dependencies/libcudacxx/docs/extended_api/synchronization_primitives/counting_semaphore.md

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grand_parent: Extended API
parent: Synchronization Primitives
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cuda::counting_semaphore

Defined in header <cuda/semaphore>:

template <cuda::thread_scope Scope,
          cuda::std::ptrdiff_t LeastMaxValue = /* implementation-defined */>
class cuda::counting_semaphore;

The class template cuda::counting_semaphore is an extended form of cuda::std::counting_semaphore that takes an additional cuda::thread_scope argument. cuda::counting_semaphore has the same interface and semantics as cuda::std::counting_semaphore.

Concurrency Restrictions

An object of type cuda::counting_semaphore or cuda::std::counting_semaphore, shall not be accessed concurrently by CPU and GPU threads unless:

• it is in unified memory and the concurrentManagedAccess property is 1, or
• it is in CPU memory and the hostNativeAtomicSupported property is 1.

Note, for objects of scopes other than cuda::thread_scope_system this is a data-race, and thefore also prohibited regardless of memory characteristics.

Under CUDA Compute Capability 6 (Pascal) or prior, an object of type cuda::counting_semaphore or cuda::std::counting_semaphore may not be used.

Implementation-Defined Behavior

For each cuda::thread_scope S and least maximum value V, counting_semaphore<S,V>::max() is as follows:

cuda::thread_scope S	Least Maximum Value V	cuda::counting_semaphore<S,V>::max()
Any	Any	cuda::std::numeric_limits<cuda::std::ptrdiff_t>::max()

Example

#include <cuda/semaphore>

__global__ void example_kernel() {
  // This semaphore is suitable for all threads in the system.
  cuda::counting_semaphore<cuda::thread_scope_system> a;

  // This semaphore has the same type as the previous one (`a`).
  cuda::std::counting_semaphore<> b;

  // This semaphore is suitable for all threads on the current processor (e.g. GPU).
  cuda::counting_semaphore<cuda::thread_scope_device> c;

  // This semaphore is suitable for all threads in the same thread block.
  cuda::counting_semaphore<cuda::thread_scope_block> d;
}

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