C++ Atomic Library - Fetch Add

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Description

It adds a non-atomic value to an atomic object and obtains the previous value of the atomic.

Declaration

Following is the declaration for std::atomic_fetch_add.

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

C++11

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

Following is the declaration for std::atomic_fetch_add_explicit.

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

C++11

 template< class T > T* atomic_fetch_add_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_add_explicit.

 #include <string> #include <thread> #include <vector> #include <iostream> #include <atomic> #include <chrono> const int N = 10; std::atomic<int> cnt = ATOMIC_VAR_INIT(N); std::vector<int> data; void reader(int id) { for(;;) { while(std::atomic_fetch_sub(&cnt, 1) <= 0) std::atomic_fetch_add(&cnt, 1); if(!data.empty()) std::cout << ( "reader " + std::to_string(id) + " sees " + std::to_string(*data.rbegin()) + '\n'); if(data.size() == 100) break; std::atomic_fetch_add(&cnt, 1); std::this_thread::sleep_for(std::chrono::milliseconds(1)); } } void writer() { for(int n = 0; n < 100; ++n) { while(std::atomic_fetch_sub(&cnt, N+1) != N) std::atomic_fetch_add(&cnt, N+1); data.push_back(n); std::cout << "writer pushed back " << n << '\n'; std::atomic_fetch_add(&cnt, N+1); std::this_thread::sleep_for(std::chrono::milliseconds(1)); } } 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(); } } 

The output should be like this −

 writer pushed back 0 reader 8 sees 0 reader 3 sees 0 reader 1 sees 0 ............... reader 2 sees 99 reader 6 sees 99 reader 1 sees 99