Issue
First of all: I know that there are a lot of web pages (including discussions on stackoverflow) where the differences between .bss and .data for the data declaration are discussed, but I have a specific question and unfortunately I did not find the answer on those pages, so I ask it here :-).
I am a big beginner in assembly, so I apologize if the question is stupid :-).
I am learning assembly on an x86 64-bit linux OS, but I think that my question is more general and probably not specific to the OS / the architecture.
I find the definition of the .bss and .data sections a bit strange. I can always declare a variable in .bss and then move a value in this variable from my code (.text section), right? So why should I declare a variable in the .data section, when I know that variables declared in this section will add to the size of my executable file?
I could ask this question in the context of C programming as well: why should I initialize my variable when I declare it, if it is more efficient to declare it uninitialized and then assign a value to it in the beginning of my code?
I suppose that my approach of memory management is naive and not correct, but I do not understand why.
Solution
.bss is where you put zero-initialized static data, like C int x; (at global scope). That's the same as int x = 0; for static / global (static storage class)1.
.data is where you put non-zero-initialized static data, like int x = 2; If you put that in BSS, you'd need a runtime static "constructor" to initalize the BSS location. Like what a C++ compiler would do for static const int prog_starttime = __rdtsc();. (Even though it's const, the initializer isn't a compile-time constant so it can't go in .rodata)
.bss with a runtime initializer would make sense for big arrays that are mostly zero or filled with the same value (memset / rep stosd), but in practice writing char buf[1024000] = {1}; will put 1MB of almost all zeros into .data, with current compilers.
Otherwise it is not more efficient. A mov dword [myvar], imm32 instruction is 10 bytes long, costing over twice as many bytes in your executable as if it were statically initialized in .data. Also, the initializer code has to be executed as well as loaded, and the 4 bytes of BSS space also take RAM.
By contrast, section .rodata (or .rdata on Windows) is where compilers put string literals, FP constants, and static const int x = 123; (Actually, x would normally get inlined as an immediate everywhere it's used in the compilation unit, letting the compiler optimize away any static storage. But if you took its address and passed &x to a function, the compiler would need it to exist in memory somewhere, and that would be in .rodata)
Footnote 1: Inside a function, int x; would be on the stack if the compiler didn't optimize it away or into registers, when compiling for a normal register machine with a stack like x86.
I could ask this question in the context of C programming as well
In C, an optimizing compiler will treat int x; x=5; pretty much identically to int x=5; inside a function. No static storage is involved. Looking at actual compiler output is often instructive: see How to remove "noise" from GCC/clang assembly output?.
Outside a function, at global scope, you can't write things like x=5;. You could do that at the top of main, and then you would trick the compiler into making worse code.
Inside a function with static int x = 5;, the initialization happens once. (At compile time). If you did static int x; x=5; the static storage would be re-initialized every time the function was entered, and you might as well have not used static unless you have other reasons for needing static storage class. (e.g. returning a pointer to x that's still valid after the function returns.)
Answered By - Peter Cordes Answer Checked By - Clifford M. (WPSolving Volunteer)