Where are static variables stored in C and C++?

Question

In what segment (.BSS, .DATA, other) of an executable file are static variables stored so that they don\'t have name collision? For example:

foo.c:                  


        

Answer 1

How to find it yourself with objdump -Sr

To actually understand what is going on, you must understand linker relocation. If you've never touched that, consider reading this post first.

Let's analyze a Linux x86-64 ELF example to see it ourselves:

#include 

int f() {
    static int i = 1;
    i++;
    return i;
}

int main() {
    printf("%d\n", f());
    printf("%d\n", f());
    return 0;
}

Compile with:

gcc -ggdb -c main.c

Decompile the code with:

objdump -Sr main.o

• -S decompiles the code with the original source intermingled
• -r shows relocation information

Inside the decompilation of f we see:

 static int i = 1;
 i++;
4:  8b 05 00 00 00 00       mov    0x0(%rip),%eax        # a 
        6: R_X86_64_PC32    .data-0x4

and the .data-0x4 says that it will go to the first byte of the .data segment.

The -0x4 is there because we are using RIP relative addressing, thus the %rip in the instruction and R_X86_64_PC32.

It is required because RIP points to the following instruction, which starts 4 bytes after 00 00 00 00 which is what will get relocated. I have explained this in more detail at: https://stackoverflow.com/a/30515926/895245

Then, if we modify the source to i = 1 and do the same analysis, we conclude that:

• static int i = 0 goes on .bss
• static int i = 1 goes on .data