void padMatrix (int **A, int n) // overwrites A with a padded matrix.
{
	int i, j;
	int **R;
	
	R = malloc((n+1)*sizeof(int *));

	for (i = 0; i < n+1; i++)
		R[i] = malloc((n+1)*sizeof(int *));
	
	for (i = 0; i < n+1; i++)
		for (j = 0; j < n+1; j++)
		{
			if (i == n)
				R[i][j] = 0;
			else if (j == n)
				R[i][j] = 0;
			else
				R[i][j] = A[i][j];
			
		}
	free(A); 
	A=R; // overwrites A
}

padMatrix(A, n);
padMatrix(B, n);

padMatrix(&A, n);

#include <stdio.h>
#include <stdlib.h>

int main()
{
  void padMatrix (int ***A, int n);

  int **x;
  
  /* do stuff with nxn matrix x */

 
  padMatrix(&x, n);
  n = n+1; 

  /* do stuff with padded nxn matrix x --- n is one greater than the original */


  for (i = 0; i < n; i++) { /* n is the dimension of the padded matrix */
    free(x[i]);
  }
  free(x);
  
  return 0;
}

/* 
 * This assumes the input matrix was nxn, and the new matrix
 * will be (n+1)x(n+1)
 */
void padMatrix (int ***A, int n)
  int i, j;
  int **R;
  
  R = malloc((n+1)*sizeof(int *)); /* allocate n+1 rows */

  /* you should probably check return value from malloc(), to make
   * sure that you don't run out of memory.
   */

  for (i = 0; i < n+1; i++) {
    R[i] = malloc((n+1)*sizeof(int)); /* for each row, allocate n+1 ints */
  }

  
  for (i = 0; i < n+1; i++) {
    for (j = 0; j < n+1; j++) {
      if ((i == n) || (j == n)) {
        R[i][j] = 0;
      }
      else {
        R[i][j] = (*A)[i][j];
      }
      
    }
  }

  for (i = 0; i < n; i++) {
    free((*A)[i]);
  }
  free(*A); 
  *A = R;
}