std::atomic<T>::fetch_or (3) - Linux Man Pages
T fetch_or( T arg,
std::memory_order order = std::memory_order_seq_cst ) noexcept; (since C++11)
T fetch_or( T arg, (member only of atomic<Integral> template specialization)
std::memory_order order = std::memory_order_seq_cst ) volatile noexcept;
Atomically replaces the current value with the result of bitwise OR of the value and arg. The operation is read-modify-write operation. Memory is affected according to the value of order.
arg - the other argument of bitwise OR
order - memory order constraints to enforce
The value immediately preceding the effects of this function in the modification_order of *this.
atomic_fetch_or_explicit replaces the atomic object with the result of bitwise OR with a non-atomic argument and obtains the previous value of the atomic