How can I perform pre-main initialization in C/C++ with avr-gcc?

Question

In order to ensure that some initialization code runs before main (using Arduino/avr-gcc) I have code such as the following:

class Init {
public         


        

Answer 1

You can use the ".init*" sections to add C code to be run before main() (and even the C runtime). These sections are linked into the executable at the end and called up at specific time during program initialization. You can get the list here:

http://www.nongnu.org/avr-libc/user-manual/mem_sections.html

.init1 for example is weakly bound to __init(), so if you define __init(), it will be linked and called first thing. However, the stack hasn't been setup, so you have to be careful in what you do (only use register8_t variable, not call any functions).

Answer 2

You can use GCC's constructor attribute to ensure that it gets called before main():

void Init(void) __attribute__((constructor));
void Init(void) { /* code */ }  // This will always run before main()

Answer 3

There is how I perform pre-main coding. There are sever init sections executed before main, refers to http://www.nongnu.org/avr-libc/user-manual/mem_sections.html initN sections.

Anyhow, this only works on -O0 optimization for some reason. I still try to find out which option "optimized" my pre-main assembly code away.

static void
__attribute__ ((naked))
__attribute__ ((section (".init8")))    /* run this right before main */
__attribute__ ((unused))    /* Kill the unused function warning */
stack_init(void) {assembly stuff}

Update, it turns out I claimed this function is unused, leading to optimize the routine away. I was intended to kill function unused warning. It is fixed to used used attribute instead.