std::atomic_fetch_sub,std::atomic_fetch_sub_explicit (3) - Linux Manuals

std::atomic_fetch_sub,std::atomic_fetch_sub_explicit: std::atomic_fetch_sub,std::atomic_fetch_sub_explicit

NAME

std::atomic_fetch_sub,std::atomic_fetch_sub_explicit - std::atomic_fetch_sub,std::atomic_fetch_sub_explicit

Synopsis

Defined in header <atomic>
template< class T >
T atomic_fetch_sub( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;
template< class T >
T atomic_fetch_sub( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;
template< class T > (1)
T atomic_fetch_sub_explicit( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,
std::memory_order order ) noexcept; (2)
template< class T >
T atomic_fetch_sub_explicit( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,
std::memory_order order ) noexcept;

Performs atomic subtraction. Atomically subtracts arg from 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_sub(arg)
2) obj->fetch_sub(arg, order)

Parameters

obj - pointer to the atomic object to modify
arg - the value to subtract from 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.

Possible implementation

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

Example

Multiple threads may use fetch_sub to concurrently process an indexed container
// Run this code

  #include <string>
  #include <thread>
  #include <vector>
  #include <iostream>
  #include <atomic>
  #include <numeric>

  const int N = 50;
  std::atomic<int> cnt;
  std::vector<int> data(N);

  void reader(int id)
  {
      for (;;) {
          int idx = atomic_fetch_sub_explicit(&cnt, 1, std::memory_order_relaxed);
          if (idx >= 0) {
              std::cout << "reader " << std::to_string(id) << " processed item "
                        << std::to_string(data[idx]) << '\n';
          } else {
              std::cout << "reader " << std::to_string(id) << " done\n";
              break;
          }
      }
  }

  int main()
  {
      std::iota(data.begin(), data.end(), 1);
      cnt = data.size() - 1;

      std::vector<std::thread> v;
      for (int n = 0; n < 5; ++n) {
          v.emplace_back(reader, n);
      }
      for (auto& t : v) {
          t.join();
      }
  }

Output:

  reader 2 processed item 50
  reader 1 processed item 44
  reader 4 processed item 46
  <....>
  reader 0 done
  reader 4 done
  reader 3 done

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