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* Rewrite CLOn12 loader in C

* Remove additional now-unneeded extern "C" guards for khrEnableTrace

* Fix trace message

Co-authored-by: Ben Ashbaugh <>

Co-authored-by: Ben Ashbaugh <>

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OpenCLTM ICD Loader

This repo contains the source code and tests for the Khronos official OpenCL ICD Loader.

CI Build Status

Windows Build Status Linux Build Status MacOS Build Status


OpenCL defines an Installable Client Driver (ICD) mechanism to allow developers to build applications against an Installable Client Driver loader (ICD loader) rather than linking their applications against a specific OpenCL implementation. The ICD Loader is responsible for:

  • Exporting OpenCL API entry points
  • Enumerating OpenCL implementations
  • Forwarding OpenCL API calls to the correct implementation

This repo contains the source code and tests for the Khronos official OpenCL ICD Loader.

Note that this repo does not contain an OpenCL implementation (ICD). You will need to obtain and install an OpenCL implementation for your OpenCL device that supports the OpenCL ICD extension cl_khr_icd to run an application using the OpenCL ICD Loader.

The OpenCL Installable Client Driver extension (cl_khr_icd) is described in the OpenCL extensions specification, which may be found on the Khronos OpenCL Registry.

Build Instructions

While the ICD Loader can be built and installed in isolation, it is part of the OpenCL SDK. If looking for streamlined build experience and a complete development package, refer to the SDK build instructions instead of the following guide.


The OpenCL ICD Loader requires:

  • the OpenCL Headers.
    • It is recommended to install the headers via CMake, however a convenience shorthand is provided. Providing OPENCL_ICD_LOADER_HEADERS_DIR to CMake, one may specify the location of OpenCL Headers. By default, the OpenCL ICD Loader will look for OpenCL Headers in the inc directory.
  • The OpenCL ICD Loader uses CMake for its build system. If CMake is not provided by your build system or OS package manager, please consult the CMake website.
  • The Windows OpenCL ICD Loader requires the Windows SDK to check for and enumerate the OpenCLOn12 ICD. An OpenCL ICD Loader can be built without a dependency on the Windows SDK using the CMake variable OPENCL_ICD_LOADER_DISABLE_OPENCLON12. This variable should only be used when building an import lib to link with, and must be enabled when building an OpenCL ICD Loader for distribution!

Example Build

For most Windows and Linux usages, the following steps are sufficient to build the OpenCL ICD Loader:

  1. Clone this repo and the OpenCL Headers:

     git clone
     git clone
  2. Install OpenCL Headers CMake package

     cmake -D CMAKE_INSTALL_PREFIX=./OpenCL-Headers/install -S ./OpenCL-Headers -B ./OpenCL-Headers/build 
     cmake --build ./OpenCL-Headers/build --target install
  3. Build and install OpenCL ICD Loader CMake package. (Note that CMAKE_PREFIX_PATH need to be an absolute path. Update as needed.)

     cmake -D CMAKE_PREFIX_PATH=/absolute/path/to/OpenCL-Headers/install -D CMAKE_INSTALL_PREFIX=./OpenCL-ICD-Loader/install -S ./OpenCL-ICD-Loader -B ./OpenCL-ICD-Loader/build 
     cmake --build ./OpenCL-ICD-Loader/build --target install


  • For x64 Windows builds, you need to instruct the default Visual Studio generator by adding -A x64 to all your command-lines.

  • Some users may prefer to use a CMake GUI frontend, such as cmake-gui or ccmake, vs. the command-line CMake.

Example Use

Example CMake invocation

cmake -D CMAKE_PREFIX_PATH="/chosen/install/prefix/of/headers;/chosen/install/prefix/of/loader" /path/to/opencl/app

and sample CMakeLists.txt

cmake_minimum_required(VERSION 3.0)
cmake_policy(VERSION 3.0...3.18.4)
add_executable(app main.cpp)
find_package(OpenCLHeaders REQUIRED)
find_package(OpenCLICDLoader REQUIRED)
target_link_libraries(app PRIVATE OpenCL::Headers OpenCL::OpenCL)

OpenCL ICD Loader Tests

OpenCL ICD Loader Tests can be run using ctest from the build directory. CTest which is a companion to CMake. The OpenCL ICD Loader Tests can also be run directly by executing icd_loader_test[.exe] executable from the bin folder.

(Note that running the tests manually requires setting up it's env manually, by setting OCL_ICD_FILENAMES to the full path of, something otherwise done by CTest.)

Registering ICDs

The method to installing an ICD is operating system dependent.

Registering an ICD on Linux

Install your ICD by creating a file with the full path to the library of your implementation in /etc/OpenCL/vendors for eg.:

echo full/path/to/ > /etc/OpenCL/vendors/test.icd

Registering an ICD on Windows

Install your ICD by adding a REG_DWORD value to the registry keys:

// For 32-bit operating systems, or 64-bit tests on a 64-bit operating system:

// For 32-bit tests on a 64-bit operating system:

// The name of the REG_DWORD value should be the full path to the library of your implementation, for eg.
// OpenCLDriverStub.dll, and the data for this value should be 0.

About Layers

Layers have been added as an experimental feature in the OpenCL ICD Loader. We do not expect the API or ABI to change significantly, but the OpenCL Working Group reserves the right to do so. The layer support can also be completely deactivated during configuration by using the ENABLE_OPENCL_LAYERS (ON by default) cmake variable:


For now, runtime configuration of layers is done using the OPENCL_LAYERS environment variable. A colon (Linux) or semicolon (Windows) list of layers to use can be provided through this environment variable.

We are looking for feedback.


Please create a GitHub issue to report an issue or ask questions.


Contributions to the OpenCL ICD Loader are welcomed and encouraged. You will be prompted with a one-time "click-through" CLA dialog as part of submitting your pull request or other contribution to GitHub.

Table of Debug Environment Variables

The following debug environment variables are available for use with the OpenCL ICD loader:

Environment Variable Behavior Example Format
OCL_ICD_FILENAMES Specifies a list of additional ICDs to load. The ICDs will be enumerated first, before any ICDs discovered via default mechanisms. export

set OCL_ICD_FILENAMES=vendor_a.dll;vendor_b.dll
OCL_ICD_VENDORS On Linux and Android, specifies a directory to scan for ICDs to enumerate in place of the default `/etc/OpenCL/vendors'. export OCL_ICD_VENDORS=/my/local/icd/search/path
OPENCL_LAYERS Specifies a list of layers to load. export

set OPENCL_LAYERS=libLayerA.dll;libLayerB.dll
OPENCL_LAYER_PATH On Linux and Android, specifies a directory to scan for layers to enumerate in place of the default `/etc/OpenCL/layers'. export OPENCL_LAYER_PATH=/my/local/layers/search/path
OCL_ICD_ENABLE_TRACE Enable the trace mechanism export OCL_ICD_ENABLE_TRACE=True

true, T, 1 can also be used here.