C: Correctly freeing memory of a multi-dimensional array

Question

Say you have the following ANSI C code that initializes a multi-dimensional array :

int main()
{
      int i, m = 5, n = 20;
      int **a = malloc(m * sizeo         


        

Answer 1

Undo exactly what you allocated:

  for (i = 0; i < m; i++) { 
      free(a[i]);
  }
  free(a);

Note that you must do this in the reverse order from which you originally allocated the memory. If you did free(a) first, then a[i] would be accessing memory after it had been freed, which is undefined behaviour.