I am using a test program for understanding C memory model on linux 6.3 with kernal version 2.6.32-279.el6.x86_64 .
First i have compile below code,
#include <stdio.h>
int main(void)
{
static int i = 100; /* Initialized static variable stored in DS*/
return 0;
}
on running size command , i got below ,
[root@rachitjain jan14]# size a.out
text data bss dec hex filename
1040 488 16 1544 608 a.out
then, after removing the intialization for static variable 'i' , my code becomes ,
include <stdio.h>
int main(void)
{
static int i ;
return 0;
}
On running size on after compiling above ,
[root@rachitjain jan14]# size a.out
text data bss dec hex filename
1040 484 24 1548 60c a.out
There is 8 byte increment in bss section but only 4 bytes are reduced in the data section. Why the size is integer in getting doubled while moving to bss segment ?
I have tested this character and float as well , observed the same behavioral.
Look, the answer is that the .bss section has a requirement to be aligned on 64 bits and the .data does not have this requirement.
How can you see this? When you build your program ld uses a default linker script in order to build your program. You can see it if you add -Wl,-verbose:
g++ main.cpp -Wl,-verbose
When you build 64 bit applicaton this is a default aligment for .bss section:
.bss :
{
*(.dynbss)
*(.bss .bss.* .gnu.linkonce.b.*)
*(COMMON)
/* Align here to ensure that the .bss section occupies space up to
_end. Align after .bss to ensure correct alignment even if the
.bss section disappears because there are no input sections.
FIXME: Why do we need it? When there is no .bss section, we don't
pad the .data section. */
. = ALIGN(. != 0 ? 64 / 8 : 1);
}
As you can see ALIGN(. != 0 ? 64 / 8 : 1); tells to align to 8 bytes
When you build 32 bit applicaton (g++ -m32 main.cpp -Wl,-verbose) this is a default aligment for .bss section:
.bss :
{
*(.dynbss)
*(.bss .bss.* .gnu.linkonce.b.*)
*(COMMON)
/* Align here to ensure that the .bss section occupies space up to
_end. Align after .bss to ensure correct alignment even if the
.bss section disappears because there are no input sections.
FIXME: Why do we need it? When there is no .bss section, we don't
pad the .data section. */
. = ALIGN(. != 0 ? 32 / 8 : 1);
}
Your .data section obviously does not have any ALIGN commands in the default linker script:
.data :
{
*(.data .data.* .gnu.linkonce.d.*)
SORT(CONSTRUCTORS)
}
Useful links: