C++ OpenCL Example

CMakeLists.txt

# Minimal version of CMake
cmake_minimum_required (VERSION 2.6)
 
# Build type
if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
	message(STATUS "Setting build type to 'Debug' as none was specified.")
	set(CMAKE_BUILD_TYPE Debug CACHE STRING "Choose the type of build." FORCE)
	# Set the possible values of build type for cmake-gui
	set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS "Debug" "Release")
endif ()
 
# Define project name
project (OpenCL_Example)

set (CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/")
 
find_package( OpenCL REQUIRED )

include_directories( ${OPENCL_INCLUDE_DIR} )

# Source code of application		
set (opencl_example_src opencl_example.cc)
 
# Compiler flags
if (CMAKE_COMPILER_IS_GNUCC)
    set (CMAKE_CXX_FLAGS "-D_REETRANT -Wall -Wextra -pedantic -Wno-long-long")
	if (CMAKE_BUILD_TYPE STREQUAL "Debug")
   	    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -ggdb -O0")
	elseif( CMAKE_BUILD_TYPE STREQUAL "Release" )
	    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNDEBUG -O3 -fno-strict-aliasing")
	endif ()
endif (CMAKE_COMPILER_IS_GNUCC)
 
# Set up executable
add_executable (opencl_example ${opencl_example_src})
target_link_libraries(opencl_example ${OPENCL_LIBRARIES})

FindOpenCL.cmake

# - Try to find OpenCL
# Once done this will define
#  
#  OPENCL_FOUND		- system has OpenCL
#  OPENCL_INCLUDE_DIR  - the OpenCL include directory
#  OPENCL_LIBRARIES	- link these to use OpenCL
#
# WIN32 should work, but is untested

IF (WIN32)
	FIND_PATH(OPENCL_INCLUDE_DIR CL/cl.h )
	
	# TODO this is only a hack assuming the 64 bit library will
	# not be found on 32 bit system
	FIND_LIBRARY(OPENCL_LIBRARIES opencl64 )
	IF( OPENCL_LIBRARIES )
		FIND_LIBRARY(OPENCL_LIBRARIES opencl32 )
	ENDIF( OPENCL_LIBRARIES )
ELSE (WIN32)
	# Unix style platforms
	# We also search for OpenCL in the NVIDIA SDK default location
	FIND_PATH(OPENCL_INCLUDE_DIR CL/cl.h /opt/AMDAPPSDK-2.9-1/include/ )
	FIND_LIBRARY(OPENCL_LIBRARIES OpenCL 
	  ENV LD_LIBRARY_PATH
	)
ENDIF (WIN32)

SET( OPENCL_FOUND "NO" )
IF(OPENCL_LIBRARIES )
	SET( OPENCL_FOUND "YES" )
ENDIF(OPENCL_LIBRARIES)

MARK_AS_ADVANCED(
  OPENCL_INCLUDE_DIR
)

opencl_example.cc

#define __CL_ENABLE_EXCEPTIONS

#if defined(__APPLE__) || defined(__MACOSX)
#include <OpenCL/cl.hpp>
#else
#include <CL/cl.hpp>
#endif
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <vector>

const char * helloStr  = "__kernel void "
		"hello(void) "
		"{ "
		"  "
		"} ";

int main(void)
{
	cl_int err = CL_SUCCESS;
	try {

		std::vector<cl::Platform> platforms;
		cl::Platform::get(&platforms);
		if (platforms.size() == 0) {
			std::cout << "Platform size 0\n";
			return -1;
		}

		// Print number of platforms and list of platforms
		std::cout << "Platform number is: " << platforms.size() << std::endl;
		std::string platformVendor;
		for (unsigned int i = 0; i < platforms.size(); ++i) {
			platforms[i].getInfo((cl_platform_info)CL_PLATFORM_VENDOR, &platformVendor);
			std::cout << "Platform is by: " << platformVendor << std::endl;
		}

		cl_context_properties properties[] =
		{
				CL_CONTEXT_PLATFORM,
				(cl_context_properties)(platforms[0])(),
				0
		};
		cl::Context context(CL_DEVICE_TYPE_ALL, properties);

		std::vector<cl::Device> devices = context.getInfo<CL_CONTEXT_DEVICES>();

		// Print number of devices and list of devices
		std::cout << "Device number is: " << devices.size() << std::endl;
		for (unsigned int i = 0; i < devices.size(); ++i) {
			std::cout << "Device #" << i << ": " << devices[i].getInfo<CL_DEVICE_NAME>() << std::endl;
		}

		cl::Program::Sources source(1,
				std::make_pair(helloStr, strlen(helloStr)));
		cl::Program program_ = cl::Program(context, source);
		program_.build(devices);

		cl::Kernel kernel(program_, "hello", &err);

		cl::Event event;
		cl::CommandQueue queue(context, devices[0], 0, &err);
		queue.enqueueNDRangeKernel(
				kernel,
				cl::NullRange,
				cl::NDRange(4,4),
				cl::NullRange,
				NULL,
				&event);

		event.wait();
	}
	catch (cl::Error err) {
		std::cerr
		<< "ERROR: "
		<< err.what()
		<< "("
		<< err.err()
		<< ")"
		<< std::endl;
	}

	return EXIT_SUCCESS;

}

Note: This code was tested only with AMD GPU at Linux.

Works with Intel GT2 on Ubuntu 17.04 installing everything here: https://askubuntu.com/a/850594/698904
Output:

Platform number is: 1
Platform is by: Intel
Device number is: 1
Device #0: Intel(R) HD Graphics Kabylake ULT GT2

works with arch installing everything
Output :
Platform number is: 3 Platform is by: Intel(R) Corporation Platform is by: NVIDIA Corporation Platform is by: Mesa Device number is: 1 Device #0: Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz

Works also on Nvidia GTX 1080 on Ubuntu 18.04. (OpenCL libs distributed with the Cuda 8.0 toolkit)
Output:
Platform number is: 1
Platform is by: NVIDIA Corporation
Device number is: 1
Device #0: GeForce GTX 1080

Works on PopOS 20.04 with Nvidia GTX 2070

The key to make my OpenCL program work was to add set (CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/") to my CMakeList.txt file. Then my program would work with the custom FindOpenCL.cmake file.

works on Ubuntu14.04, nvidia GTX 1060. minor changes by setting cuda/include as OPENCL_INCLUDE_DIR and cuda/lib64/libOpenCL.so as OPENCL_LIBRARIES. output:

Platform number is: 1
Platform is by: NVIDIA Corporation
Device number is: 1
Device #0: GeForce GTX 1060 6GB