OpenCL/C: results always zero

Question

I’m using Ubntu 20.04 to run my Opencl program with C but whatever the function i execute the results is always zero or other illogical results, it’s like kernel not working, Please if anyone has OpenCL, please try this program i want to know if the problem is in the program or in the installation of OpenCL. This is the programme i use.

#define CL_USE_DEPRECATED_OPENCL_1_2APIS
#include <stdio.h>
#include <stdlib.h>
 
#include <CL/cl.h>

#define MAX_SOURCE_SIZE (0x100000)
 
int main(void) {
    // Create the two input vectors
    int i;
    const int LIST_SIZE = 10;
    int* A = (int*)malloc(sizeof(int)*LIST_SIZE);
    int* B = (int*)malloc(sizeof(int)*LIST_SIZE);
    int* C = (int*)malloc(sizeof(int)*LIST_SIZE);
    for(i = 0; i < LIST_SIZE; i++) {
        A[i] = i;
        B[i] = LIST_SIZE - i;
        C[i] = 0;
    }
    
    // Load the kernel source code into the array source_str
    /*FILE *fp;
    char *source_str;
   
 
    fp = fopen("vector_add_kernel.cl", "r");
    if (!fp) {
        fprintf(stderr, "Failed to load kernel.\n");
        exit(1);
    }
    source_str = (char*)malloc(MAX_SOURCE_SIZE);
    size_t source_size;
    source_size = fread( source_str, 1, MAX_SOURCE_SIZE, fp);
    fclose( fp );*/
    //size_t source_size;

    const char* source_str =
        "__kernel void vector_add(__global int *A, __global int *B, __global int *C) {\n"
        "    int i = get_global_id(0);\n"
        "    if(i>=10) return;\n"
        "    C[i] = A[i]+B[i];\n"
        "}"
    ;
    // 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_ALL, 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 = clCreateCommandQueueWithProperties(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, NULL, &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 local_item_size = 64; // Divide work items into groups of 64
    size_t global_item_size = ((LIST_SIZE+local_item_size-1)/local_item_size)*local_item_size;  // Process the entire lists
    ret = clEnqueueNDRangeKernel(command_queue, kernel, 1, NULL, 
            &global_item_size, &local_item_size, 0, NULL, NULL);
     clFinish(command_queue);
    // Read the memory buffer C on the device to the local variable C
    
    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);
    free(A);
    free(B);
    free(C);
    return 0;
}

Answer 1

The code looks oddly familiar to me... With my PC it works as intended. I suppose the issue is that on your system no OpenCL device is detected. With this code snippet, you can select a platform and device:

// Get platform and device information
    const int select_platform = 0;
    const int select_device = 0;
    cl_platform_id* platform_ids = NULL;
    cl_uint nr_platforms = 0;
    cl_device_id* device_ids = NULL;   
    cl_uint nr_devices = 0;
    cl_device_id device_id = NULL;
    cl_int ret = clGetPlatformIDs(0, NULL, &nr_platforms);
    if(ret!=CL_SUCCESS || nr_platforms==0) std::cerr << "no OpenCL platforms found" << std::endl;
    platform_ids = (cl_platform_id*)malloc(nr_platforms * sizeof(*platform_ids));
    ret = clGetPlatformIDs(nr_platforms, platform_ids, &nr_platforms);
    ret = clGetDeviceIDs(platform_ids[select_platform], CL_DEVICE_TYPE_ALL, 0, device_ids, &nr_devices);
    if(ret!=CL_SUCCESS || nr_devices==0) std::cerr << "no OpenCL devices found on that platform" << std::endl;
    device_ids = (cl_device_id*)malloc(nr_devices * sizeof(*device_ids));
    ret = clGetDeviceIDs(platform_ids[select_platform], CL_DEVICE_TYPE_ALL, nr_devices, device_ids, &nr_devices);
    device_id = device_ids[select_device];
    char name[1024];
    clGetDeviceInfo(device_id, CL_DEVICE_NAME, 1024, name, NULL);
    std::cout << "selected device: " << name << std::endl;

Check different values of select_platform and select_device. If there is no device found on any platform, make sure to have compatible hardware and the latest graphics drivers or CPU runtime installed.

---

To make OpenCL development much less painful, consider this OpenCL-Wrapper. This automatically selects the fastest available OpenCL device for you in 1 line of code:

Device device(select_device_with_most_flops());

Alternatively, you can autiomatically select the device with most memory

Device device(select_device_with_most_memory());

or a device with specified ID:

Device device(select_device_with_id(1));