Simple OpenCL example

Adapted from here:

But tidied up a little and made to work on MacOS. It creates a OpenCL executor “thing” and tells it to run a kernel which writes “Hello World” to a buffer, this gets sent back to the main C++ program.

#include “cl.hpp”

std::string prog(
“#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enable\n” \
“__constant char hw[] = \”Hello World\”; \n” \
“__kernel void hello(__global char * out) {\n” \
” size_t tid = get_global_id(0); \n” \
” out[tid] = hw[tid]; \n” \

int main(void) {
cl_int err;

// Get list of platforms (things that can execute OpenCL on this host), get a “context” on the first executor.
std::vector platformList;
cl_context_properties cprops[3] = {CL_CONTEXT_PLATFORM, (cl_context_properties)(platformList[0])(), 0};
cl::Context context( CL_DEVICE_TYPE_CPU, cprops, NULL, NULL, &err);

// Allocate 100 bytes of memory which we will use to communicate with the OpenCL context, executor, whatever.
size_t mem_size = 100;
char * outH = new char[mem_size];
cl::Buffer outCL( context, CL_MEM_WRITE_ONLY | CL_MEM_USE_HOST_PTR, mem_size, outH, &err);

// Give the OpenCL program embedded in the string above to OpenCL.
cl::Program::Sources source(1, std::make_pair(prog.c_str(), prog.length()+1));

// Get devices used in this “context”
std::vector devices;
devices = context.getInfo();

// Compile program against device
cl::Program program(context, source);
err =,””);

// create a kernel object, tell it we are using the kernel called “hello”, give it an argument which is the memory we alloc’d above.
cl::Kernel kernel(program, “hello”, &err);
err = kernel.setArg(0, outCL);

// Queue the kernel up to run
cl::CommandQueue queue(context, devices[0], 0, &err);
cl::Event event;
err = queue.enqueueNDRangeKernel( kernel, cl::NullRange, cl::NDRange(mem_size), cl::NDRange(1, 1), NULL, &event);

// Use the event object above to block until processing has completed

// Read the results out of the shared memory area.
err = queue.enqueueReadBuffer( outCL, CL_TRUE, 0, mem_size, outH);

// Write to screen
std::cout << outH; } [/sourcecode] On MacOS compile it with: g++ ./simple.cpp -framework OpenCL -o simple You'll also need to C++ bindings for OpenCL which are available here: Tarball containing all this is HERE untar it and type make to build.