Dada una matris cuadrada invertir su diagonal secundaria

#include <stdio.h>
#include <conio.h>
void llenar (int M[20][20], int d)
{
for (int i = 1 ; i <= d ; i++)
{
for (int j = 1 ; j <= d ; j++)
{
printf ("Inserte pos[%d][%d]: ",i, j);
scanf("%d",&M [i] [j]);
}
}
}

void mostrar (int M[20][20], int d)
{
for (int i = 1 ; i <= d ; i++)
{
printf("\n");
for (int j = 1 ; j <= d ; j++)
{
printf ("[%d]",M [i] [j]);
}
}
}

void invierte (int M[20][20], int d)
{
int fin = d;
for (int i = 1 ; i <= d / 2 ; i++)
{
int aux = M [i] [d];
M [i] [d] = M [d] [i];
M [d] [i] = aux;
fin--;
}
}

int main()
{
int M [20] [20];
int d;
printf ("Inserte dimen. de la matris cuadrada: ");
scanf("%d",&d);
llenar (M, d);
printf ("\nMATRIS ORIGINAL: ");
mostrar (M, d);
printf ("\n\nMATRIS CON LA DIAGONAL SECUNDARIA INVERTIDA: ");
invierte (M, d);
mostrar (M, d);
getch();
}