Determining BSS Size on Object File - c

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