OpenCL alignment issue - c++

Question

I want to fill an array of glm::vec3 with an OpenCL kernel.
All I want to do is fill the array with [1.0, 2.0, 3.0].
So upon success I should get the triplet repeated 256 times.
[1.0, 2.0, 3.0][1.0, 2.0, 3.0][1.0, 2.0, 3.0] ... [1.0, 2.0, 3.0]
However the result looks like this
[1.0, 2.0, 2.0][2.0, 2.0, 2.0] ... [2.0, 2.0, 2.0]
Why?
Here is the code for the kernel
__kernel void fill_array(__global float *output_values)
{
int i = get_global_id(0);
float3 pos = (float3)(1.0, 2.0, 3.0);
vstore3(pos, 0, &(output_values[i]));
}
And here is the code to run it
#include <stdio.h>
#include <stdlib.h>
#include <vector>
#include "glm/glm.hpp"
#ifdef __APPLE__
#include <OpenCL/opencl.h>
#else
#include <CL/cl.h>
#endif
#define MAX_SOURCE_SIZE (0x100000)
int main(void)
{
std::vector<glm::vec3> values;
values.resize(256);
// Load the kernel source code into the array source_str
FILE *fp;
char *source_str;
size_t source_size;
fp = fopen("E:/Dev/fill_array_kernel.cl", "r");
if (!fp) {
fprintf(stderr, "Failed to load kernel.\n");
exit(1);
}
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_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 = clCreateCommandQueue(context, device_id, 0, &ret);
// Create memory buffers on the device for each vector
cl_mem output_mem = clCreateBuffer(context, CL_MEM_WRITE_ONLY, values.size() * sizeof(glm::vec3), NULL, &ret);
// 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);
if(ret != CL_SUCCESS)
{
cl_build_status build_status;
ret = clGetProgramBuildInfo(program, device_id, CL_PROGRAM_BUILD_STATUS, sizeof(cl_build_status), &build_status, NULL);
size_t ret_val_size;
ret = clGetProgramBuildInfo(program, device_id, CL_PROGRAM_BUILD_LOG, 0, NULL, &ret_val_size);
char *build_log = (char*)malloc(sizeof(char)*(ret_val_size + 1));
ret = clGetProgramBuildInfo(program, device_id, CL_PROGRAM_BUILD_LOG, ret_val_size, build_log, NULL);
build_log[ret_val_size] = '\0';
printf("%s\n", build_log);
free(build_log);
return -1;
}
// Create the OpenCL kernel
cl_kernel kernel = clCreateKernel(program, "fill_array", &ret);
// Set the arguments of the kernel
ret = clSetKernelArg(kernel, 0, sizeof(cl_mem), (void *)&output_mem);
// Execute the OpenCL kernel on the list
size_t global_item_size = values.size(); // Process the entire lists
size_t local_item_size = 64; // Process in 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
ret = clEnqueueReadBuffer(command_queue, output_mem, CL_TRUE, 0, values.size() * sizeof(glm::vec3), values.data(), 0, NULL, NULL);
// Clean up
ret = clFlush(command_queue);
ret = clFinish(command_queue);
ret = clReleaseKernel(kernel);
ret = clReleaseProgram(program);
ret = clReleaseMemObject(output_mem);
ret = clReleaseCommandQueue(command_queue);
ret = clReleaseContext(context);
return 0;
}

Answer 1

I was misusing the vstore function.
I should have used the 2nd parameter to specify the index in the array.
https://www.khronos.org/registry/OpenCL/sdk/1.0/docs/man/xhtml/vstoren.html
__kernel void fill_array(__global float *output_values)
{
int i = get_global_id(0);
float3 pos = (float3)(1.0, 2.0, 3.0);
vstore3(pos, i, output_values);
}