Data section in a.out

Question

here is a simple code that I executed

int a;
int main()
{
    return 0;
}

Then after compiling with gcc I did

size a.out

I got some output in bss and data section...Then I changed my code to this

int a;
int main()
{
    char *p = "hello";
    return 0;
}

Again when I saw the output by size a.out after compiling , size of data section remained same..But we know that string hello will be allocated memory in read only initialized part..Then why size of data section remained same?

Probably either because it's never being used or because the data section is padded. — Nick, Sep 27 '12 at 07:52

Jeegar Patel · Answer 1 · 2012-09-27T08:27:32.053

#include <stdio.h>

int main()
{
return 0;
}

It gives

   text    data     bss     dec     hex filename
    960     248       8    1216     4c0 a.out

when you do

int a;
int main()
{
    char *p = "hello";
    return 0;
}

it gives

   text    data     bss     dec     hex filename
    982     248       8    1238     4d6 a.out

at that time hello is stored in .rodata and the location of that address is stored in char pointer p but here p is stored on stack

and size doesnt shows stack. And i am not sure but .rodata is here calculated in text or dec.

when you write

int a;
char *p = "hello";
int main()
{
    return 0;
}

it gives

   text    data     bss     dec     hex filename
    966     252       8    1226     4ca a.out

now here again "hello" is stored in .rodata but char pointer takes 4 byte and stored in data so data is increment by 4

For more info http://codingfreak.blogspot.in/2012/03/memory-layout-of-c-program-part-2.html

score 2 · Answer 2 · answered Sep 27 '12 at 07:53

2

Actually, that's an implementation detail. The compiler works by an as-is principle. Meaning that as long as the behavior of the program is the same, it's free to exclude any piece of code it wants. In this case, it can skip char* p = "hello" altogether.

answered Sep 27 '12 at 07:53

Luchian Grigore

253,575
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but when i added printf("%s\",s) then size only increased by 4 – user1205088 Sep 27 '12 at 08:06
@user1205088 the call to printf probably. – Luchian Grigore Sep 27 '12 at 08:11
As long as you use -O0 (default) on gcc, it will not exclude code (at least as far as I know). – dbrank0 Sep 27 '12 at 08:20

score 2 · Answer 3 · answered Sep 27 '12 at 07:53

2

The string "hello" is allocated in the section .rodata

answered Sep 27 '12 at 07:53

Serge

6,088
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27

@user1205088 just do "grep hello" on your binary file and see if it is thrown away by compiler or not :) – Serge Sep 27 '12 at 07:58

Michael Mera · Answer 4 · 2012-09-27T08:20:37.580

Even if the total size doesn't changed, it doesn't mean that the code didn't.

I tested your example. The string "hello" is a constant data, thus it is stored in the readonly .rodata section. You can see this particular section using objdump, for example:
objdump -s -j .rodata <yourbinary>

With gcc 4.6.1 without any options, I got for your second code:

Contents of section .rodata:
 4005b8 01000200 68656c6c 6f00               ....hello.

score 0 · Answer 5 · answered Sep 27 '12 at 07:53

0

Since you don't use that char * in your code, the compiler optimized it away.

answered Sep 27 '12 at 07:53

Fredrik Pihl

44,604
7
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Data section in a.out

5 Answers5