Ambiguous behaviour of .bss segment in C program - c

Question

I wrote the simple C program (test.c) below:-
#include<stdio.h>
int main()
{
return 0;
}
and executed the follwing to understand size changes in .bss segment.
gcc test.c -o test
size test
The output came out as:-
text data bss dec hex filename
1115 552 8 1675 68b test
I didn't declare anything globally or of static scope. So please explain why the bss segment size is of 8 bytes.
I made the following change:-
#include<stdio.h>
int x; //declared global variable
int main()
{
return 0;
}
But to my surprise, the output was same as previous:-
text data bss dec hex filename
1115 552 8 1675 68b test
Please explain.
I then initialized the global:-
#include<stdio.h>
int x=67; //initialized global variable
int main()
{
return 0;
}
The data segment size increased as expected, but I didn't expect the size of bss segment to reduce to 4 (on the contrary to 8 when nothing was declared). Please explain.
text data bss dec hex filename
1115 556 4 1675 68b test
I also tried the comands objdump, and nm, but they too showed variable x occupying .bss (in 2nd case). However, no change in bss size is shown upon size command.
I followed the procedure according to:
http://codingfox.com/10-7-memory-segments-code-data-bss/
where the outputs are coming perfectly as expected.

Answer 1

When you compile a simple main program you are also linking startup code.
This code is responsible, among other things, to init bss.
That code is the code that "uses" 8 bytes you are seeing in .bss section.
You can strip that code using -nostartfiles gcc option:
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used
To make a test use the following code
#include<stdio.h>
int _start()
{
return 0;
}
and compile it with
gcc -nostartfiles test.c
Youll see .bss set to 0
text data bss dec hex filename
206 224 0 430 1ae test

Answer 2

Your first two snippets are identical since you aren't using the variable x.
Try this
#include<stdio.h>
volatile int x;
int main()
{
x = 1;
return 0;
}
and you should see a change in .bss size.
Please note that those 4/8 bytes are something inside the start-up code. What it is and why it varies in size isn't possible to tell without digging into all the details of mentioned start-up code.