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?

Answer 1

#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

Answer 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.

Answer 3

字符串“hello”在.rodata部分中.rodata

Answer 4

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.

Answer 5

由于您不在代码中使用该char * ，因此编译器对其进行了优化。