Which is the address printed by printf() with a %p format in c?

Question

I'm having simple code as follows:

#include<stdio.h>

int glob;

int main(void)
{
   int a;
   printf("&a is : %p \n", &a);
   printf("glob is : %p \n", &glob);
   return 0;
}

Output of above program is: First run:

&a is : 0x7fff70de91ec
glob is : 0x6008f4

Second run :

&a is : 0x7fff38c4c7ac
glob is : 0x6008f4

I'm studying about virtual & physical addresses. I have following question:

1. Which is the printed address(physical/virtual) of variable "a"?
2. If it is virtual then, How it changes in each run of same program? As i understood compiler provides virtual address to variables at compile time?
3. Why the address of global variable is constant in each run of program?

In executed this program on Linux : 2.6.18-308.el5 x86_64 GNU/Linux

Compiled using : gcc version 4.1.2 20080704 (Red Hat 4.1.2-52)

Answer 1

Both addresses are virtual.

Modern systems uses stack randomization to prevent so-called stack-smashing attacks, which is why the local variable can change its location every run. However the global variable is stored in the executable and is loaded at the same offset every time.