explanation of the output of the code

Question

I was recently asked to consider the output of the following code:

#include <iostream>
int gvar;
int main (void)
{
     cout << &gvar << endl;
     while (true);
     return 0;
}

This code is compiled and an executable is generated. If this executable in run 2 terminals in parallel. what would it print.

My reply was that it would print 2 different addresses. But on running, I found out in both the terminals same address was printed.

Can somebody explain why this is happening. I know that might be due to virtual memory but I am not able to put the pieces together.

More than likely you are getting a memory address that is in the executable's stack space and if that is the case then it should be the same address. — NathanOliver, May 05 '15 at 14:21
@NathanOliver Not that it matters, but no that variable shouldn't be on the stack. It's a global variable, after all. — unwind, May 05 '15 at 14:22
@unwind I didn't mean the actual stack. I couldn't think of the way to say the processes memory space at the time. — NathanOliver, May 05 '15 at 14:23

score 5 · Answer 1 · answered May 05 '15 at 14:21

5

It is very clearly due to virtual memory, yes.

The addresses seen by code like this are virtual. Since each process has its own table of virtual-to-physical mappings, the virtual addresses need not be unique inside each process.

answered May 05 '15 at 14:21

unwind

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but does that mean that every time it would return same address. As in, it could have printed a totally different address which on mapping would have returned a different physical memory address. ? – ShReYaNsH May 05 '15 at 14:23
@ShReYaNsH Yes, it could, but those are implementation details in the operating system. It can choose to randomize the virtual addresses used, or not. – unwind May 05 '15 at 14:25
so what I understand is that it is merely a co-incidence that the same address is getting printed. Though seemingly same addresses will be mapped to different addresses in physical memory. Is that correct ? – ShReYaNsH May 05 '15 at 14:28
@ unwind Thanks for pointing to the correct question. Helped a lot ! – ShReYaNsH May 05 '15 at 14:41

explanation of the output of the code

1 Answers1