cuda- code doesnt enter the kernel - cuda

Question

I am trying to learn cuda. I am trying to run a simple code
#include <stdlib.h>
#include <stdio.h>
__global__ void kernel(int *array)
{
int index = blockIdx.x * blockDim.x + threadIdx.x;
array[index] = 7;
}
int main(void)
{
int num_elements = 256;
int num_bytes = num_elements * sizeof(int);
// pointers to host & device arrays
int *device_array = 0;
int *host_array = 0;
// malloc a host array
host_array = (int*)malloc(num_bytes);
// cudaMalloc a device array
cudaMalloc((void**)&device_array, num_bytes);
int block_size = 128;
int grid_size = num_elements / block_size;
kernel<<<grid_size,block_size>>>(device_array);
// download and inspect the result on the host:
cudaMemcpy(host_array, device_array, num_bytes, cudaMemcpyDeviceToHost);
// print out the result element by element
for(int i=0; i < num_elements; ++i)
{
printf("%d ", host_array[i]);
}
// deallocate memory
free(host_array);
cudaFree(device_array);
}
It is supposed to print 7's but it prints 0's
This statement doesn't seem to get executed
"kernel<<>>(device_array);"
It doesn't give any compilation error also.
Any help ??

Answer 1

The code runs fine on my machine, but make sure you add cudaDeviceSynchronize and error checking after the kernel call.
Change the code as follows to check for errors:
kernel<<<grid_size,block_size>>>(device_array);
// wait until tasks are completed
cudaDeviceSynchronize();
// check for errors
cudaError_t error = cudaGetLastError();
if (error != cudaSuccess) {
fprintf(stderr, "ERROR: %s \n", cudaGetErrorString(error));
}