Question

I’m writing a simple variadic function that adds together a series of ints and returns the sum. I’m having a bit of trouble understanding how it works and my code doesn’t seem to work though I feel I’m in the right direction with my code. (Posted below) The specifications of this function are that it takes at least one param and the last param is always a zero (as seen called in the main). I was also told, based upon my machine, that I wouldn’t necessarily get the output I’m looking for which, as you could imagine, further complicates my situation. Some assistance with correcting my Sum() function would be greatly appreciated.

EDIT:
This is supposed to be done w/o the use of stdarg.h header and thus no va_arg functions.

int Sum(int a, ... ) {
   int sum = 0, *addy = &a;

   while (*addy) {
      sum += *addy;
      addy += sizeof(a);
   }

   return sum;
}


int main() {
   printf("%d %d %d %d\n", Sum(0), Sum(3, 5, 6, 7, 0),
   Sum(7, 2, 42, 3, 5, -4, 0), Sum(-1, 9, 12, 123, -213, 42, 7, 2, 0));
}
//Expected output: 0 21 55 -19
//My output: 0 32770 32770 32776

Answer 1

When you add a number to an int pointer (as in addy += sizeof(a)) the number you add is automatically multiplied by the size of whatever type the pointer is declared as (in this case int). To fix this, just use

addy += 1;

instead. However, I would recommend using variadic macros instead of this method, they are clearer and less error prone.