Can some one check this for me please..... Newbie needs help learning

Can some one check and confirm that this kernel does what i think it does? Im having trouble getting to grips with threads…


  • in main function, a block of threads is created with dimensions 16x1

  • a 1x1 grid is then made

then in testThreads function:

make i = x data of the thread block and thread id

make j the same but for y dimensions

index is then identifyer the different threads

then if (i <N && j <1)

induvidual thread does the maths for each value in the array

back to main function, memcopy to host


I think this is correct however i am getting an odd error with the output here it is:

0; 12; 7; 16; 13; 29; 18; 15; 19; 22; 27; 9; 11; 23; 17; 21;

i dont know why the very first value is not computed? not even set to 5 in the testThreads function? why is that? im guessing something im doing with the threads is making it do that?

Thanks in advance for any help

heres the code…

#include <stdio.h>

#include <cutil.h>

#define OUTFILE "output.txt"

#define blocksize 16

#define N 16

__global__ void testThreads(unsigned int deviceBuff[]);

__constant__ unsigned int numbers[16] = {3,7,2,11,8,24,13,10,14,17,22,4,6,18,12,16}; 

__global__ void testThreads(unsigned int deviceBuff[]){

	int i = blockDim.x * blockIdx.x + threadIdx.x;

	int j = blockDim.y * blockIdx.y + threadIdx.y;

	int index = i+j*N;

	if (i<N && j< 1){



  deviceBuff[index] = 5;

  deviceBuff[index] = deviceBuff[index]+numbers[index];





int main (int argc, char** argv){

	unsigned int * hostBuff;

	unsigned int * deviceBuff;

	hostBuff = (unsigned int*) malloc(sizeof(unsigned int)*N);

	printf("\nAllocated %d host memory bytes...", sizeof(unsigned int)*N);

	CUDA_SAFE_CALL(cudaMalloc((void**)&deviceBuff, sizeof(unsigned int)*N));

	printf("\nAllocated %d device memory bytes...", sizeof(unsigned int)*N);


	dim3 dimBlock (blocksize, 1);

	dim3 dimGrid (N/dimBlock.x,1);


	CUDA_SAFE_CALL(cudaMemcpy(hostBuff, deviceBuff, sizeof(unsigned int)*N, cudaMemcpyDeviceToHost));

	printf("\nCopied %d device memory bytes to host...", sizeof(unsigned int)*N);



	printf("\nFreed memory on host and device...");



	FILE* output;

	output=fopen(OUTFILE, "w");

	for (int x = 0; x< N; x++){


  fprintf(output, "%d; ", hostBuff[x]);



	printf("\nPrinted output to file...");


	return 0;


It’s something strange in your code.

You first free(hostBuff);then you use the hostBuffer write the result into the file?

As i move the free(hostBuff);to the end ,it works fine.And the first element is 8.Maybe you can check it out in Emudebug mode.