Will memory addresses be the same if I run a program in a VM from two different computers?

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Fairly new to C and I learned that addresses depend on a few things like the operating system and the CPU. I have a lab for one of my C courses that asks us if we run a program and print out the address for each variable will they have the same address and value as another student's (exact same program). They are local variables, stored on the stack. Normally I would say no but all of us are required to ssh to our University's lab and our programs are being run on the same machines with the same specs. This is where I'm confused, pretty sure that the values will be the same however, I don't know what exactly determines these addresses. Here is a piece of code from the program:

int g2(int a, int b)
{
 int c = g1(a + 3, b - 11);
 printf("g2: %d %d %d \n", a,b,c);
 printf("a's address is %p b's address is %p C's address is %p\n", &a, &b, &c);
 return c - b;
}

For me a's address is 0x7ffe9bce4a0c. Also not just looking for a homework answer, asking here because none of my Teammates have sent me their addresses which we were allowed to do. Have researched it but can't find an answer that matches this sort of situation, any help is greatly appreciated, thank you!

virtualization virtual-machine memory-address memory c
3 Answers

"Will memory addresses be the same if I run a program in a VM from two different computers?"

No, they probably won´t even be the same when running only in the same environment and on the same machine. There is nothing like a guarantee that it will have the same address.

A modern-day OS assigns the memory arbitrarily (within certain sections of course).

And this has a good reason: To protect against the exploitation of memory vulnerabilities a hacker could use to harm the program or even the OS.

This technique is called Address Space Layout Randomization. You can read more about it here.

It could be that the variables may have the same address on several executions, but there is no guarantee that this will happen again, already on the next run. In fact, if the OS supports ASLR, It is more likely, that there is the "almost-guarantee" that the addresses will be unequal.

The virtual machine shall have no influence on that behavior. Maybe you should read more in the documentation about the memory storage for your particular virtual machine (if it supports ASLR), but it shall follow the same guidelines.

user_121391790

short answer, no.

  1. operating system loads program in different position every time.
  2. the address that you see is not the actual address in the memory. There is an abstract address layer, supplied by the operating system. You can read about virtual memory addresses if you would like you. You will probably learn it in a course on Operating Systems
user_135014680

Whether you get the same address or varying addresses depends on the operating system.

Not too many years ago, if a program printed the address of one of the local variables in its function, that address would be the same every time the program was run, as long as the function was called in the same point in program execution with the same program input and other circumstances. (Which functions are called, including recursive calls, and how much stack space they use could be affected by program input and other factors.) This was true because, when the program was loaded and initialized, its stack was always started at the same memory address.

This behavior was exploited by malicious people—if there were bugs in the program, they might be exploited, and knowing which addresses were used in the program helps some exploits. So common operating systems have changed it. Now, when a program is started, the locations of its stack and other parts of its memory layout are adjusted randomly. This is called Address Space Layout Randomization (ASLR).

So, in common modern operating systems, you will get varying addresses from run to run when printing the address of a local variable. In specialized operating systems, such as for embedded devices, you may get the same address every time.

The title of your question asks about “a VM,” presumably for virtual machine, but this is not mentioned in the body of your question. To the extent that a virtual machine implements a machine properly, it should produce identical behavior. So whether a program is running in a virtual machine or not should be irrelevant to this question.

user_2982250
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