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std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

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

#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

See also

atomically subtracts the argument from the value stored in the atomic object and obtains the value held previously
(public member function of std::atomic)
(C++11)(C++11)
adds a non-atomic value to an atomic object and obtains the previous value of the atomic
(function template)
C documentation for atomic_fetch_sub, atomic_fetch_sub_explicit

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