Memory allocation for Data segment not happening properly for each static and global(uninitialized/ initialized variable)

Question

I am trying to understand how memory allocation works for any C code which is executed or compiled.

I have written following 5 separate small codes which will help me understand the memory allocation process.

segment_size1.c

int main(){
    printf("hellow world");
    return 0;
}

segment_size2.c

//Adding uninitialized static int variable This would increase the size of BSS by 4 bytes.
int main()
{
    static int mystatic;
    printf("Hellow world");
    return 0;
}

segment_size3.c

// Adding initialized static int variable, this would increase the size of initialized data
// segment by 4 bytes.

int main()
{
        static int mystatic;
        static int mystatic1 = 100;
        printf("Hellow world");
        return 0;
}

segment_size4.c

// Adding un-initialized global int variable, this would increase the size of BSS segment by 4 bytes.

int myglobal;
int main()
{
        static int mystatic;
        static int mystatic1 = 100;
        printf("Hellow world");
        return 0;
}

segment_size5.c

// Adding initialized global int variable, this would increase the size of data segment by 4 bytes.

int myglobal;
int myglobal2 = 500;
int main()
{
    static int mystatic;
    static int mystatic1 = 100;
    printf("Hellow world");
    return 0;
}

According to my understanding, if we compile above files then after compiling segment_size2.c BSS size should get incremented by 4 bytes and after compiling segment_size3.c Data size should get incremented by 4 bytes. But when I hit size command, I got following results,

    size segment_size1 segment_size2 segment_size3 segment_size4 segment_size5
       text    data     bss     dec     hex filename
       1217     560       8    1785     6f9 segment_size1
       1217     560       8    1785     6f9 segment_size2
       1217     564      12    1793     701 segment_size3
       1217     564      12    1793     701 segment_size4
       1217     568      16    1801     709 segment_size5

Here We can clearly see that bss and data segment is simultaneously getting updated after compiling segment_size3. How is this possible?

score 0 · Answer 1 · answered Jan 09 '19 at 10:21

We can clearly see that bss and data segment is simultaneously getting updated

This is the distinction between where the initial values != 0 are saved and where the static/global variables are

Before the main execution the initial values !=0 are copied into the global/static variables, the global/static variables having the initial value 0 are also set to 0 of course

Memory allocation for Data segment not happening properly for each static and global(uninitialized/ initialized variable)

1 Answers1