nate@haswell:~/tmp$ cat deref.c
  /* gcc -fno-inline -O3 -g -Wall deref.c -o deref */
  /* (or as desired to show particular outcome) */

  #include <stdio.h>
  #include <stdlib.h>

  void deref_and_print(int *ptr) {
    int i = *ptr;  /* undefined behavior if ptr is NULL */
    if (ptr == NULL) printf("NULL\n");
    else printf("%d\n", i);
  }

  int main(int argc, char **argv) {
    int *ptr = NULL;
    if (argc <= 1) {
      printf("Usage: %s [arg]\n", argv[0]);
      printf(" Demonstrates undefined behavior if 'arg'");
      printf(" is not a positive integer\n");
      exit(1);
    }
    int i = atoi(argv[1]);
    if (i > 0) ptr = &i;
    deref_and_print(ptr);
    exit(0);
  }