch_add( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;

template< class T >
T atomic_fetch_add( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;

template< class T >
T atomic_fetch_add_explicit( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,
std::memory_order order ) noexcept;

template< class T >
T atomic_fetch_add_explicit( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,
std::memory_order order ) noexcept;

#include <atomic>
#include <chrono>
#include <iostream>
#include <string>
#include <thread>
#include <vector>
 
using namespace std::chrono_literals;
 
// meaning of cnt:
//  5: readers and writer are in race. There are no active readers or writers.
//  4...0: there are 1...5 active readers, The writer is blocked.
// -1: writer won the race and readers are blocked.
 
const int N = 5; // four concurrent readers are allowed
std::atomic<int> cnt(N);
 
std::vector<int> data;
 
void reader(int id)
{
    for (;;)
    {
        // lock
        while (std::atomic_fetch_sub(&cnt, 1) <= 0)
            std::atomic_fetch_add(&cnt, 1);
 
        // read
        if (!data.empty())
            std::cout << ("reader " + std::to_string(id) +
                          " sees " + std::to_string(*data.rbegin()) + '\n');
        if (data.size() == 25)
            break;
 
        // unlock
        std::atomic_fetch_add(&cnt, 1);
 
        // pause
        std::this_thread::sleep_for(1ms);
    }
}
 
void writer()
{
    for (int n = 0; n < 25; ++n)
    {
        // lock
        while (std::atomic_fetch_sub(&cnt, N + 1) != N)
            std::atomic_fetch_add(&cnt, N + 1);
 
        // write
        data.push_back(n);
        std::cout << "writer pushed back " << n << '\n';
 
        // unlock
        std::atomic_fetch_add(&cnt, N + 1);
 
        // pause
        std::this_thread::sleep_for(1ms);
    }
}
 
int main()
{
    std::vector<std::thread> v;
    for (int n = 0; n < N; ++n)
        v.emplace_back(reader, n);
    v.emplace_back(writer);
 
    for (auto& t : v)
        t.join();
}