Getting started with OpenCL and GPU Computing

OpenCL (Open Computing Language) is a new framework for writing programs that execute in parallel on different compute devices (such as CPUs and GPUs) from different vendors (AMD, Intel, ATI, Nvidia etc.). The framework defines a language to write “kernels” in. These kernels are the functions which are to run on the different compute devices. In this post I explain how to get started with OpenCL and how to make a small OpenCL program that will compute the sum of two lists in parallel.

Installing and setting up OpenCL on your computer

First of all you need to download the newest drivers to your graphics card. This is important because OpenCL will not work if you don’t have drivers that support OpenCL.

To install OpenCL you need to download an implementation of OpenCL. The major graphic vendors NVIDIA and AMD have both released implementations of OpenCL for their GPUs. Other processor manufacturers such as Intel, also have their own OpenCL implementations. Note, if you are using Apple Mac OS X, you need to use Apple’s OpenCL implementation, which should already be installed on your system.

For AMD GPUs and CPUs download the AMD APP SDK
For NVIDIA GPUs download the CUDA Toolkit
For Intel CPUs/GPUs download the Intel OpenCL SDK

The installation steps differ for each SDK and the OS you are running. Follow the installation manual of the SDK carefully. You can have multiple OpenCL implementations installed on your system.

Your first OpenCL program – Vector addition

To demonstrate OpenCL I explain how to perform the simple task of vector addition. Suppose we have two lists of numbers, A and B, of equal size. The task of vector addition is to add the elements of A with the elements of B and put the result in the element of a new list called C of the same size. The figure below explains the operation.

Two lists A and B and the result list C of vector addition on A and B

The naive way of performing this operation is to simply loop through the list and perform the operation on one element at a time like the C++ code below:

for(int i = 0; i < LIST_SIZE; i++) {
    C[i] = A[i] + B[i];

This algorithm is simple but has a linear time complexity, O(n) where n is the size of the list. But since each iteration of this loop is independent on the other iterations this operation is data parallel, meaning that each iteration can be computed simultaneously. So if we have n cores on a processor this operation can be performed in constant time O(1).

To make OpenCL perform this operation in parallel we need to make the kernel. The kernel is the function which will run on the compute device.

The kernel

The kernel is written in the OpenCL language which is a subset of C and has a lot of math and vector functions included. The kernel to perform the vector addition operation is defined below.

__kernel void vector_add(__global const int *A, __global const int *B, __global int *C) {
    // Get the index of the current element to be processed
    int i = get_global_id(0);
    // Do the operation
    C[i] = A[i] + B[i];

The host program

The host program controls the execution of kernels on the compute devices. The host program is written in C, but bindings for other languages like C++ and Python exists. The OpenCL API is defined in the cl.h (or opencl.h for apple) header file. Below is the code for the host program that executes the kernel above on compute device. I will not go into details on each step as this is supposed to be an introductory article. The main steps of a host program is as follows:

  • Get information about the platform and the devices available on the computer (line 42)
  • Select devices to use in execution (line 43)
  • Create an OpenCL context (line 47)
  • Create a command queue (line 50)
  • Create memory buffer objects(line 53-58)
  • Transfer data (list A and B) to memory buffers on the device (line 61-64)
  • Create program object (line 67)
  • Load the kernel source code (line 24-35) and compile it (line 71) (online exeuction) or load the precompiled binary OpenCL program (offline execution)
  • Create kernel object (line 74)
  • Set kernel arguments (line 77-79)
  • Execute the kernel (line 84)
  • Read memory objects. In this case we read the list C from the compute device (line 88-90)
#include <stdio.h>
#include <stdlib.h>
#ifdef __APPLE__
#include <OpenCL/opencl.h>
#include <CL/cl.h>
#define MAX_SOURCE_SIZE (0x100000)
int main(void) {
    // Create the two input vectors
    int i;
    const int LIST_SIZE = 1024;
    int *A = (int*)malloc(sizeof(int)*LIST_SIZE);
    int *B = (int*)malloc(sizeof(int)*LIST_SIZE);
    for(i = 0; i < LIST_SIZE; i++) {
        A[i] = i;
        B[i] = LIST_SIZE - i;
    // Load the kernel source code into the array source_str
    FILE *fp;
    char *source_str;
    size_t source_size;
    fp = fopen("", "r");
    if (!fp) {
        fprintf(stderr, "Failed to load kernel.\n");
    source_str = (char*)malloc(MAX_SOURCE_SIZE);
    source_size = fread( source_str, 1, MAX_SOURCE_SIZE, fp);
    fclose( fp );
    // Get platform and device information
    cl_platform_id platform_id = NULL;
    cl_device_id device_id = NULL;   
    cl_uint ret_num_devices;
    cl_uint ret_num_platforms;
    cl_int ret = clGetPlatformIDs(1, &platform_id, &ret_num_platforms);
    ret = clGetDeviceIDs( platform_id, CL_DEVICE_TYPE_DEFAULT, 1, 
            &device_id, &ret_num_devices);
    // Create an OpenCL context
    cl_context context = clCreateContext( NULL, 1, &device_id, NULL, NULL, &ret);
    // Create a command queue
    cl_command_queue command_queue = clCreateCommandQueue(context, device_id, 0, &ret);
    // Create memory buffers on the device for each vector 
    cl_mem a_mem_obj = clCreateBuffer(context, CL_MEM_READ_ONLY, 
            LIST_SIZE * sizeof(int), NULL, &ret);
    cl_mem b_mem_obj = clCreateBuffer(context, CL_MEM_READ_ONLY,
            LIST_SIZE * sizeof(int), NULL, &ret);
    cl_mem c_mem_obj = clCreateBuffer(context, CL_MEM_WRITE_ONLY, 
            LIST_SIZE * sizeof(int), NULL, &ret);
    // Copy the lists A and B to their respective memory buffers
    ret = clEnqueueWriteBuffer(command_queue, a_mem_obj, CL_TRUE, 0,
            LIST_SIZE * sizeof(int), A, 0, NULL, NULL);
    ret = clEnqueueWriteBuffer(command_queue, b_mem_obj, CL_TRUE, 0, 
            LIST_SIZE * sizeof(int), B, 0, NULL, NULL);
    // Create a program from the kernel source
    cl_program program = clCreateProgramWithSource(context, 1, 
            (const char **)&source_str, (const size_t *)&source_size, &ret);
    // Build the program
    ret = clBuildProgram(program, 1, &device_id, NULL, NULL, NULL);
    // Create the OpenCL kernel
    cl_kernel kernel = clCreateKernel(program, "vector_add", &ret);
    // Set the arguments of the kernel
    ret = clSetKernelArg(kernel, 0, sizeof(cl_mem), (void *)&a_mem_obj);
    ret = clSetKernelArg(kernel, 1, sizeof(cl_mem), (void *)&b_mem_obj);
    ret = clSetKernelArg(kernel, 2, sizeof(cl_mem), (void *)&c_mem_obj);
    // Execute the OpenCL kernel on the list
    size_t global_item_size = LIST_SIZE; // Process the entire lists
    size_t local_item_size = 64; // Divide work items into groups of 64
    ret = clEnqueueNDRangeKernel(command_queue, kernel, 1, NULL, 
            &global_item_size, &local_item_size, 0, NULL, NULL);
    // Read the memory buffer C on the device to the local variable C
    int *C = (int*)malloc(sizeof(int)*LIST_SIZE);
    ret = clEnqueueReadBuffer(command_queue, c_mem_obj, CL_TRUE, 0, 
            LIST_SIZE * sizeof(int), C, 0, NULL, NULL);
    // Display the result to the screen
    for(i = 0; i < LIST_SIZE; i++)
        printf("%d + %d = %d\n", A[i], B[i], C[i]);
    // Clean up
    ret = clFlush(command_queue);
    ret = clFinish(command_queue);
    ret = clReleaseKernel(kernel);
    ret = clReleaseProgram(program);
    ret = clReleaseMemObject(a_mem_obj);
    ret = clReleaseMemObject(b_mem_obj);
    ret = clReleaseMemObject(c_mem_obj);
    ret = clReleaseCommandQueue(command_queue);
    ret = clReleaseContext(context);
    return 0;

To make OpenCL run the kernel on the GPU you can change the constant CL_DEVICE_TYPE_DEFAULT to CL_DEVICE_TYPE_GPU in line 43. To run on CPU you can set it to CL_DEVICE_TYPE_CPU. This shows how easy OpenCL makes it to run different programs on different compute devices.

The source code for this example can be downloaded here.

Compiling an OpenCL program

If the OpenCL header and library files are located in their proper folders (/usr/include and /usr/lib) the following command will compile the vectorAddition program.

gcc main.c -o vectorAddition -l OpenCL

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152 Responses

  1. mariam says:

    I couldn’t build the program first, got different errors, finally figured out the problem was the comments in the kernel using //, it works if I use /*comment*/, does anyone know why?

  2. shilpa says:

    Hi Erik,

    Your program is pretty good to understand . I ran this program on a octacore machine but could not see the expected parallel processing of instructions.
    The time taken for this code was more than a sequential code for vector addition. I doubt , this program is not running on multiple cores parallely on my system. Could you plz help ?


    • Erik Smistad says:

      This is probably because the vectors are so small (only 1024 items). Try to increase the size of the vector to lets say 1024*1024*1024, and you will probably see a speedup.

      • celio says:

        i changed item_size to 1024*1024*32 but i did not defferene between GPU &CPU

        • anonymous says:

          this is because it has to be COPIED. The memory access is what takes the most time in this case. If it were a O(n^2) operation or even a more expensive linear operation, it would make much more sense to have it run on the GPU.

  3. KSSR says:

    hello all,
    I need instruction about setting up opencl environemnt for Multicore system i.e on GPU.

  4. kevinkit says:

    Sry I was wrong with the cl_mem stuff but nevertheless the “+1” is missing. (THIS I NOT C++!)

  5. kevinkit says:

    Doesn’t it has to be

    int *A = (int*)malloc(sizeof(int)*(LIST_SIZE +1));

    and this in every other memory allocation furthermore when you allocate the memory objects it should be

    cl_mem a_mem_obj = clCreateBuffer(context, CL_MEM_READ_ONLY,
    (LIST_SIZE+1) * sizeof(cl_mem), NULL, &ret);

    instead of cl_int.

  6. Fabio says:

    Hi, I have a question.
    When you call clEnqueueReadBuffer, you don’t have to pass as parameter also the list of events to wait before read the buffer? I mean, you can get the event identifier from clEnqueueNDRangeKernel and pass it to clEnqueueReadBuffer, otherwise the program may read the results before the sum is completed.
    If it’s not needed, why?

    • Erik Smistad says:

      The third argument of clEnqueueReadBuffer with the value CL_TRUE ensures that this call is blocking. That means that the function will not return until it has read the buffer and thus explicit synchronization is not needed. However, if you set this argument to CL_FALSE you have to do explicit synchronization using events as you suggest.

      • Fabio says:

        I know that the third argument make the call blocking, but I think that means that (as you say) “the function will not return until it has read the buffer”. Howerer the documentation doesn’t say anything about the previous enqueued operations for this argument. Maybe is not so clear.
        The documentation says also that you must ensure that “All commands that use this buffer object have finished execution before the read command begins execution”

        • Erik Smistad says:

          Ah yes, that is a good point. The clue here is that your command queue is created with in-order execution (this is default and most devices doesn’t support out-of-order execution). In-order execution guarantees that all of the queued commands will be executed in the order in which they were added to the queue. Thus, for clEnqueueReadBuffer to finish, all other queued operations have to finish first.

          See for more info on this

        • Fabio says:

          Aaahh sorry 🙂
          Your queue is not out-of-order.

          I’m working with an aout-of-order queue and I have some problems so I’m trying to understand…

  7. Anonymous says:

    Have you found out how to fix the failure?

    Name (required):

    I added a printf() to your code after line 43:if (ret != CL_SUCCESS) {printf(“Error: Failed to query platforms! (%d)\n”, ret);return EXIT_FAILURE;}
    after compiling, running it gives me this error: “Failed to query platforms (-1001)”

  8. meenu says:

    I am currently using ubuntu13.04 and have a VGA compatible controller: NVIDIA Corporation GK107 [GeForce GT 630 OEM] (rev al) … My open CL samples are running fine for CL_DEVICE_TYPE_DEFAULT and CL_DEVICE_TYPE_CPU… But they are not able to find OPENCL devices for CL_DEVICE_TYPE_GPU…

    • Erik Smistad says:

      The NVIDIA OpenCL platform only support GPUs. So most likely you have more than one platform installed (or the NVIDIA platform is not installed). Try to select the correct platform. You can do this by increasing the number of entries in the clGetPlatformIDs function, see

      • Anonymous says:

        Okie i will tell u what exactly my configuration is and what i have observed then probably u might help me out… My CPU is intel core processor -i7 and i have an inbuilt gpu of nvidia. Now i have installed both intel sdk for opencl and nvidia propeitary drivers for ubuntu13.04. How can i create a run time so that it identifies both CPU and GPU. Currently i feel that only intel platform is getting recognised … hence opencl is working fine for option CPU and not GPU. Is there a way around where both my devices are identified and probably i can transfer data between my CPU and GPU. Also in my vendor directory i can observe both intelocl64.icd and nvidia.icd.

        • Erik Smistad says:

          An OpenCL context can only be associated with ONE platform.You have TWO platforms installed and the code above only selects ONE platform, whichever one comes first, in your case the intel platform. To select the NVIDIA platform you need to increase the number of entries:

          cl_platform_id platform_ids[2];
          clGetPlatformIDs(2, platform_ids, &ret_num_platforms);

          and then select the other platform like this:

          clGetDeviceIDs( platform_ids[1], CL_DEVICE_TYPE_DEFAULT, 1, &device_id, &ret_num_devices);

  9. Gustavo Rozolin da Silva says:

    Excellent post Erik,

    Erik what I need to change in this code for to pass __local arg to kernel.

    Thanks you.

    • Erik Smistad says:

      Local memory, or shared memory as it is also called, is not accessible from the host. So if you want to use it you have to transfer data from global to local explicitly in a kernel.

  10. Laxator2 says:

    Here is how it worked on my machine, using a (rather old) Nvidia card under PCLinuxOS :

    gcc main.c -I/usr/local/cuda-5.0/include/ -L/usr/lib/nvidia-current -l OpenCL -o vectorAddition

    Great example, short and to the point.

  11. Kareti says:

    Hi Erik, The blog was very helpful. Thanks for that.

    I have no GPU on my laptop, so is there a way to practise the opencl programs by emulating a GPU! I am using Fedora 18, 64 bit !

    Thank you.

    • Erik Smistad says:

      Yes, you can use your CPU instead. That’s the nice thing about OpenCL: The code can run on different types of processors. To do so simply install the Intel or AMD OpenCL runtime, depending on which type of processor you have. Afterwards execute the example above and it should run on the CPU.

  12. Victor says:

    Hi Erik. Could u tell me what does this mean?
    gcc -c -I /usr/include/CL main.c -o main.o
    gcc -o local16 main.o -L /usr/lib -l OpenCL
    /usr/lib64/gcc/x86_64-suse-linux/4.7/../../../../x86_64-suse-linux/bin/ld: skipping incompatible /usr/lib/ when searching for -lOpenCL
    /usr/lib64/gcc/x86_64-suse-linux/4.7/../../../../x86_64-suse-linux/bin/ld: skipping incompatible /usr/lib/ when searching for -lc

    I dont know if I successfuly link to the lib. What’s weird is that the result keeps the same even if I comment the whole kernel code.

    • Erik Smistad says:

      This is a problem with the linking, not the source code. Not sure what the problem is, never seen that error message before

  13. Xocoatzin says:

    Wow, it just worked. Awesome!

  14. Anonymous says:

    Very nice article, thanks!

    I’m wondering whether the statements:

    ret = clFlush(command_queue);
    ret = clFinish(command_queue);

    are actually needed. If I’m getting it right, since the command queue is in-order, when clEnqueueReadBuffer (which is blocking thanks to the CL_TRUE parameter) returns, the command queue should be empty.

    Another point that would be worth explaining is why there is no clFinish
    between clEnqueueNDRangeKernel and clEnqueueReadBuffer.

    • Erik Smistad says:

      Since the program terminates right after all the clean up statements, none of them are actually needed.

      In either case, you are correct, the flush and finish statements are not necessary.

      When the command queue is in-order, OpenCL will make sure that all the enqueue commands are performed in the order in which they are called in your C program.

  15. Rohit Vashistha says:

    All those getting ‘zero’ result change the option “GPU” to “CPU” or vise versa

  16. Jai says:

    I have a desktop pc with configuration as:
    Intel(R) Core(TM) 2 Duo CPU E700@2.93GHz, 2GB RAM 32 bit OS

    and I am willing to purchase a graphic card with config as:
    Sapphire AMD/ATI RADEON HD 6450 2GB RAM

    Can you please tell me is it compatible for my pc…?
    Thanks in advance.

  17. Jack says:

    Can i run openCL on a CPU? (I do not have a GPU but want to experiment with openCL before buying one).
    I have an intel i3 processor with a dual boot for Windows 7 and Ubuntu 12.04

  18. Anonymous says:

    The sample code all worked fine.
    But when I changed CL_DEVICE_TYPE_DEFAULT into CL_DEVICE_TYPE_GPU, it runs, but give me:
    0 + 1024 = 763461144
    1 + 1023 = 32716
    2 + 1022 = 763461144
    3 + 1021 = 32716
    4 + 1020 = 15489024
    5 + 1019 = 0
    6 + 1018 = 15489024
    7 + 1017 = 0
    8 + 1016 = 0
    9 + 1015 = 0
    10 + 1014 = 0
    11 + 1013 = 0
    12 + 1012 = 0
    13 + 1011 = 0
    14 + 1010 = 0
    15 + 1009 = 0
    16 + 1008 = 0
    17 + 1007 = 0
    18 + 1006 = 0
    19 + 1005 = 0
    20 + 1004 = 0
    21 + 1003 = 0
    22 + 1002 = 0
    23 + 1001 = 0
    24 + 1000 = 0
    25 + 999 = 0
    26 + 998 = 124817
    27 + 997 = 0
    28 + 996 = 1801415779
    29 + 995 = 1717531240
    30 + 994 = 540292720
    31 + 993 = 1633643619
    32 + 992 = 1717527661
    33 + 991 = 540292720
    34 + 990 = 1801415779
    35 + 989 = 1734308456
    36 + 988 = 1633841004
    37 + 987 = 1852399468
    38 + 986 = 1597125492
    39 + 985 = 1702060386
    40 + 984 = 1869898079
    41 + 983 = 1935894893
    42 + 982 = 1600938784
    43 + 981 = 1601333355
    44 + 980 = 1651469415
    45 + 979 = 1767861345
    46 + 978 = 842232942
    47 + 977 = 1954047327
    48 + 976 = 1701080677
    49 + 975 = 1952538468
    50 + 974 = 1667853679
    i tried CL_DEVICE_TYPE_CPU, and it worked fine.

    why is GPU not working?

  19. safwan says:

    Thank you for this briefly example and its work with me but I have a problem when I change the value of LIST_SIZE to another value, the program execute but she doesn’t execute the kernel fonction an finaly the result i have for C[i]=0 :

    how can I resolve this problem?

    • Erik Smistad says:

      If you change the LIST_SIZE variable to another variable that is not dividable with 64 it will not run because local size is set to 64 (see line 83). This means that 64 work-items are grouped together.

      If you only get 0, you probably haven’t installed OpenCL correctly.

  20. Yaknan says:

    Hi Erik,
    I am enjoying your tutorials on OpenCl. Thanks for the good work. Please, am testing a hypothesis here for my thesis work on enhancing graphic rendering of the ray tracing algorithm. Am wondering if it is possible to integrate cilk++ with openCL. The idea is to see if cilk++ will take full and better advantage of CPUs while OpnCL takes advantage of the GPUs more efficiently. Thanks!

    • Erik Smistad says:

      As far as I know cilk++ can run regular C/C++ code as well. And OpenCL is written in C, so I think it should work..

  21. prince says:

    i am using gpu of type nvidia,i am using opencl but when i run the program using ctrl+F5(start without debugging )then i get result in which gpu takes more time than cpu but when i run the program cpu takes more time than gpu and result is also i am giving
    start without debugging -> cpu time=6127 ms gpu time= 6240 ms
    start with debug-> cpu time= 18354 ms gpu time= 9125 ms

    wt is the reason in this difference……
    visual studio 2010 i am using
    the code is here. wt is going wrong.?..thanks

    // Hello.cpp : Defines the entry point for the console application.

    #include “CL/cl.h”
    #define DATA_SIZE 100000
    const char *KernelSource =
    “kernel void hello(global float *input , global float *output)\n”\
    ” size_t id =get_global_id(0);\n”\
    “output[id] =input[id]*input[id];\n”\
    “} ”
    //float start_time,end_time;

    int main(void)
    double start_time,end_time;
    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 input,output;
    size_t global;
    float inputData[100000];
    for(int j=0;j<100000;j++)

    float results[DATA_SIZE];//={0};

    // int i;

    //retrieve a list of platform variable
    printf("Unable to get platform_id\n");
    return 1;

    //try to get supported GPU DEvice
    printf("unable to get device_id\n");
    return 1;

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

    //create a context with the GPU device

    //create command queue using the context and device

    //create a program from the kernel source code
    program=clCreateProgramWithSource(context,1,(const char**)

    //compile the program
    printf("build error \n",err);
    size_t len;
    char buffer[4096];
    //get the build log
    printf("—-build Log—\n%s\n",buffer);

    // return 1;

    //specify which kernel from the program to execute

    //create buffers for the input and output


    //load data into the input buffer


    //set the argument list for the kernel command

    //enqueue the kernel command for execution

    //copy the results from out of the buffer

    //print the results
    for(int i=0;i<DATA_SIZE;i++)
    //printf("no. of times loop run %d\n",count);

    //cleanup-release OpenCL resources

    printf("execution time is%f",end_time-start_time);
    return 0;


    • Erik Smistad says:

      It is normal for the execution time to increase when debugging an application. It doesn’t mean that anything is wrong with the program

  22. swap says:

    How will i execute same program on windows 7 with intel HD 4000 GPU.
    I have installed Intel opencl SDK

    • Erik Smistad says:

      Just remember to link to the lib files included in the Intel OpenCL SDK and add the include folder. If you are using Visual Studio you can add these things in the project settings menu.

  23. swap says:

    how will i execute same program on windows 7 with intel HD 4000.
    I have downloaded Intel Opencl SDK.

  24. Lennart says:


    You officially got me started with OpenCL

    (Hilsen fra Bergen)

  25. Ricardas says:

    Great Tutorial. Thanks ! 🙂

  26. Shinchan says:

    I followed your steps and was able to get main.o.
    But when i did
    gcc main.o -o host -L /home/mydir/Downloads/ati-stream-sdk-v2.1-lnx64/lib/x86_64 -l OpenCL

    I got
    /usr/bin/ld: cannot find -lopenCL
    clooect2: id returned 1 eit status

    I have no idea what this means. Please help!

    • Erik Smistad says:

      It means that the linker can’t find the libOpenCL file which should lie in your /home/mydir/Downloads/ati-stream-sdk-v2.1-lnx64/lib/x86_64 folder. Make sure you are using a large O in “-lOpenCL” and not “-lopenCL” as it says in your error message: “/usr/bin/ld: cannot find -lopenCL”

  27. spandei says:

    Thank you!
    You write very understandable code:)
    To tell the truth your article helped me to understand the OpenCL Mechanism better than the whole AMD SKD. Keep it up!

  28. Bishoy Mikhael says:

    i failed to compile the example,can you please review my configuration for the IDE, i’ve tried MS Visual Studio 2010, NetBeans 7.1 and Eclipse Indigo using both AMD and NVIDIA SDKs on Windows 7 x64 with Nvidia GeForce 330M graphics card.
    i’ve declared the environment variables for CUDA SDK as follows $(CUDA_LIB_PATH), $(CUDA_BIN_PATH) and $(CUDA_INC_PATH) for (CUDA installation path\lib\x64), (CUDA installation path\bin), (CUDA installation path\include) respectively. in NetBeans TOOLS–>Options in C/C++ Code Assistance tab i’ve added the include directory for CUDA SDK, then in the project properties in “C Compiler” tab i’ve added the include directory path in “Include Directory”, and in the “Linker” tab i’ve added the library path in “Additional Library Directories” then “opencl.lib” in “Additional Dependencies”, i din’t know what to add in the “Compilation Line” or if there is another settings i’m missing.

    when i build the project i get an error:

    “/bin/sh: -c: line 0: syntax error near unexpected token `(‘
    /bin/sh: -c: line 0: `gcc.exe -m64 -c -I(CUDA_INC_PATH\) -MMD -MP -MF build/Release/MinGW-Windows/_ext/141055651/vector_add.o.d -o build/Release/MinGW-Windows/_ext/141055651/vector_add.o /C/Users/Arch/Documents/NetBeansProjects/vector_add/vector_add.c’
    make[2]: *** [build/Release/MinGW-Windows/_ext/141055651/vector_add.o] Error 2”

    • Erik Smistad says:

      Always a nightmare to compile on windows… but it should work on Visual Studio 2010 and setting include and library settings should be enough. Don’t know what’s wrong in your case

      • Bishoy Mikhael says:

        i’ve uninstalled VS 2010, netbeans, MinGW, cygwin and the newly installed Microsoft Visual C++ 2008 Redistributables and .NET frameworks, then installed Code::Blocks then i copied the CL directory from the include directory of NVIDIA Toolkit and set the additional libraries in Code::Blocks to point at the OpenCL.lib, guess what, it worked fine without any errors

  29. Hao Wang says:


    I’m trying to run OpenCL program on a simulator (gem5).

    The simulator supports Full-System mode, that is, first boot Linux from a disk-image and then run the program.

    I borrowed the ICD record and from AMD SDK, and put them into the proper place in the image file.

    But the simulation trace shows that, it fails to find a platform, and then crashes when trying to create a context.

    Do you have any suggestions on my situation>
    Thank you.

    • Erik Smistad says:


      Make sure the ICD files can be read by the program and that the AMD APP SDK and display drivers are properly installed.

  30. rupam says:

    hi, I an pretty new in GPU.Currently I am working on AMD RADEON 6500 series…. I have written some program on MATLAB 2011b…I want to run the codes on GPU…can u plz instruct me how to run .m codes on GPU…thanx in advance…

  31. vegihat says:

    Hello Erik,

    i try to understand what is the physical meaning of below clEnqueueNDRangeKernel’s arguments

    const size_t *global_work_size
    const size_t *local_work_size

    you used values
    size_t global_item_size = LIST_SIZE;
    size_t local_item_size = 64

    which means that we have LIST_SIZE/64 work-groups,right?

    what’s the difference between local_item_size=1 and local_item_size = 64?

    i want to understand which is the perfect value of the local_item_size .

    • Erik Smistad says:

      The “perfect” value of local_item_size is very system and application dependent. You can omit this parameter and let OpenCL decide by itself. Note that for NVIDIA GPUs the local_item_size should be a multiple of 32 (one warp) or else some work-items will be idle. The same applies to AMD GPUs, but with a multiple of 64 instead (one wavefront as they call it). This is because NVIDIA and AMD schedules 32 and 64 work-items atomically on each compute unit.

  32. The Ham says:

    Hi, to all people that get wrong output (garbage or all 0)
    I used this code under gForce M8400 GS and get garbage and error -52 in clEnqueueNDRangeKernel which is wrong kernel arguments.

    that is what i changed in this code:
    add 4-th argument for the kernel

    ret = clSetKernelArg(kernel, 3, sizeof(int), &LIST_SIZE);

    remember that in this code LIST_SIZE must be 64 * m (m is integer)

    This code works ok on my AMD HD 6670 without any changes,
    dont know why (just started with OpenCL)

    (cant rly add comment on this site!!)

  33. The Ham says:

    Hi, to all people that get wrong output (garbage or all 0)
    I used this code under gForce M8400 GS and get garbage and error -52 in clEnqueueNDRangeKernel which is wrong kernel arguments.

    that is what i changed in this code:
    add 4-th argument for the kernel

    ret = clSetKernelArg(kernel, 3, sizeof(int), &LIST_SIZE);

    remember that in this code LIST_SIZE must be 64 * m (m is integer)

    This code works ok on my AMD HD 6670 without any changes,
    dont know why (just started with OpenCL)

    • The Ham says:

      i just can get it right…

      another thing in the kernel
      change output buffer type
      __global int *C to __global float *C
      or you get all = 0

  34. LaKing says:

    Hello. ..

    I compiled some sample applications, and get this error when running any OpenCL application. …

    OpenCL SW Info:

    Error -1001 in clGetPlatformIDs Call !!!

    I was googling for several hours, got some useful info’s but could not solve the problem yet. Any ideas? … Thanks.

    • Erik Smistad says:


      I think it means that it can’t find any OpenCL platforms. Check to see if the .icd files are properly installed. They should exist under /etc/OpenCL/vendors. If no .icd files exists there you have to find them in the downloaded SDK and extract them manually to this location.

  35. Otto says:

    The OpenCL library seems to slow initial execution down a bit. The example above does not really work the GPU. I created the same thing in pure C and it’s almost 3 times faster. 😐

    time (./cpu >dump)

    real 0m0.320s
    user 0m0.292s
    sys 0m0.024s

    time (./gpu >dump)

    real 0m0.825s
    user 0m0.736s
    sys 0m0.100s

    • Erik Smistad says:

      There is always some overhead with using the GPU such as transferring data back and forth and setup time compared to the CPU. But if you try the same example with a VERY large list/vector you will definitely see a speedup. So if your data parallel problem is very small, using the GPU is usually not faster, but large problems such as volume processing where you have several million elements to be processed you get an enormous speedup.

  36. Otto says:

    Create a Makefile
    CFLAGS= -c -I/opt/AMDAPP/include
    LDFLAGS= -L/opt/AMDAPP/lib/x86_64 -L/usr/lib -lOpenCL

    all: host

    ${GCC} ${CFLAGS} main.c -o main.o
    ${GCC} main.o -o host ${LDFLAGS}

    rm -rf host main.o

    I’m using Ubuntu 11.10 64bit + AMD-APP-SDK 2.6 + 11.12 drivers and got it working with the Makefile. In Eriks exmaple above, the linking params are in the wrong order, therefore it fails every time.

    • Erik Smistad says:

      I can swear that my compilation command worked before, but when I tried it myself just now on the same system it failed. Maybe its a new version of gcc or something… anyway thanks for letting me know, I’ve updated the post.

  37. Ajay says:

    Hey Erik,
    Is there a way to write functions inside the opencl kernel?

    • Erik Smistad says:

      I don’t now why you would want to write a function inside a kernel… but you can create a function in OpenCL that is inside the same file as the kernels and then call that function in the kernel (see below). If this is not what you want you may have to use macros or something (like “#define MAX(a,b) a > b ? a : b”).

      int myFunction() {

      __kernel void myKernel() {

  38. José says:


    I tried to install and run your example but the answer is that it can’t find the file cl.h
    What should I look for?
    I tried to reinstall but it simply don’t want to run.
    The icd files and the .h are where they are supposed to be.
    The instalation of the openCL is on the /opt/AMDAPP/
    Also, running on a Phenom II 1090T + HD6850

    Forgot this information:

    The comand lines that I used:

    main.c @ ~/openCL-teste/

    comand line used:
    gcc -I /opt/AMDAPP/include/CL/ main.c -o main.o

    • Erik Smistad says:

      It is because the include line is “#include “, but your include path is inside the CL directory. Try this instead: “gcc -I /opt/AMDAPP/include/ main.c -o main.o”

  39. José says:

    I tried to install and run your example but the answer is that it can’t find the file cl.h

    What should I look for?

    I tried to reinstall but it simply don’t want to run.

    The icd files and the .h are where they are supposed to be.

    The instalation of the openCL is on the /opt/AMDAPP/

    Also, running on a Phenom II 1090T + HD6850

  40. Jesse says:

    This is a good guide, so much easier on linux than windows

  41. Anjil says:

    I tried with CPP vesrion of your example and it simply displays: clGetPlatformIDs(-1001)

    Does this mean the OpenCL is not installed properly? Should I re install the NVIDIA driver?

  42. Anjil says:

    The program compiles fine but the results are not as expected:

    989 + 35 = 0
    990 + 34 = 0
    991 + 33 = 0
    992 + 32 = 0
    993 + 31 = 0
    994 + 30 = 0
    995 + 29 = 0
    996 + 28 = 0
    997 + 27 = 0
    998 + 26 = 0
    999 + 25 = 0
    1000 + 24 = 0
    1001 + 23 = 0
    1002 + 22 = 0
    1003 + 21 = 0
    1004 + 20 = 0
    1005 + 19 = 0
    1006 + 18 = 0
    1007 + 17 = 0
    1008 + 16 = 0
    1009 + 15 = 0
    1010 + 14 = 0
    1011 + 13 = 0
    1012 + 12 = 0
    1013 + 11 = 0
    1014 + 10 = 0
    1015 + 9 = 0
    1016 + 8 = 0
    1017 + 7 = 0
    1018 + 6 = 0
    1019 + 5 = 0
    1020 + 4 = 0
    1021 + 3 = 0
    1022 + 2 = 0
    1023 + 1 = 0

    Please let me know whats the issue here?

  43. blogger says:

    Great tutorial! Thanks a lot, it helped me get started with opencl

  44. Guilherme says:

    Great tutorial! Congrats dude.

    I’m having just a little problem. I’m trying to use M$ Visual C++ 2010 Express: I created a new empty project, included the path to the necessary files (AMD SDK – wich are located on C:\Program Files (x86)\AMD APP\include) on the “include settings” of the project, created two files on this project with the names test.cpp and with the exact same codes on this tutorial. I get a lot of errors:
    (first line in the output)- 1>classe.obj : error LNK2019: unresolved external symbol _clReleaseContext@4 referenced in function _main

    Am I doing something wrong? Should I use another IDE? I’m really newbie on OpenCL (and a little bit in C++). And sorry for my english ’cause I’m brazilian.

    • Erik Smistad says:

      You have forgot to add additional library paths in Visual Studio. You need to go to project settings, c++ linker, and then add C:\Program Files (x86)\AMD APP\lib/x86 to additional libraries, and on input write OpenCL.lib…

      I think I will add some notes on how to set up OpenCL in visual studio in this post in a few days

  45. Ben says:

    Yours is probably the clearest and most concise “getting started” guide I’ve seen, so many thanks for this.

    Incidentally, both your C and C++ example programs also work perfectly on Windows using AMD’s OpenCL implementation with MinGW-w64.

  46. rwi says:

    Hi Erik,
    your guide helped me getting started. I’m using the nVIDIA CUDA Toolkit 4 on Ubuntu 10.10. I had trouble compiling with gcc and ended up with:

    gcc -lOpenCL -I /usr/lib64/ -I /usr/local/cuda/include main.c -o host

    where again lOpenCL starts with a lowercase L and the two locations are prefixed with -(capital i) (just in case). This worked for me.

    Thanks a lot!

    • Jeffin says:

      Hi rwi,
      Thanks a lot for your post.The command options that you provided was very helpful for my build issues while trying with Netbeans IDE.
      Thanks again

  47. none says:

    I feel with you and did not read your desperate endeavours (trying to help the ppl here) till the end. But one thing i had to fight with a year ago was that the nvidia drivers that came with ubuntu (i mean the propietary ones) did not place two links right. One of them was the opencl… (as above). One solution was to install the driver directly from nvidia. The other one is to search the net and the answer will come. It’s just two softlinks.

  48. Dan says:

    Can we run OpenCl without installing one of the SDKs? if so, what is the best way to do it?

  49. michael says:

    Here is my result, it’s not correct. why?
    0 + 1000 = 0
    1 + 999 = 0
    2 + 998 = 0
    3 + 997 = 0
    4 + 996 = 0
    5 + 995 = 0
    6 + 994 = 0
    7 + 993 = 0
    8 + 992 = 0
    9 + 991 = 0
    10 + 990 = 0
    11 + 989 = 0
    12 + 988 = 0

  50. Harshit says:

    Hi Erik,
    I’m a noob..
    I get this error when i Compile the program using
    Documents/cuda/Assignment/Ex. 2$ gcc -c -Wall -I /usr/local/cuda/include cpu.c -o main

    cpu.c:8: warning: return type defaults to ‘int’
    cpu.c: In function ‘main’:
    cpu.c:52: warning: control reaches end of non-void function

    But I’m getting the right result.. 🙂

    Is something wrong??

    • Erik Smistad says:

      They are only warnings… and it seems you only need to add a “return 0;” in the end of your main method to get rid of it.

  51. Ajay says:

    I changed it. But I want to know that whether the program is running on the GPU or not..
    Will I get error message if the program is not running on GPU or will it run on CPU if it doesn’t find the GPU after changing CL_DEVICE_TYPE_DEFAULT to CL_DEVICE_TYPE_GPU.

    I’m asking this coz I wanted to measure performance of an OpenCL program by using clGetEventProfilingInfo. I have done everything right but I’m getting timings only if i set CL_DEVICE_TYPE_CPU. If i set it to CL_DEVICE_TYPE_DEFAULT or CL_DEVICE_TYPE_GPU, then I’m getting answer as 0.00.
    Why am I getting timings only on CPU?? Is the program running on CPU even after setting CL_DEVICE_TYPE_GPU?? Thats my doubt..

    • Erik Smistad says:

      There is no error checking in the example above, because I wanted the example to be as simple as possible. So it can happen that it doesn’t run at all. You can print out ret_num_devices and see if it is larger than 0.

      I recommend the free OpenCL/OpenGL profiler tool gDEBugger –

      With this program you should be able to see exactly where your code is run and how fast.

  52. Ajay says:

    thanks Erik,
    I have another doubt..
    How can i find whether the code is actually running on GPU or not??

    • Erik Smistad says:

      Change CL_DEVICE_TYPE_DEFAULT to CL_DEVICE_TYPE_GPU on line 43 and it should only run on GPUs.

  53. Ajay says:

    I am new to OpenCL programming. I successfully compiled the sample program. then I tried ‘./host’ to run it. but I’m getting an error..
    ./host: error while loading shared libraries: cannot open shared object file: No such file or directory

    • Erik Smistad says:

      That error means that you have forgot to set the LD_LIBRARY_PATH, see the description in the article above.

  54. Kurt says:

    Hi Erik – Thanks for your blog. Has given me good help.
    Now on a slightly different topic: ATI plus NVidia plus CPU.
    The reason I want to use OpenCL is so my software can work on any of these parallel platforms – I might even create a driver for high-end DSPs after I learn a bit more.

    However, it seems difficult to find platform-neutral info on how to get set up. Is the platform (NV vs ATI) entirely based on what LD_LIBRARY_PATH points to? If so, is it impossible to use –device cpu in a machine equipped with NVidia GPU?

    IBM has what I think is intended to be a solution called OpenCLCommonRuntime (aplhaworks), but I can’t seem to get it to work with AMD –device cpu. Do you have any experience with this? IBM is, at least, a neutral source.

    • Erik Smistad says:

      Hi Kurt

      My personal experience with NVidia’s and ATI/AMD’s OpenCL implementation is that NVidia still only has a buggy version of OpenCL 1.0. Their implementation has some serious issues when using images. ATI/AMD’s implementation on the other hand has worked great and is updated to 1.1.

      The thing about OpenCL is that it is to some degree portable, (it is portable if you don’t take into account all of NVidia’s bugs), but it is not necessary performance portable. Some OpenCL code might be fast on an ATI card while it may not be on an NVidia card. Because of their different architectures. This might and hopefully will be less true when their OpenCL compilers have matured.

      It is possible to use the CPU with OpenCL on a machine with a NVidia GPU.

      I haven’t tried OpenCLCommonRuntime, but it looks quite promising

  55. Krish says:

    hi erik
    it worked ……


  56. Krish says:

    Hi erik
    (for the post no:28)
    even after specifying cuda installation path
    [root@ghost OpenCL]# gcc -l /usr/local/cuda/include/CL/ -lOpenCL hellocl.c
    In file included from hellocl.c:12:0:
    cl.h:32:28: fatal error: CL/cl_platform.h: No such file or directory compilation terminated.

    i dont understand what is happening…… how to correct the errror……..

  57. paolo says:

    Hi Erik,
    thanks for posting these info

    I tried to compile your example on a Mac OSX 10.6 with Envidia GEFORCE 9400M.

    I have the drivers installed correctly (I believe), and so it the opencl toolkit.
    When I call the linking part of it, I get the error on “host”

    it says: “host: no such file or directory”
    what is host exactly?
    thank you

    • Erik Smistad says:

      Host is just the name of the executable you are creating when compiling. The name of the executable is not important. There most be something wrong with your compile command. Paste your compile command here

  58. Krish says:

    i am just started learning open cl
    i have installed nvidia sdk etc… as given on Nvidia website…
    i am using Fedora 14 – 64 bit edition
    when i am compiling
    $ gcc hello.c -lopenCL

    it gives me
    cl.h:32:28: fatal error: CL/cl_platform.h: No such file or directory; compilation terminated.

    pls need help from u to rectify it ….

    • Erik Smistad says:

      Have you remembered to specify the include path -I to the include folder of your CUDA installation?

  59. Freddan says:

    main.c:(.text+0x129): undefined reference to `clGetPlatformIDs’
    main.c:(.text+0x150): undefined reference to `clGetDeviceIDs’
    main.c:(.text+0x17b): undefined reference to `clCreateContext’
    main.c:(.text+0x19e): undefined reference to `clCreateCommandQueue’
    main.c:(.text+0x1cc): undefined reference to `clCreateBuffer’
    main.c:(.text+0x1fa): undefined reference to `clCreateBuffer’
    main.c:(.text+0x228): undefined reference to `clCreateBuffer’
    main.c:(.text+0x27c): undefined reference to `clEnqueueWriteBuffer’
    main.c:(.text+0x2cc): undefined reference to `clEnqueueWriteBuffer’
    main.c:(.text+0x2ef): undefined reference to `clCreateProgramWithSource’
    main.c:(.text+0x31f): undefined reference to `clBuildProgram’
    main.c:(.text+0x33a): undefined reference to `clCreateKernel’
    main.c:(.text+0x361): undefined reference to `clSetKernelArg’
    main.c:(.text+0x384): undefined reference to `clSetKernelArg’
    main.c:(.text+0x3aa): undefined reference to `clSetKernelArg’
    main.c:(.text+0x411): undefined reference to `clEnqueueNDRangeKernel’
    main.c:(.text+0x47f): undefined reference to `clEnqueueReadBuffer’
    main.c:(.text+0x4e7): undefined reference to `clFlush’
    main.c:(.text+0x4f6): undefined reference to `clFinish’
    main.c:(.text+0x508): undefined reference to `clReleaseKernel’
    main.c:(.text+0x51a): undefined reference to `clReleaseProgram’
    main.c:(.text+0x529): undefined reference to `clReleaseMemObject’
    main.c:(.text+0x538): undefined reference to `clReleaseMemObject’
    main.c:(.text+0x54a): undefined reference to `clReleaseMemObject’
    main.c:(.text+0x559): undefined reference to `clReleaseCommandQueue’
    main.c:(.text+0x568): undefined reference to `clReleaseContext’

    • Erik Smistad says:

      You need to link the OpenCL library when you compile: (example: gcc -L /path-to-the-lib-folder-with-OpenCL-libfile/ -l OpenCL main.o -o host)

      • Rohan says:

        I too got the same errors and it actually happens during the first command not the second. I even created the symlinks as stated above by a user, but nothing works 🙁

        • Erik Smistad says:

          What OS and OpenCL implementation are you using? And what is the exact commands you are running? Because this works perfectly on my system…

        • snk says:

          I got these errors, but the solution was simple. Try the -c option for the first command. This option will prevent the gcc from linking, and then the linking can be done with the second command.
          It worked for me. I hope it will work for you too.

          • Erik Smistad says:

            Thanks for clearing that up snk. It seems that the -c flag was missing in the first example.

          • Lucas Campos says:

            I had these same problems on linking. I’m using nVidia’s SDK, on Mint 10 x64 and this solution did not work for me. But, when I compiled using just

            gcc -I path-to-opencl-common-inc -lopenCL main.c -o test, worked like a charm. Using SDK 4.0 was

            gcc -lOpenCL -I ~/NVIDIA_GPU_Computing_SDK/OpenCL/common/inc/ main.c -o test

            Important to notice that its lOpenCL (lower case L) and -I (upper case -i)

  60. tom says:

    0 + 1000 = 1561096336
    1 + 999 = 32714
    2 + 998 = 1561096336
    3 + 997 = 32714
    4 + 996 = 6311776
    5 + 995 = 0
    6 + 994 = 6311776
    7 + 993 = 0
    8 + 992 = 2147483647
    9 + 991 = 0
    10 + 990 = 12370275
    11 + 989 = 0
    12 + 988 = 788225073

  61. tom says:

    once everything compiles, running the program, it actually returns garbage !?
    the vectors are not added but garbage is displayed.

  62. tom says:

    then I get: “undefined reference to `clGetPlatformIDs'” and similar errors all relating to the functions defined in cl_platform.h (which gcc finds, I checked). any hints appreciated.

    • tom says:

      symlinks need to be created. comes from nvidia. and are needed symlinks that need to be created (command ‘ln’)

  63. tom says:

    also, the link above to download the source code leads to a 404 file not found.

    • Erik Smistad says:

      Sorry about that. The link has been fixed now

      • tom says:

        using your files exactly, still getting garbage output of the vector addition.

        • Erik Smistad says:

          Then I’m out of ideas.. it works fine on all of my ATI cards. I will try to run the code on some machines with nvidia cards at the university and see if I get the same problem there.

          • I added a printf() to your code after line 43:
            if (ret != CL_SUCCESS) {
            printf(“Error: Failed to query platforms! (%d)\n”, ret);
            return EXIT_FAILURE;

            after compiling, running it gives me this error: “Failed to query platforms (-1001)”

          • tom says:

            have you had a chance to try it out at your university?

            it’s still not working on my side 🙁

            • Erik Smistad says:

              Yes, I tested it just now on a geforce 8400 at the university. The installation was no problem.

              First I downloaded the CUDA toolkit and installed that. Then I installed the CUDA devdriver, set LD_LIBRARY_PATH to /usr/local/cuda/lib64 and ran the example, no problem… I will add the procedure for nvidia to the post soon.

              I don’t understand why it doesn’t work for you.. you have a nvidia.icd file in your /etc/OpenCL/vendors folder, right?

            • Anjil says:

              Hi Erik,

              Is the garbage value issue posted by few developers above fixed? Is there any solution available. Please let me know, I am facing the same problem, Every time I execute the sample, it simply returns either zero some garbage value which is not relevant. I am working with Ubuntu 11.04 with NVIDIA graphics driver installed.

            • Erik Smistad says:

              There is no error checking in the example above, because I wanted the example to be as simple as possible. So it can happen that the kernel doesn’t run at all, maybe because OpenCL is not properly installed. Try using the C++ bindings in this post as this example has error checking. See what error message you get, maybe that can help you find out what the problem is. Post it here afterwards. I got this error myself once and found that it was because OpenCL wasn’t properly installed, was lacking icd files or something like that. Hope that helps!

  64. tom says:

    gcc question: gcc can’t find the cl.h, even though it’s in the same directory as the source file, also it can’t find it even after explicitly specifying the path!!!???

  65. Achtkraut says:

    SSE2 support was only added in ATI Stream SDK v2.2

  66. Is it the case that the CPU must be sse3? I followed your blog but the OpenCL samples don’t run successfully (have sse2).
    Also, when you say to decompress the registration in the root folder, I assumed you meant the sdk root folder … might want to clarify (for dummies like me) that it is the file system root folder.
    Thanks for the article … nicely written.

    • Erik Smistad says:

      I don’t believe that the CPU must support SSE3. I don’t see why it shouldn’t support CPUs with SSE2. Can you compile the example and does it run without any errors? What exactly happens when you try to run the example?

      I don’t mean that you should decompress the SDK to the root folder of your system. What I mean is that you should extract the .icd files of the icd-registration.tgz file to the folder /etc/OpenCL/vendors. This is offcourse only if you are running on linux, I haven’t tried setting OpenCL on windows just yet. You have to do this to make it work.

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