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Defined in header <cuda/atomic>:

template <typename T, cuda::thread_scope Scope = cuda::thread_scope_system>
class cuda::atomic_ref;

The class template cuda::atomic_ref is an extended form of cuda::std::atomic_ref that takes an additional cuda::thread_scope argument, defaulted to cuda::std::thread_scope_system. It has the same interface and semantics as cuda::std::atomic_ref, with the following additional operations. This class additionally deviates from the standard by being backported to C++11.


cuda::atomic_ref<T> and cuda::std::atomic_ref<T> may only be instantiated with a T that are either 4 or 8 bytes.

No object or subobject of an object referenced by an atomic_­ref shall be concurrently referenced by any other atomic_­ref that has a different Scope.

For cuda::atomic_ref<T> and cuda::std::atomic_ref<T> the type T must satisfy the following:

  • 4 <= sizeof(T) <= 8.
  • T must not have “padding bits”, i.e., ‘T`’s object representation must not have bits that do not participate in it’s value representation.

Atomic Extrema Operations

cuda::atomic_ref::fetch_min Atomically find the minimum of the stored value and a provided value. (member function)
cuda::atomic_ref::fetch_max Atomically find the maximum of the stored value and a provided value. (member function)

Concurrency Restrictions

See memory model documentation for general restrictions on atomicity.

With CUDA Compute Capability 6 (Pascal), an object of type atomic_ref may not be used:

  • with a reference to an object with a automatic storage duration in a GPU thread, or
  • if is_always_lock_free() is false.

For CUDA Compute Capability prior to 6 (Pascal), objects of type cuda::atomic_ref or cuda::std::atomic_ref may not be used.

Implementation-Defined Behavior

For each type T and cuda::thread_scope S, the value of cuda::atomic_ref<T, S>::is_always_lock_free() and cuda::std::atomic_ref<T>::is_always_lock_free() is as follows:

Type T cuda::thread_scope S cuda::atomic_ref<T, S>::is_always_lock_free()
Any Any sizeof(T) <= 8


#include <cuda/atomic>

__global__ void example_kernel(int *gmem, int *pinned_mem) {
  // This atomic is suitable for all threads in the system.
  cuda::atomic_ref<int, cuda::thread_scope_system> a(pinned_mem);

  // This atomic has the same type as the previous one (`a`).
  cuda::atomic_ref<int> b(pinned_mem);

  // This atomic is suitable for all threads on the current processor (e.g. GPU).
  cuda::atomic_ref<int, cuda::thread_scope_device> c(gmem);

  __shared__ int shared_v;
  // This atomic is suitable for threads in the same thread block.
  cuda::atomic_ref<int, cuda::thread_scope_block> d(&shared);

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