If a static variable is declared out side of a function, will the memory address be the same as if it's declared in a function

kingnesay 0
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I was asked these 2 questions during an interview. I guess the address of a static variable will be the same no matter where it is declared. It will also have the same address from run to run. Correct me if I am wrong.

  1. If a static variable is declared out side of a function, will the memory address be the same as if it's declared in a function?

  2. If a static variable is declared out side of a function, will it have the same address every time you run the program?

static-memory-allocation c
2 Answers

Cunningham's Law will ensure quick correction of my mistakes:

  1. global and local static variables are treated similarly so they may indeed end up at the same address when moved:
#include <stdio.h>

#ifdef GLOBAL
static int i;
#endif
// static int j;

#ifndef GLOBAL
void f(void) {
    static int i;
    printf("local:  %p\n" , (void *) &i);
}
#endif

int main() {
#ifdef GLOBAL
    printf("global: %p\n" , (void *) &i);
#else
    f();
#endif
}

You need to disable address randomization to observe it:

$ sudo bash -c 'echo 0 >  /proc/sys/kernel/randomize_va_space';\
gcc 1.c && ./a.out &&\
gcc -DGLOBAL 1.c && ./a.out;\
sudo bash -c 'echo 2 >  /proc/sys/kernel/randomize_va_space'
local:  0x555555558034
global: 0x555555558034

If you add another global static int j (commented out above) after the global i then the effect goes away:

local:  0x555555558038
global: 0x555555558034

So call me maybe!?

  1. No. Modern operating systems, specifically the program loader, use address space layout randomization (ASLR) so the address changes on every execution:
$ ./a.out && ./a.out
global: 0x563bcce14034
global: 0x55cd4a497034
Allan Wind0

static variables may or may not have the same address in memory when declared globally or locally in the body of a function. The fact is nothing can be assumed about these addresses: the C Standard only guarantees that they compare different from the null pointer and different from the addresses of other data objects defined in a C module (albeit compiler extensions can force some symbols to alias one another).

Modern operating systems use address randomisation to make it more difficult for attackers to exploit software flaws, so the addresses may indeed differ from one run to another.

Answering such questions in an interview is tricky: the interviewer's assumptions might be outdated, but a substantiated answer should get you a good mark.

chqrlie0
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