Std::atomic_fetch_add,std::atomic_fetch_add_explicit (3) Linux Manual Page

std::atomic_fetch_add,std::atomic_fetch_add_explicit – std::atomic_fetch_add,std::atomic_fetch_add_explicit

Synopsis

Defined in header <atomic>

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 >                                        (1)

T atomic_fetch_add_explicit( std::atomic<T>* obj,

typename std::atomic<T>::difference_type arg,

std::memory_order order ) noexcept;                            (2)

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;

Performs atomic addition. Atomically adds arg to the value pointed to by obj and returns the value obj held previously. The operation is performed as if the following was executed:
1) obj->fetch_add(arg)
2) obj->fetch_add(arg, order)

Parameters

obj – pointer to the atomic object to modify
arg – the value to add to the value stored in the atomic object
order – the memory synchronization 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.

Possible implementation

  template< class T >
  T atomic_fetch_add( std::atomic<T>* obj, typename std::atomic<T>::difference_type arg )
  {
      return obj->fetch_add(arg);
  }

Example

Single-writer/multiple-reader lock can be made with fetch_add. Note that this simplistic implementation is not lockout-free
// Run this code

#include <string>

#include <thread>

#include <vector>

#include <iostream>

#include <atomic>

#include <chrono>
// meaning of cnt:

// 5: there are no active readers or writers.

// 1...4: there are 4...1 readers active, The writer is blocked

// 0: temporary value between fetch_sub and fetch_add in reader lock

// -1: there is a writer active. The 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(std::chrono::milliseconds(1));
    }
}
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(std::chrono::milliseconds(1));
    }
}
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();
    }
}

Output:

  writer pushed back 0
  reader 2 sees 0
  reader 3 sees 0
  reader 1 sees 0
  <…>
  reader 2 sees 24
  reader 4 sees 24
  reader 1 sees 24

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P0558R1 C++11 exact type match required because T is deduced from multiple arguments T is deduced from the atomic argument only

std::atomic_fetch_add,std::atomic_fetch_add_explicit (3) Linux Manual Page