Question

This is my code:

#include <stdio.h>

void add(int num, ...);

int main(void)
{
  int a=100, b=200, c=300;

  add(1, a);
  add(2, a, b);
  add(3, a, b, c);
  return 0;
}

void add(int num, ...)
{
  int *p=NULL;
  p=&num+1;
  printf("%x \n", p);
  if(num==1)
    {
      printf("%d \n", p[0]);
      printf("num is: %d \n", num);
    }
  else if (num==2)
    {
      printf("%d \n", p[0]+p[1]);
      printf("num is: %d \n", num);
    }
  else
    {
      printf("%d \n", p[0]+p[1]+p[2]);
      printf("num is: %d \n", num);
    }
}

From my understanding, p initially points to a, which is 10. Thus, it should print 10, 30, 60, respectively. Nonetheless, it prints

6786db50 
1736891264 
num is: 1 
6786db50 
1736924031 
num is: 2 
6786db50 
1867401241 
num is: 3 

Is p pointing to a wrong address? How can I correctly read the arguments passed as ...?

Answer 1

That’s not how you use variadic function calls, you need to use the va_* function calls to extract the parameters.

See http://unixhelp.ed.ac.uk/CGI/man-cgi?stdarg+3 or http://en.wikipedia.org/wiki/Variadic_function#Variadic_functions_in_C.2C_Objective-C.2C_C.2B.2B.2C_and_D