Create 2D Array with CUDA - cuda

Question

in cuda c programming guide document there is a sample that show a 2d array:
// Kernel definition
__global__ void MatAdd(float A[N][N], float B[N][N], float C[N][N])
{
int i = blockIdx.x * blockDim.x + threadIdx.x;
int j = blockIdx.y * blockDim.y + threadIdx.y;
if (i < N && j < N)
C[i][j] = A[i][j] + B[i][j];
}
int main()
{
...
// Kernel invocation
dim3 threadsPerBlock(16, 16);
dim3 numBlocks(N / threadsPerBlock.x, N / threadsPerBlock.y);
MatAdd<<<numBlocks, threadsPerBlock>>>(A, B, C);
...
}
i use 2d array with below form and works correctly:
dim3 grid[COLUMNS][ROWS];
kernel_Matrix<<<grid,1>>>(dev_strA, dev_strB, dev_Matrix);
__global__ void add(int *a, int *b, int *c)
{
int x = blockIdx.x;
int y = blockIdx.y;
int i = (COLUMNS*y) + x;
c[i] = a[i] + b[i];
}
there is a way that implement 2d array with [ ][ ] definition? i tested this way but not works.

Answer 1

dim3 is not array but structure defined in CUDA header file (vector_types.h). This structure is used to specify dimensions of GRID in execution configuration of global functions, i.e. in <<< >>>. It doesn't keep the 'real' blocks it just configures a number of blocks that will be executed.
The only two ways (to my knowledge) to initialize this structure are:
1. dim3 grid(x, y, z);
2. dim3 grid = {x, y, z};
EDIT:
Host code with dim3 initialization and with passing the arrays to kernel function in a way you will be able to access its elements via [][]:
float A[N][N];
float B[N][N];
float C[N][N];
float (*d_A)[N]; //pointers to arrays of dimension N
float (*d_B)[N];
float (*d_C)[N];
for(int i = 0; i < N; i++) {
for(int j = 0; j < N; j++) {
A[i][j] = i;
B[i][j] = j;
}
}
//allocation
cudaMalloc((void**)&d_A, (N*N)*sizeof(float));
cudaMalloc((void**)&d_B, (N*N)*sizeof(float));
cudaMalloc((void**)&d_C, (N*N)*sizeof(float));
//copying from host to device
cudaMemcpy(d_A, A, (N*N)*sizeof(float), cudaMemcpyHostToDevice);
cudaMemcpy(d_B, B, (N*N)*sizeof(float), cudaMemcpyHostToDevice);
cudaMemcpy(d_C, C, (N*N)*sizeof(float), cudaMemcpyHostToDevice);
// Kernel invocation
dim3 threadsPerBlock(16, 16);
dim3 numBlocks(N / threadsPerBlock.x, N / threadsPerBlock.y);
MatAdd<<<numBlocks, threadsPerBlock>>>(d_A, d_B, d_C);
//copying from device to host
cudaMemcpy(A, (d_A), (N*N)*sizeof(float), cudaMemcpyDeviceToHost);
cudaMemcpy(B, (d_B), (N*N)*sizeof(float), cudaMemcpyDeviceToHost);
cudaMemcpy(C, (d_C), (N*N)*sizeof(float), cudaMemcpyDeviceToHost);