Code

 Header...
long factorial (int i);
 int main(){
   ... 

»  printf("%ld\n", factorial(3));
»  return 0;
}
// The actual factorial function
 long factorial_real(int i){
   ... 

»  if (i<=1) 
    return 1;
»  return i*factorial_real(i-1);
}

// Publically-callable front-end to the 
// factorial function with error checking
 long factorial(int i){
   ... 

»  if (i<0) {// Error check
»    fprintf(stderr, "Factorial must be positive\n");
»    return -1;
  }
» return factorial_real(i);
}