lassie% cat if2.c
/* File: if2.c
   This program demonstrates how
   #ifndef works.
*/
#include <stdio.h>
#include "genlib.h"
#include "simpio.h"

main() {

#ifndef MARKER
   printf("MARKER has not been defined\n") ;
#else
   printf("MARKER has been defined\n") ;
#endif

}
lassie%

lassie% cc201 if2.c
lassie%

lassie% a.out
MARKER has not been defined
lassie%