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std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

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Defined in header <atomic>
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template< class Integral >
Integral atomic_fetch_sub( std::atomic<Integral>* obj, Integral arg );
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<td > (1) </td> <td > (since C++11) </td> </tr> <tr class="t-dcl-sep"><td></td><td></td><td></td></tr>

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<td >
template< class Integral >
Integral atomic_fetch_sub( volatile std::atomic<Integral>* obj, Integral arg );
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<td > (2) </td> <td > (since C++11) </td> </tr> <tr class="t-dcl-sep"><td></td><td></td><td></td></tr>

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template< class Integral >

Integral atomic_fetch_sub_explicit( std::atomic<Integral>* obj, Integral arg,

                                    std::memory_order order );
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<td >
template< class Integral >

Integral atomic_fetch_sub_explicit( volatile std::atomic<Integral>* obj, Integral arg,

                                    std::memory_order order);
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<td >
template< class T >
T* atomic_fetch_sub( std::atomic<T*>* obj, std::ptrdiff_t arg );
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<td >
template< class T >
T* atomic_fetch_sub( volatile std::atomic<T*>* obj, std::ptrdiff_t arg );
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<td >
template< class T >

T* atomic_fetch_sub_explicit( std::atomic<T*>* obj, std::ptrdiff_t arg,

                              std::memory_order order );
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<td > (7) </td> <td > (since C++11) </td> </tr> <tr class="t-dcl-sep"><td></td><td></td><td></td></tr>

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<td >
template< class T >

T* atomic_fetch_sub_explicit( volatile std::atomic<T*>* obj, std::ptrdiff_t arg,

                              std::memory_order order );
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<td > (8) </td> <td > (since C++11) </td> </tr> Template:ddcl list end

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)

Contents

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
  

Possible implementation

First version
template< class T >
typename std::enable_if<std::is_integral<T>::value && !std::is_same<T, bool>::value, T>::type
atomic_fetch_sub( std::atomic<T>* obj, T arg );
{
    return obj->fetch_sub(arg);
}
Second version
template< class T >
T* atomic_fetch_sub( std::atomic<T*>* obj, std::ptrdiff_t arg)
{
    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 = 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
C documentation for atomic_fetch_sub, atomic_fetch_sub_explicit