Initialisation during load time vs runtime vs compile-time

Question

In the below example, I get an error on compilation

int main() {

    int x = 10; // guess this is runtime initialisation
    static int y = x; //guess this is loadtime initialisation

    printf("x = %d, y = %d", x, y);

}

Error:

    error: initializer element is not constant

My understanding is this is related to the way static and global members are initialised as against auto variables. I wanted to know the difference between the load time , runtime and compile-time initialisation I also wanted to know about the element's value getting stored in data section and stack and its effect on initialisation.

I think load-time and compile-time are the same. At least from the point of view that both occur before run time. — Alexey Frunze, Apr 17 '13 at 11:51
You should really look your questions up before asking because they may have been already asked and answered. Type in the relevant keywords (`[c]`, `static`, `initialize` here), use google too. — Alexey Frunze, Apr 17 '13 at 11:58

score 0 · Answer 1 · answered Apr 17 '13 at 11:56

I'd say in C there is no difference between compile-time and load-time. The same is not true of C++: POD constants are initialized at compile time, while global/static objects with constructors are initialized at load time.

In your example, you try to initialize a compile-time value with a run-time variable's contents, so it fails. In C++, if x were declared const, it would be recognized as a compile-time constant and succeed.

score 0 · Answer 2 · answered Apr 17 '13 at 11:57

0

Static variables came to life before the automatic variable or say local variable so u can't use any variables for initialization of static variable u can only use constants for initialization of static variable.

Load time initialization and compile time initialization is same in C and runtime initialization is may be not supported in C ..it is supported in C++ but i don't know about C

answered Apr 17 '13 at 11:57

umang2203

78
5

Runtime initialization is definitely supported in C. That `x` in `main()` does not even exist outside of run time, so it's created and initialized at run time. – Alexey Frunze Apr 17 '13 at 12:17

score 0 · Answer 3 · answered Apr 17 '13 at 12:22

0

constants are initialized at compile time, while global/static variables are initialized at load time so you cannot initialise global/static variable with load time with run time stack variables if we can convert x to const(c++) it would run ok.

answered Apr 17 '13 at 12:22

anshul garg

473
3
7

some answers below say "there is no difference between compile-time and load-time" ?? – Aadishri Apr 17 '13 at 12:25
yes in C, order of loading instructions in the code will be determined at compile time – anshul garg Apr 17 '13 at 12:29

Initialisation during load time vs runtime vs compile-time

3 Answers3