C++ Atomic Library - Fetch Sub

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Description

It subtracts a non-atomic value from an atomic object and obtains the previous value of the atomic.

Declaration

Following is the declaration for std::atomic_fetch_sub.

 template< class Integral > Integral atomic_fetch_sub( volatile std::atomic<Integral>* obj, Integral arg ); 

C++11

 template< class Integral > Integral atomic_fetch_sub( std::atomic<Integral>* obj, Integral arg ); 

Following is the declaration for std::atomic_fetch_sub_explicit.

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

C++11

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

Parameters

• obj − It is used in pointer to the atomic object to modify.

• desr − It is used to store the value in the atomic object.

• order − It is used to synchronise the memory ordering for this operation.

Return Value

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

Exceptions

No-noexcept − this member function never throws exceptions.

Example

In below example for std::atomic_fetch_sub_explicit.

 #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(); } } 

The output should be like this −

 reader 1 processed item 9997 reader 1 processed item 9992 reader 1 processed item 9991 reader 1 processed item 9990 reader 1 processed item 9989 reader 1 processed item 9988 reader 1 processed item 9987 reader 1 processed item 9986 reader 1 processed item 9985 reader 1 processed item 9984 reader 1 processed item 9983 reader 1 processed item 9982 reader 1 processed item 9981 reader 1 processed item 9980 reader 1 processed item 9979 reader 1 processed item 9978 reader 1 processed item 9977 reader 1 processed item 9976 reader 1 processed item 9975 reader 1 processed item 9974 reader 1 processed item 9973 ............................