IBM example code, non re-entrant functions doesn't work in my system

I was studying re-entrancy in programming. On this site of IBM (really good one). I have founded a code, copied below. It's the first code that comes rolling down the website.

The code tries showing the issues involving shared access to variable in a non linear development of a text program (asynchronicity) by printing two values that constantly change in a "dangerous context".

#include <signal.h>
#include <stdio.h>

struct two_int { int a, b; } data;

void signal_handler(int signum){
   printf ("%d, %d\n", data.a, data.b);
   alarm (1);
}

int main (void){
   static struct two_int zeros = { 0, 0 }, ones = { 1, 1 };

   signal (SIGALRM, signal_handler); 
   data = zeros;
   alarm (1);
   while (1){
       data = zeros;
       data = ones;
   }
}

The problems appeared when I tried to run the code (or better, didn't appear). I was using gcc version 6.3.0 20170516 (Debian 6.3.0-18+deb9u1) in default configuration. The misguided output doesn't occurs. The frequency in getting "wrong" pair values is 0!

What is going on after all? Why there is no problem in re-entrancy using static global variables?