std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

Defined in header <atomic>
template< class Integral > Integral atomic_fetch_sub( std::atomic<Integral>* obj, Integral arg );	(1)	(since C++11)
template< class Integral > Integral atomic_fetch_sub( volatile std::atomic<Integral>* obj, Integral arg );	(2)	(since C++11)
template< class Integral > Integral atomic_fetch_sub_explicit( std::atomic<Integral>* obj, Integral arg,                                     std::memory_order order );	(3)	(since C++11)
template< class Integral > Integral atomic_fetch_sub_explicit( volatile std::atomic<Integral>* obj, Integral arg,                                     std::memory_order order);	(4)	(since C++11)
template< class T > T* atomic_fetch_sub( std::atomic<T*>* obj, std::ptrdiff_t arg );	(5)	(since C++11)
template< class T > T* atomic_fetch_sub( volatile std::atomic<T*>* obj, std::ptrdiff_t arg );	(6)	(since C++11)
template< class T > T* atomic_fetch_sub_explicit( std::atomic<T*>* obj, std::ptrdiff_t arg,                               std::memory_order order );	(7)	(since C++11)
template< class T > T* atomic_fetch_sub_explicit( volatile std::atomic<T*>* obj, std::ptrdiff_t arg,                               std::memory_order order );	(8)	(since C++11)

1-2) Atomically subtracts arg from the value pointed to by obj and returns the value obj held previously, as if by obj->fetch_sub(arg)

3-4) Atomically subtracts arg from the value pointed to by obj and returns the value obj held previously, as if by obj->fetch_sub(arg, order)

5-6) Atomically decrements the pointer value, pointed to by obj, by arg, and returns the value obj held previously, as if by obj->fetch_sub(arg)

7-8) Atomically decrements the pointer value, pointed to by obj, by arg, and returns the value obj held previously, as if by 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 held previously by the atomic object pointed to by obj

Exceptions

noexcept specification:  

noexcept

  (since C++11)

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 = 10000;
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 < 10; ++n) {
        v.emplace_back(reader, n);
    }
    for (auto& t : v) {
        t.join();
    }
}

Output:

reader 2 processed item 10000
reader 6 processed item 9994
reader 4 processed item 9996
reader 6 processed item 9992
<....>
reader 0 done
reader 5 done
reader 3 done
reader 9 done

See also

Template:cpp/atomic/atomic/dcl list fetch subTemplate:cpp/atomic/dcl list atomic fetch add