NULL pointer points to nothing.
But dangling pointers are those pointers which points to
invalid location (e.g. still points to those memory
locations which are already freed)

int *p = NULL;
   Is mere a pointer which is not initialised to valid
memroy location. it points to nothing

int *q ;
q -> 0x1A
free (q);

still we can access this pointer using q. Still memory has
not been released to the system.