Vector Addition – OpenCL

In this post, I will show you how to write a vector addition code using OpenCL . The code is listed below:

#include <stdio.h>
#include <stdlib.h>
#include <iostream>

#ifdef __APPLE__
#include <OpenCL/opencl.h>
#include <CL/cl.h>

#define DATA_SIZE 10

using namespace std;

const char *ProgramSource =
"__kernel void add(__global float *inputA, __global float *inputB, __global float *output)\n"\
"  size_t id = get_global_id(0);\n"\
"  output[id] = inputA[id] + inputB[id];\n"\

int main(void)
cl_context context;
cl_context_properties properties[3];
cl_kernel kernel;
cl_command_queue command_queue;
cl_program program;
cl_int err;
cl_uint num_of_platforms=0;
cl_platform_id platform_id;
cl_device_id device_id;
cl_uint num_of_devices=0;
cl_mem inputA, inputB, output;

size_t global;

float inputDataA[DATA_SIZE]={1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
float inputDataB[DATA_SIZE]={1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
float results[DATA_SIZE]={0};

int i;

// retreive a list of platforms avaible
if (clGetPlatformIDs(1, &platform_id, &num_of_platforms)!= CL_SUCCESS)
printf("Unable to get platform_id\n");
return 1;

// try to get a supported GPU device
if (clGetDeviceIDs(platform_id, CL_DEVICE_TYPE_GPU, 1, &device_id, &num_of_devices) != CL_SUCCESS)
printf("Unable to get device_id\n");
return 1;

// context properties list - must be terminated with 0
properties[0]= CL_CONTEXT_PLATFORM;
properties[1]= (cl_context_properties) platform_id;
properties[2]= 0;

// create a context with the GPU device
context = clCreateContext(properties,1,&device_id,NULL,NULL,&err);

// create command queue using the context and device
command_queue = clCreateCommandQueue(context, device_id, 0, &err);

// create a program from the kernel source code
program = clCreateProgramWithSource(context,1,(const char **) &ProgramSource, NULL, &err);

// compile the program
if (clBuildProgram(program, 0, NULL, NULL, NULL, NULL) != CL_SUCCESS)
printf("Error building program\n");
return 1;

// specify which kernel from the program to execute
kernel = clCreateKernel(program, "add", &err);

// create buffers for the input and ouput

inputA = clCreateBuffer(context, CL_MEM_READ_ONLY, sizeof(float) * DATA_SIZE, NULL, NULL);
inputB = clCreateBuffer(context, CL_MEM_READ_ONLY, sizeof(float) * DATA_SIZE, NULL, NULL);
output = clCreateBuffer(context, CL_MEM_WRITE_ONLY, sizeof(float) * DATA_SIZE, NULL, NULL);

// load data into the input buffer
clEnqueueWriteBuffer(command_queue, inputA, CL_TRUE, 0, sizeof(float) * DATA_SIZE, inputDataA, 0, NULL, NULL);
clEnqueueWriteBuffer(command_queue, inputB, CL_TRUE, 0, sizeof(float) * DATA_SIZE, inputDataB, 0, NULL, NULL);

// set the argument list for the kernel command
clSetKernelArg(kernel, 0, sizeof(cl_mem), &inputA);
clSetKernelArg(kernel, 1, sizeof(cl_mem), &inputB);
clSetKernelArg(kernel, 2, sizeof(cl_mem), &output);


// enqueue the kernel command for execution
clEnqueueNDRangeKernel(command_queue, kernel, 1, NULL, &global, NULL, 0, NULL, NULL);

// copy the results from out of the output buffer
clEnqueueReadBuffer(command_queue, output, CL_TRUE, 0, sizeof(float) *DATA_SIZE, results, 0, NULL, NULL);

// print the results
printf("output: ");

for(i=0;i<DATA_SIZE; i++)
printf("%f ",results[i]);

// cleanup - release OpenCL resources

return 0;


To compile the code on a Mac, open terminal and type

g++ -o add add.c -framework OpenCL

The output is :

output: 2.000000

Take a look at line number 53, CL_DEVICE_TYPE_GPU is used to select a GPU device. Other alternatives for this flag include CL_DEVICE_TYPE_CPUCL_DEVICE_TYPE_ACCELERATORCL_DEVICE_TYPE_ALL etc . Refer to OpenCL documentation for more details (see here).


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