Function xchg 

pub unsafe fn xchg<T: AtomicType, Ordering: Ordering>(
    ptr: *mut T,
    new: T,
    o: Ordering,
) -> T
where
    T::Repr: AtomicExchangeOps,

Atomic exchange over raw pointers.

This function provides a short-cut of Atomic::from_ptr().xchg(..), and can be used to work with C side on synchronizations.

Safety

• ptr is a valid pointer to T and aligned to align_of::<T>().
• If there is a concurrent access from kernel (C or Rust), it has to be atomic.