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std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

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< cpp‎ | atomic
 
 
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Atomic types
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Initialization of atomic types
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Memory ordering
Free functions for atomic operations
atomic_fetch_subatomic_fetch_sub_explicit
(C++11)(C++11)
Free functions for atomic flags
 
Defined in header <atomic>
template< class T >

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

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

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

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

T atomic_fetch_sub_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_sub_explicit( volatile std::atomic<T>* obj,
                             typename std::atomic<T>::difference_type arg,

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

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:

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

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

Contents

[edit] 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 synchronization ordering

[edit] Return value

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

[edit] Example

Multiple threads may use std::atomic_fetch_sub to concurrently process an indexed container.

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

[edit] 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

[edit] 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<T>) [edit]
adds a non-atomic value to an atomic object and obtains the previous value of the atomic
(function template) [edit]
C documentation for atomic_fetch_sub, atomic_fetch_sub_explicit