cis-lclient20:~/2012/c/make>more testfac.c
/* my include file */
#include <stdio.h>
#define INTTYPE unsigned char
INTTYPE factorial(INTTYPE);
INTTYPE input();
void output(INTTYPE);

void main() {
   INTTYPE f, n;
   n = input();
   f = factorial(n);
   output(f);
}

INTTYPE input() {
   int x;
   printf("input a number - ");
   scanf("%i", &x);
   return (INTTYPE) x;
}

INTTYPE factorial(INTTYPE n){
    if(n!=1){
        return(n * factorial(n-1));
    }
    else return (INTTYPE) 1;
}

void output(INTTYPE f) {
printf("The length of f is %d\n", sizeof(INTTYPE));
   if (sizeof(INTTYPE) == 8) {
      printf("The factorial is %llu\n", f);
   } else {
      printf("The factorial is %u\n", f);
   }
   return;
}

cis-lclient20:~/2012/c/make>gcc testfac.c
cis-lclient20:~/2012/c/make>./a.out
input a number - 5
The length of f is 1
The factorial is 120
cis-lclient20:~/2012/c/make>