How to track linker issue for global variable re-definition issue - c

Question

I am trying to find the issue with below code, need to know how linker is able to track global variable multiple definition issue in below program, I am not sure much on how linker internally performs checking so requesting help to clarify.
**hello.h**
#ifndef __HELLO_
#define __HELLO_
static int s = 20;
int g = 10;
#endif
**cat hello1.c**
#include <stdio.h>
#include "hello.h"
//int g = 10;
int main()
{
g++;
s++;
printf("g : %d s : %d\n", g, s);
function();
return 0;
}
**cat hello2.c**
#include <stdio.h>
#include "hello.h"
//extern int g;
void function()
{
g++;
s++;
printf("g : %d s : %d\n", g, s);
}
**gcc -save-temps hello1.c hello2.c -o hello**
hello2.o:(.data+0x4): multiple definition of `g'
hello1.o:(.data+0x4): first defined here
**collect2: error: ld returned 1 exit status**
I don't need solution to fix this error, I know above program will throw multiple definition error of g as above but can anyone please explain how it tracks this error at linking stage?.
Here is the output of assembly code.
**cat hello1.s**
.file "hello1.c"
.data
.align 4
.type s, #object
.size s, 4
s:
.long 20
.globl g
.align 4
.type g, #object
.size g, 4
g:
.long 10
.section .rodata
.LC0:
.string "g : %d s : %d\n"
.text
.globl main
.type main, #function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl g(%rip), %eax
addl $1, %eax
movl %eax, g(%rip)
movl s(%rip), %eax
addl $1, %eax
movl %eax, s(%rip)
movl s(%rip), %edx
movl g(%rip), %eax
movl %eax, %esi
movl $.LC0, %edi
movl $0, %eax
call printf
movl $0, %eax
call function
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-44)"
.section .note.GNU-stack,"",#progbits
**cat hello2.s**
.file "hello2.c"
.data
.align 4
.type s, #object
.size s, 4
s:
.long 20
.globl g
.align 4
.type g, #object
.size g, 4
g:
.long 10
.section .rodata
.LC0:
.string "g : %d s : %d\n"
.text
.globl function
.type function, #function
function:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl g(%rip), %eax
addl $1, %eax
movl %eax, g(%rip)
movl s(%rip), %eax
addl $1, %eax
movl %eax, s(%rip)
movl s(%rip), %edx
movl g(%rip), %eax
movl %eax, %esi
movl $.LC0, %edi
movl $0, %eax
call printf
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size function, .-function
.ident "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-44)"
.section .note.GNU-stack,"",#progbits
Please share me the reference if this query is already posted earlier, unable to track the similar link.

Answer 1

Internally compiler generates two distinct object files hello1.o and hello2.o. Each of them exports variable g:
$ readelf -sW hello1.o | grep GLOBAL
8: 0000000000000000 4 OBJECT GLOBAL DEFAULT 2 g
$ readelf -sW hello2.o | grep GLOBAL
10: 0000000000000000 4 OBJECT GLOBAL DEFAULT 2 g
Linker detects that there is more then one definition of g when building a global symbol table for the executable and reports an error.