Std::atomic_fetch_sub,std::atomic_fetch_sub_explicit (3) Linux Manual Page

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

std::atomic_fetch_sub,std::atomic_fetch_sub_explicit (3) Linux Manual Page