## 3/12/2015

### Meaning of threadIdx, blockIdx, blockDim, gridDim in the cuda (2D)

If you make kernel like that

dim3 blocks(2,3);

36 threads are made and gridDim and blockDim is (2,3) and (3,2).

problem is now...
See the this figure..

Do you understand?
We have to do indexing calculation, because threadIdx.x, threadIdx.y is only indicate indexing in their block.

For more detail, refer to below figure that represent the index list of tid calculation result.

```#include "cuda_runtime.h"
#include "device_launch_parameters.h"

#include < stdio.h>

#define N 40

__global__ void increase(int *c){
int x = threadIdx.x + blockIdx.x * blockDim.x;
int y = threadIdx.y + blockIdx.y * blockDim.y;
int tid = x + y*blockDim.x * gridDim.x;
if(tid < N)
c[tid] = tid;
}

int main(void)
{
int c[N];
int *dev_c;

cudaMalloc( (void**)&dev_c, N*sizeof(int) );

for(int i=0; i< N; ++i)
{
c[i] = -1;
}

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

dim3 blocks(2,3);

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

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

cudaFree( dev_c );
}

```
...

In the source code, threads are made only 36. so 37th 38th 39th 40th array have left initial value -1.