c - If a global variable is initialized to 0, will it go to BSS?

Question

All the initialized global/static variables will go to initialized data section. All the uninitialized global/static variables will go to uninitialed data section(BSS). The variables in BSS will get a value 0 during program load time.

If a global variable is explicitly initialized to zero (int myglobal = 0), where that variable will be stored?

Answer 1

Compiler is free to put such variable into bss as well as into data. For example, GCC has a special option controlling such behavior:

-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. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

The default is -fzero-initialized-in-bss.

Tried with the following example (test.c file):

int put_me_somewhere = 0;

int main(int argc, char* argv[]) { return 0; }

Compiling with no options (implicitly -fzero-initialized-in-bss):

$ touch test.c && make test && objdump -x test | grep put_me_somewhere
cc     test.c   -o test
0000000000601028 g     O .bss   0000000000000004              put_me_somewhere

Compiling with -fno-zero-initialized-in-bss option:

$ touch test.c && make test CFLAGS=-fno-zero-initialized-in-bss && objdump -x test | grep put_me_somewhere
cc -fno-zero-initialized-in-bss    test.c   -o test
0000000000601018 g     O .data  0000000000000004              put_me_somewhere