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carmdeclarationelfsections

Process sections: does a declaration add also something to .text? If yes, what does it add?

I have a C code like this one, that will be possibly compiled in an ELF file for ARM:

int a;
int b=1;

int foo(int x) {
    int c=2;
    static float d=1.5;
    // .......
}

I know that all the executable code goes into the .text section, while .data , .bss and .rodata will contain the various variables/constants. My question is: does a line like int b=1; here add also something to the .text section, or does it only tell the compiler to place a new variable initialized to 1 in .data (then probably mapped in RAM memory when deployed on the final hardware)?

Moreover, trying to decompile a similar code, I noticed that a line such as int c=2;, inside the function foo(), was adding something to the stack, but also some lines of .text where the value '2' was actually memorized there.

So, in general, does a declaration always imply also something added to .text at an assembly level? If yes, does it depends on the context (i.e. if the variable is inside a function, if it is a local global variable, ...) and what is actually added?

Thanks a lot in advance.

Solution

  • does a line like int b=1; here add also something to the .text section, or does it only tell the compiler to place a new variable initialized to 1 in .data (then probably mapped in RAM memory when deployed on the final hardware)?

    You understand that this is likely to be implementation specific, but the likelihood is that that you will just get initialised data in the data section. Were it a constant, it might, instead go into the text section.

    Moreover, trying to decompile a similar code, I noticed that a line such as int c=2;, inside the function foo(), was adding something to the stack, but also some lines of .text where the value '2' was actually memorized there.

    Automatic variables that are initialised, have to be initialised each time the function's scope is entered. The space for c is reserved on the stack (or in a register, depending on the ABI) but the program has to remember the constant from which it is initialised and this is best placed somewhere in the text segment, either as a constant value or as a "move immediate" instruction.

    So, in general, does a declaration always imply also something added to .text at an assembly level?

    No. If a static variable is initialised to zero or null or not initialised at all, it is often just enough to reserve space in bss. If a static non constant variable is initialised to a non zero value, it will just be put in the data segment.