_global_ void add(int *a, int *b, int *c){    int index = threadIdx.x + blockIdx.x * blockDim.x;    c[index] = a[index] + b[index];}#define N (2048*2048)#define THREAD_PER_BLOCK 512int main(){    int *a, *b, *c;            //host copies of a, b, c    int *d_a, *d_b, *d_c;    //device copies of a, b, c    int size = N * sizeof(int);        //Alloc space for device copies of a, b, c    cudaMalloc((void **)&d_a, size);    cudaMalloc((void **)&d_b, size);    cudaMalloc((void **)&d_c, size);        //Alloc space for host copies of a, b, c and setup input values    a = (int *)malloc(size); random_ints(a, N);    b = (int *)malloc(size); random_ints(b, N);    c = (int *)malloc(size);        //Copy the data into device    cudeMemcpy(d_a, a, size, cudaMemcpyHostToDevice);    cudaMemcpy(d_b, b, size, cudaMemcpyHostToDevice);        //Launch add() kernel on GPU with N blocks    add<<<N/THREADS_PER_BLOCK,THREADS_PER_BLOCK>>>(d_a, d_b, d_c);        //Copy result back to host    cudaMemcpy(c, d_c, size, cudaMemcpyDeviceToHost);        //Cleanup    free(a); free(b); free(c);    cudeFree(d_a); cudaFree(d_b); cudaFree(d_c);    return 0;}/**** What‘s the function of random_ints****/void random_ints(int* a, int N){ int i; for (i = 0; i < N; ++i) a[i] = rand();}