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

Addresses seen in a program are always virtual and the behaviour described by the OP is a Linux counter-measure to avoid buffer overflow attacks.

Just to try, you can disable it with

sysctl -w kernel.randomize_va_space=0

then run again your program and watch.

The global one is in another space of memory that can't be harmful in an hackish-wise point of view. That's because it is not randomized every time.