Undefined array in struct

Hi there.
I am confused as to why the following piece of code does work:

Code:

---

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

struct my_struct
{
      char my_word[];
      int num;
};
     
int main()
{
 
    struct my_struct eg;

    eg.my_word[3] = 'y';
    eg.my_word[67] = 'w';

    printf("\n %c \n %c \n %c \n", eg.my_word[3], eg.my_word[67], eg.my_word[54]);
   
    system("pause");
}

---

The array in the struct is not assigned a maximum size. Outside of a struct this is not allowed. This behaviour is not mentioned in my text book.
Why is this?

Many thanks,
WVM

C99 Extension: Flexible array members.

http://david.tribble.com/text/cdiffs.htm#C99-fam

note that in this case my_word[3] overwrites some byte in the num member, and my_word[64] acesses some memory out of bounds...

On some compilers it works, on some will crash

Thanks guys.
WVM