$ cat a.c
#include <stdio.h>

int main() {
   int i = 0;
   printf("%d %d %d\n", i++, i++, i++);
   return 0;
}

$ gcc -Wall -o a a.c && a
a.c: In function ‘main’:
a.c:5: warning: operation on ‘i’ may be undefined
a.c:5: warning: operation on ‘i’ may be undefined
2 1 0