std:: atomic_fetch_add, std:: atomic_fetch_add_explicit

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atomic_fetch_add atomic_fetch_add_explicit
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Defined in header <atomic>
template < class T >

T atomic_fetch_add ( std:: atomic < T > * obj,

typename std:: atomic < T > :: difference_type arg ) noexcept ;
(1) (since C++11)
template < class T >

T atomic_fetch_add ( volatile std:: atomic < T > * obj,

typename std:: atomic < T > :: difference_type arg ) noexcept ;
(2) (since C++11)
template < class T >

T atomic_fetch_add_explicit ( std:: atomic < T > * obj,
typename std:: atomic < T > :: difference_type arg,

std:: memory_order order ) noexcept ;
(3) (since C++11)
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 ;
(4) (since C++11)

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:

1,2) obj - > fetch_add ( arg )
3,4) obj - > fetch_add ( arg, order )

If std::atomic<T> has no fetch_add member (this member is only provided for integral , floating-point (since C++20) and pointer types except bool ), the program is ill-formed.

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

Return value

The value immediately preceding the effects of this function in the modification order of * obj .

Example

Single-writer/multiple-reader lock can be made with std::atomic_fetch_add . Note that this simplistic implementation is not lockout-free.

Run this code 
#include <atomic>
#include <chrono>
#include <iostream>
#include <string>
#include <thread>
#include <vector>
 
using namespace std::chrono_literals;
 
// meaning of cnt:
//  5: readers and writer are in race. There are no active readers or writers.
//  4...0: there are 1...5 active readers, The writer is blocked.
// -1: writer won the race and 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(1ms);
    }
}
 
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(1ms);
    }
}
 
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 was required because
T was deduced from multiple arguments 		T is only deduced
from obj

See also

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