c程序涉及链接得到奇怪的输出

In s1.c:

#include <stdio.h>
int foo();
struct t1 {
  int j; int k; int i;
} a; 
int main() {
  foo();
  printf("%d %d %d\n", a.i, a.j, a.k); }
#include <stdio.h>
int foo();
struct