how about .bss section not zero initialized

Question

as we know .bss contains un-initialized variables. if in c code, programer initialize the variables before using them. then .bss is not necessary to be zero before executing C code.

Am I right?

Thanks

Answer 1

In C code, any variable with static storage duration is defined to be initialized to 0 by the spec (Section 6.7.8 Initialization, paragraph 10):

If an object that has static storage duration is not initialized explicitly, then:

• if it has pointer type, it is initialized to a null pointer;
• if it has arithmetic type, it is initialized to (positive or unsigned) zero;
• if it is an aggregate, every member is initialized (recursively) according to these rules;
• if it is a union, the first named member is initialized (recursively) according to these rules.

Some program loaders will fill the whole section with zeroes to start with, and others will fill it 'on demand' as a perfomance improvement. So while you are technically correct that the .bss section may not really contain all zeroes when the C code starts executing, it logically does. In any case, assuming you have a standard compliant toolchain, you can think of it as being all zero.

Any variables that are initialized to non-zero values will never end up in the .bss section; they are handled in the .data or .rodata sections, depending on their particular characteristics.

Answer 2

That depends on where the variable is in code. For instance if you're talking about a local variable in main() or any other function, then variables are pushed onto the stack (unless you use other modifying keywords). If your variable is global AND uninitialized then it should be kept in .bss. Note that compiler optimization and so forth may change things around a bit. If you want to know for sure use readelf to investigate an ELF binary on linux.

Answer 3

The ELF specification says:

.bss This section holds uninitialized data that contribute to the program’s memory image. By definition, the system initializes the data with zeros when the program begins to run. The section occupies no file space, as indicated by the section type, SHT_NOBITS.

It therefore follows that a C global variable which has a value assigned to it cannot be put into the .bss section and will have to go into the .data section. The .data section contains the initial valued for all the variables assigned to it.

Answer 4

It seems like you might be confused about the mechanism by which the .bss section ends up zero initialized. The code that you compile doesn'thave to explicitly initialize the region to zero because when the operating system first allocates a new page of memory to a process the OS makes sure that the page is zero initialized. This is done for security reasons, so that a process can't go looking for secrets that were left in memory when other processes exited.