Malloc a 2D array in C

like this : int (*arr)[M] = malloc(sizeof(int[N][M]));

arr is pointer to int[M].

use like arr[0][M-1];

and free(arr);

int ** arr = malloc(N*sizeof(int[M]));

is incorrect C code, if you simulate it by allocating once
int *arr = malloc(N*M*sizeof(int));
and access it by
arr[i*M + j],
this is an analog to arr[I][j] in your first case.