std::atomic_fetch_add,std::atomic_fetch_add_explicit (3) - Linux Manuals

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

NAME

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