# Result of simple vector summation is not correct.

Hello, there.

Thank you for reading this topic!
However, its result is not matched real result as below.

Where do I have to change for calculating results come correct.

``````#include <stdio.h>
#include <stdlib.h>

__global__ void add( int *a, int *b, int *c)
{

c[tid] = a[tid] + b[tid];
}

int main()
{
int N = 1500;
int i;
int sum;
int *a, *b, *c;
int *dev_a, *dev_b, *dev_c;

// memory allocation in CPU
a = (int *)malloc(N*sizeof(int));
b = (int *)malloc(N*sizeof(int));
c = (int *)malloc(N*sizeof(int));

// allocation each parameter space in GPU
cudaMalloc( (void**)&dev_a, N*sizeof(int) );
cudaMalloc( (void**)&dev_b, N*sizeof(int) );
cudaMalloc( (void**)&dev_c, N*sizeof(int) );

// each value is stored into a and b vector
for (int i=0; i<N; ++i)
{
a[i] = i;
b[i] = i*2;
c[i] = 0;
}

// Copy data into device(e.g. GPU) memory from host memory
cudaMemcpy( dev_a, a, N*sizeof(int), cudaMemcpyHostToDevice );
cudaMemcpy( dev_b, b, N*sizeof(int), cudaMemcpyHostToDevice );

// Call function name as add
// <<< , this number concerns iteration number >>>

cudaMemcpy( c, dev_c, N*sizeof(int), cudaMemcpyDeviceToHost);

// The result
for (int i=0;i<N;++i)
{
printf("%d + %d = %d\n",a[i],b[i],c[i]);
}

// free memory
cudaFree(dev_a);
cudaFree(dev_b);
cudaFree(dev_c);

return 1;
}
``````

This line is not executed if N is bigger than 1024 (512 for older cards). If you would add some errror checking you would get an error.
Try this:

``````// make the host block until the device is finished with foo

// check for error
cudaError_t error = cudaGetLastError();
if(error ! = cudaSuccess)
{
// print the CUDA error message and exit
printf("CUDA error: %s\n", cudaGetErrorString(error));
exit(-1);
``````

Dear pasoleatis,

Thank you for a comment.
I will check it!

Yeah, I just executed above code with changing N.
Thank you!

Sincerely,
Albert