Determining BSS Size on Object File

Question

Wikipedia mentions that "the bss section typically includes all uninitialized variables declared at file scope." Given the following file:

int uninit;

int main() {
    uninit = 1;
    return 0;
}

When I compile this to an executable I see the bss segment filled properly:

$ gcc prog1.c -o prog1
$ size prog1
text       data     bss     dec     hex filename
1115        552       8    1675     68b prog1

However if I compile it as an object file I don't see the bss segment (I'd expect it to be 4):

$ gcc -c prog1.c
$ size prog1.o 
text       data     bss     dec     hex filename
  72          0       0      72      48 prog1.o

Is there something obvious I am missing?

I am using gcc version 4.8.1.

Answer 1

If we use readelf -s to look at the symbol table, we'll see:

$ readelf -s prog1.o

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS bss.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    7 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     8: 0000000000000004     4 OBJECT  GLOBAL DEFAULT  COM uninit      <<<<
     9: 0000000000000000    16 FUNC    GLOBAL DEFAULT    1 main

We see that your uninit symbol ("variable") is, at this stage, a "common" symbol. It has not yet been "assigned" to the BSS.

See this question for more information on "common" symbols: What does "COM" means in the Ndx column of the .symtab section?

Once your final executable is linked together, it will be put in the BSS as you expected.

---

You can bypass this behavior by passing the -fno-common flag to GCC:

$ gcc -fno-common -c bss.c
$ size bss.o
   text    data     bss     dec     hex filename
     72       0       4      76      4c bss.o

---

Instead, you could mark uninit as static. This way, the compiler will know that no other .o file can refer to it, so it will not be a "common" symbol. Instead, it will be placed into the BSS immediately, as you expected:

$ cat bss.c

static int uninit;

int main() {
    uninit = 1;
    return 0;
}

$ gcc -c bss.c 
$ size bss.o
   text    data     bss     dec     hex filename
     72       0       4      76      4c bss.o