Memory Layout of a C program - Part 2

Continuation of PART-1

As we have seen so much theory in the PART-1 now let us see a real-time example to understand about these segments. we will use size(1) command to list various section sizes in a C code.

A simple C program is given below

#include <stdio.h>

int main()
{
    return 0;
}

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    836     260       8    1104     450 a.out

Now add a global variable as shown below

#include <stdio.h>

int global; /* Uninitialized variable stored in bss*/

int main()
{
    return 0;
}

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    836     260      12    1108     454 a.out

As you can see BSS is incremented by 4 bytes.

Let us include a static variable as shown below.

#include <stdio.h>

int global; /* Uninitialized variable stored in bss*/

int main()
{
    static int i; /* Uninitialized static variable stored in bss */
    return 0;
}

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    836     260      16    1112     458 a.out

As you can see BSS is incremented by 4 bytes.

Now let us initialize the static variable so that it is saved in the initialized data segment.

#include <stdio.h>

int global; /* Uninitialized variable stored in bss*/

int main()
{
    static int i=10; /* Initialized static variable stored in DS*/
    return 0;
}

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    836     264      12    1112     458 a.out

Now you can see that data section is incremented by 4 bytes and BSS is decremented by 4 bytes.

Let us even initialize the Global variable which makes it part of Data Segment.

#include <stdio.h>

int global=10; /* Initialized variable stored in DS*/

int main()
{
    static int i=10; /* Initialized static variable stored in DS*/
    return 0;
}

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    836     268      8    1112     458 a.out

We can see that data section is incremented by 4 bytes and BSS is decremented by 4 bytes.

Let us see what happens when we add a const variable.

#include <stdio.h>

const int i = 1;

int global=10;

int main()
{
    static int i=10;
    return 0;
}

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    840     268       8    1116     45c a.out

Now we can see that TEXT segment is incremented by 4 bytes.

References:
1. Narendra Kangralkar