How do you know the exact address of a variable?

Question

So I'm looking through my C programming text book and I see this code.

#include <stdio.h>

int j, k;
int *ptr;

int main(void)
{
    j = 1;
    k = 2;
    ptr = &k;
    printf("\n");
    printf("j has the value %d and is stored at %p\n", j, (void *)&j);
    printf("k has the value %d and is stored at %p\n", k, (void *)&k);
    printf("ptr has the value %p and is stored at %p\n", (void *)ptr, (void *)&ptr);
    printf("The value of the integer pointed to by ptr is %d\n", *ptr);

    return 0;
}

I ran it and the output was:

j has the value 1 and is stored at 0x4030e0

k has the value 2 and is stored at 0x403100

ptr has the value 0x403100 and is stored at 0x4030f0

The value of the integer pointed to by ptr is 2

My question is if I had not ran this through a compiler, how would you know the address to those variables by just looking at this code? I'm just not sure how to get the actual address of a variable. Thanks!

Answer 1

Here's my understanding of it:

The absolute addresses of things in memory in C is unspecified. It's not standardised into the language. Because of this, you can't know the locations of things in memory by looking at just the code. (However, if you use the same compiler, code, compiler options, runtime and operating system, the addresses may be consistent.)

When you're developing applications, this is not behaviour you should rely on. You may rely on the difference between the locations of two things in some contexts, however. For example, you can determine the difference between the addresses of pointers to two array elements to determine how many elements apart they are.

By the way, if you are considering using the memory locations of variables to solve a particular problem, you may find it helpful to post a separate question asking how to so without relying on this behaviour.