#include <stdio.h>
void troca(int *end_x, int *end_y);

int main(){
 int x=4, y=5;
 troca(&x, &y);
 printf("x = %d  e y = %d\n", x, y);
}
void troca(int *end_x, int *end_y) {
  int aux;
  
  if(end_x != NULL && end_y != NULL){ //se endereços forem válidos
     aux = *end_x;                     //faz a troca
     *end_x = *end_y;
     *end_y = aux;
   }
}