memory management - How to free 2d array in C?

Question

I have the following code:

int **ptr = (int **)malloc(sizeof(int*)*N); 
for(int i=0;i<N;i++) 
     ptr[i]=(int*)malloc(sizeof(int)*N));

How can I free ptr using free? Should I loop over ptr and free ptr[i] or should I just do

free(ptr) 

and ptr will be freed?

Answer 1

Just the opposite of allocation:

for(int i = 0; i < N; i++)
    free(ptr[i]);
free(ptr);