std::atomic_fetch_or

Defined in header <atomic>

(1) (since C++11)

template< class Integral >
Integral atomic_fetch_or( std::atomic<Integral>* obj, Integral arg );

template< class Integral >
Integral atomic_fetch_or( volatile std::atomic<Integral>* obj, Integral arg );

(2) (since C++11)

template< class Integral >
Integral atomic_fetch_or_explicit( std::atomic<Integral>* obj,
                                   Integral arg, 
                                   std::memory_order order );

template< class Integral >
Integral atomic_fetch_or_explicit( volatile std::atomic<Integral>* obj, 
                                   Integral arg, 
                                   std::memory_order order );

Atomically replaces the value pointed by arg with the result of bitwise OR between the old value of obj and arg. Returns the value obj held previously.

The operation is performed as if the following is executed:

1) obj->fetch_or(arg)

2) obj->fetch_or(arg, order)

Parameters

obj	-	pointer to the atomic object to modify
arg	-	the value to bitwise OR to the value stored in the atomic object
order	-	the memory sycnhronization ordering for this operation: all values are permitted.

Return value

The value immediately preceding the effects of this function in the modification order of *obj.

Exceptions

noexcept specification:

noexcept

Possible implementation

template< class T >
typename std::enable_if<std::is_integral<T>::value && !std::is_same<T, bool>::value, T>::type
atomic_fetch_or( std::atomic<T>* obj, T arg );
{
    return obj->fetch_or(arg);
}

Example

#include <iostream>
#include <atomic>
#include <thread>
#include <chrono>
#include <functional>
  
// Binary semaphore for demonstrative purposes only
// This is a simple yet meaningful example: atomic operations
// are unnecessary without threads. 
class Semaphore {
    std::atomic_char m_signaled;
  public:
    Semaphore(bool initial = false)
    {
        m_signaled = initial;
    }
    // Block until semaphore is signaled
    void take() 
    {
        while (!std::atomic_fetch_and(&m_signaled, false)) {
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }
  
    void put() 
    {
        std::atomic_fetch_or(&m_signaled, true);
    }
};
  
class ThreadedCounter {
    static const int N = 100;
    static const int REPORT_INTERVAL = 10;
    int m_count;
    bool m_done;
    Semaphore m_count_sem;
    Semaphore m_print_sem;
  
    void count_up() 
    {
        for (m_count = 1; m_count <= N; m_count++) {
            if (m_count % REPORT_INTERVAL == 0) {
                if (m_count == N) m_done = true;
                m_print_sem.put(); // signal printing to occur
                m_count_sem.take(); // wait until printing is complete proceeding
            }
        }
        std::cout << "count_up() done\n";
        m_done = true;
        m_print_sem.put();
    }
  
    void print_count() 
    {
        do {
            m_print_sem.take();
            std::cout << m_count << '\n';
            m_count_sem.put();
        } while (!m_done);
        std::cout << "print_count() done\n";
    }
  
  public:
    ThreadedCounter() : m_done(false) {}
    void run() 
    {
        auto print_thread = std::thread(&ThreadedCounter::print_count, this);
        auto count_thread = std::thread(&ThreadedCounter::count_up, this);
        print_thread.join();
        count_thread.join();
    }
};
  
int main() 
{
    ThreadedCounter m_counter;
    m_counter.run();
}

Output:

10
20
30
40
50
60
70
80
90
100
print_count() done
count_up() done

fetch_or (C++11)	atomically performs bitwise OR between the argument and the value of the atomic object and obtains the value held previously (public member function of `std::atomic`)
atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11)	replaces the atomic object with the result of logical AND with a non-atomic argument and obtains the previous value of the atomic (function template)
atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11)	replaces the atomic object with the result of logical XOR with a non-atomic argument and obtains the previous value of the atomic (function template)
C documentation for `atomic_fetch_or, atomic_fetch_or_explicit`

Links:

http://en.cppreference.com/w/cpp/atomic/atomic_fetch_or

doc_CPP

2025-01-10 15:47:30

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