Code Example

The following example shows a small hello word example written with alpaka that can be run on different processors.

/* Copyright 2023 Benjamin Worpitz, Erik Zenker, Bernhard Manfred Gruber, Jan Stephan
 * SPDX-License-Identifier: ISC

#include <alpaka/alpaka.hpp>
#include <alpaka/example/ExampleDefaultAcc.hpp>

#include <iostream>

//! Hello World Kernel
//! Prints "[x, y, z][gtid] Hello World" where tid is the global thread number.
struct HelloWorldKernel
    template<typename TAcc>
    ALPAKA_FN_ACC auto operator()(TAcc const& acc) const -> void
        using Dim = alpaka::Dim<TAcc>;
        using Idx = alpaka::Idx<TAcc>;
        using Vec = alpaka::Vec<Dim, Idx>;
        using Vec1 = alpaka::Vec<alpaka::DimInt<1u>, Idx>;

        // In the most cases the parallel work distibution depends
        // on the current index of a thread and how many threads
        // exist overall. These information can be obtained by
        // getIdx() and getWorkDiv(). In this example these
        // values are obtained for a global scope.
        Vec const globalThreadIdx = alpaka::getIdx<alpaka::Grid, alpaka::Threads>(acc);
        Vec const globalThreadExtent = alpaka::getWorkDiv<alpaka::Grid, alpaka::Threads>(acc);

        // Map the three dimensional thread index into a
        // one dimensional thread index space. We call it
        // linearize the thread index.
        Vec1 const linearizedGlobalThreadIdx = alpaka::mapIdx<1u>(globalThreadIdx, globalThreadExtent);

        // Each thread prints a hello world to the terminal
        // together with the global index of the thread in
        // each dimension and the linearized global index.
        // Mind, that alpaka uses the mathematical index
        // order [z][y][x] where the last index is the fast one.
            "[z:%u, y:%u, x:%u][linear:%u] Hello World\n",

auto main() -> int
    // Define the index domain
    // Depending on your type of problem, you have to define
    // the dimensionality as well as the type used for indices.
    // For small index domains 16 or 32 bit indices may be enough
    // and may be faster to calculate depending on the accelerator.
    using Dim = alpaka::DimInt<3>;
    using Idx = std::size_t;

    // Define the accelerator
    // It is possible to choose from a set of accelerators:
    // - AccGpuCudaRt
    // - AccGpuHipRt
    // - AccCpuThreads
    // - AccCpuOmp2Threads
    // - AccCpuOmp2Blocks
    // - AccCpuTbbBlocks
    // - AccCpuSerial
    // Each accelerator has strengths and weaknesses. Therefore,
    // they need to be chosen carefully depending on the actual
    // use case. Furthermore, some accelerators only support a
    // particular workdiv, but workdiv can also be generated
    // automatically.

    // By exchanging the Acc and Queue types you can select where to execute the kernel.
    // using Acc = alpaka::AccCpuSerial<Dim, Idx>;
    using Acc = alpaka::ExampleDefaultAcc<Dim, Idx>;
    std::cout << "Using alpaka accelerator: " << alpaka::getAccName<Acc>() << std::endl;

    // Defines the synchronization behavior of a queue
    // choose between Blocking and NonBlocking
    using QueueProperty = alpaka::Blocking;
    using Queue = alpaka::Queue<Acc, QueueProperty>;

    // Select a device
    // The accelerator only defines how something should be
    // parallelized, but a device is the real entity which will
    // run the parallel program. The device can be chosen
    // by id (0 to the number of devices minus 1) or you
    // can also retrieve all devices in a vector (getDevs()).
    // In this example the first devices is chosen.
    auto const platformAcc = alpaka::Platform<Acc>{};
    auto const devAcc = alpaka::getDevByIdx(platformAcc, 0);

    // Create a queue on the device
    // A queue can be interpreted as the work queue
    // of a particular device. Queues are filled with
    // tasks and alpaka takes care that these
    // tasks will be executed. Queues are provided in
    // non-blocking and blocking variants.
    // The example queue is a blocking queue to a cpu device,
    // but it also exists as non-blocking queue for this
    // device (QueueCpuNonBlocking).
    Queue queue(devAcc);

    // Define the work division
    // A kernel is executed for each element of a
    // n-dimensional grid distinguished by the element indices.
    // The work division defines the number of kernel instantiations as
    // well as the type of parallelism used by the kernel execution task.
    // Different accelerators have different requirements on the work
    // division. For example, the sequential accelerator can not
    // provide any thread level parallelism (synchronizable as well as non synchronizable),
    // whereas the CUDA accelerator can spawn hundreds of synchronizing
    // and non synchronizing threads at the same time.
    // The workdiv is divided in three levels of parallelization:
    // - grid-blocks:      The number of blocks in the grid (parallel, not synchronizable)
    // - block-threads:    The number of threads per block (parallel, synchronizable).
    //                     Each thread executes one kernel invocation.
    // - thread-elements:  The number of elements per thread (sequential, not synchronizable).
    //                     Each kernel has to execute its elements sequentially.
    // - Grid     : consists of blocks
    // - Block    : consists of threads
    // - Elements : consists of elements
    // Threads in the same grid can access the same global memory,
    // while threads in the same block can access the same shared
    // memory. Elements are supposed to be used for vectorization.
    // Thus, a thread can process data element size wise with its
    // vector processing unit.
    using Vec = alpaka::Vec<Dim, Idx>;
    auto const elementsPerThread = Vec::all(static_cast<Idx>(1));
    auto const threadsPerGrid = Vec{4, 2, 4};
    using WorkDiv = alpaka::WorkDivMembers<Dim, Idx>;
    WorkDiv const workDiv = alpaka::getValidWorkDiv<Acc>(

    // Instantiate the kernel function object
    // Kernels can be everything that is trivially copyable, has a
    // callable operator() and takes the accelerator as first
    // argument. So a kernel can be a class or struct, a lambda, etc.
    HelloWorldKernel helloWorldKernel;

    // Run the kernel
    // To execute the kernel, you have to provide the
    // work division as well as the additional kernel function
    // parameters.
    // The kernel execution task is enqueued into an accelerator queue.
    // The queue can be blocking or non-blocking
    // depending on the chosen queue type (see type definitions above).
    // Here it is synchronous which means that the kernel is directly executed.
        /* put kernel arguments here */);

    return EXIT_SUCCESS;

Use alpaka in your project

We recommend to use CMake for integrating alpaka into your own project. There are two possible methods.

Use alpaka via find_package

The find_package method requires alpaka to be installed in a location where CMake can find it.


If you do not install alpaka in a default path such as /usr/local/ you have to set the CMake argument -Dalpaka_ROOT=/path/to/alpaka/install.

The following example shows a minimal example of a CMakeLists.txt that uses alpaka:

cmake_minimum_required(VERSION 3.22)
project("myexample" CXX)

find_package(alpaka REQUIRED)
alpaka_add_executable(${PROJECT_NAME} helloWorld.cpp)
target_link_libraries(${PROJECT_NAME} PUBLIC alpaka::alpaka)

In the CMake configuration phase of the project, you must activate the accelerator you want to use:

cd <path/to/the/project/root>
mkdir build && cd build
cmake .. -Dalpaka_ACC_GPU_CUDA_ENABLE=ON
cmake --build .

A complete list of CMake flags for the accelerator can be found here.

If the configuration was successful and CMake found the CUDA SDK, the C++ template accelerator type alpaka::AccGpuCudaRt is available.

Use alpaka via add_subdirectory

The add_subdirectory method does not require alpaka to be installed. Instead, the alpaka project folder must be part of your project hierarchy. The following example expects alpaka to be found in the project_path/thirdParty/alpaka:

cmake_minimum_required(VERSION 3.22)
project("myexample" CXX)

alpaka_add_executable(${PROJECT_NAME} helloWorld.cpp)
target_link_libraries(${PROJECT_NAME} PUBLIC alpaka::alpaka)

The CMake configure and build commands are the same as for the find_package approach.