Getting ellipses function parameters without an initial argument
- by Tox1k
So I've been making a custom parser for a scripting language, and I wanted to be able to pass only ellipses arguments. I don't need or want an initial variable, however Microsoft and C seem to want something else. FYI, see bottom for info.
I've looked at the va_* definitions
#define _crt_va_start(ap,v) ( ap = (va_list)_ADDRESSOF(v) + _INTSIZEOF(v) )
#define _crt_va_arg(ap,t) ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define _crt_va_end(ap) ( ap = (va_list)0 )
and the part I don't want is the v in va_start. As a little background I'm competent in goasm and I know how the stack works so I know what's happening here. I was wondering if there is a way to get the function stack base without having to use inline assembly.
Ideas I've had:
#define im_va_start(ap) (__asm { mov [ap], ebp })
and etc... but really I feel like that's messy and I'm doing it wrong.
struct function_table {
const char* fname;
(void)(*fptr)(...);
unsigned char maxArgs;
};
function_table mytable[] = {
{ "MessageBox", &tMessageBoxA, 4 } };
... some function that sorts through a const char* passed to it to find the matching function in mytable and calls tMessageBoxA with the params. Also, the maxArgs argument is just so I can check that a valid number of parameters is being sent. I have personal reasons for not wanting to send it in the function, but in the meantime we can just say it's because I'm curious.
This is just an example; custom libraries are what I would be implementing so it wouldn't just be calling WinAPI stuff.
void tMessageBoxA(...) {
// stuff to load args passed
MessageBoxA(arg1, arg2, arg3, arg4);
}
I'm using the __cdecl calling convention and I've looked up ways to reliably get a pointer to the base of the stack (not the top) but I can't seem to find any. Also, I'm not worried about function security or typechecking.
