std::atomic_fetch_sub, std::atomic_fetch_sub_explicit

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Atomic operations library

Types

atomic (C++11)

atomic_is_lock_free (C++11)

Functions

atomic_storeatomic_store_explicit (C++11)(C++11)

atomic_loadatomic_load_explicit (C++11)(C++11)

atomic_exchangeatomic_exchange_explicit (C++11)(C++11)

atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11)

atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11)

atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11)

atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11)

atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11)

atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11)

Atomic flags

atomic_flag (C++11)

atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11)

atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11)

Initialization

atomic_init (C++11)

ATOMIC_VAR_INIT (C++11)

ATOMIC_FLAG_INIT (C++11)

Memory ordering

memory_order (C++11)

kill_dependency (C++11)

atomic_thread_fence (C++11)

atomic_signal_fence (C++11)

 

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

Performs atomic subtraction.

1-2) 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)

3-4) Atomically decrements the pointer value, pointed to by obj, by arg, and returns the value obj held previously. The operation is performed as if the following was executed:

3) obj->fetch_sub(arg)

4) 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.

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

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

fetch_sub	atomically subtracts the argument from the value stored in the atomic object and obtains the value held previously (public member function of std::atomic)
atomic_fetch_addatomic_fetch_add_explicit (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