Question

When compiling this:

// external definitions
int value1 = 0;
static int value2 = 0;

the gcc compiler generates the following assembly:

.globl value1
        .bss
        .align 4
        .type   value1, @object
        .size   value1, 4
value1:
        .zero   4
        .local  value2
        .comm   value2,4,4

However, when i initialize the variables to a value other than zero such as:

// external definitions
int value1 = 1;
static int value2 = 1;

the gcc compiler generated the following:

.globl value1
        .data
        .align 4
        .type   value1, @object
        .size   value1, 4
value1:
        .long   1
        .align 4
        .type   value2, @object
        .size   value2, 4
value2:
        .long   1

My questions are:

1. Why in the first case the values are allocated in the bss segment while in the second case in the data segment.
2. Why value2 variable is defined as .local and .comm in the first case, while not in the second.

Answer 1

Generally speaking, the bss section contains uninitialized values and the data section contains initialized values. However, gcc places values that are initialized to zero into the bss section instead of the data section, as the bss section is zeroed out in runtime anyway, it doesn’t make much sense to store zeros in the data section, this saves some disk space, from man gcc:

-fno-zero-initialized-in-bss If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This
can save space in the resulting code. This option turns off this
behavior because some programs explicitly rely on variables going to
the data section

I’m not sure why .comm is used with static storage which is local to an object file, it is usually used to declare common symbols that, if not defined/initialized, should be merged by the linker with symbol that have the same name from other object files and that’s why it’s not used in the second example because the variables are initialized, from the as manual

.comm declares a common symbol named symbol. When linking, a common
symbol in one object file may be merged with a defined or common
symbol of the same name in another object file