va_arg always runs 4 times - c

Question

I am learning stdarg.h in c i am trying to print all arguments passed to function without knowing how many arguments are there but yet i have not come up with solution, during this this happened, no matter what i pass to strtest.
It always print 0. 1. 2. 3.
void strtest(char *fmt, ...){
va_list argp;
int i = 0;
va_start(argp, fmt);
while(va_arg(argp, char*))
printf("%d\t", i++ );
va_end(argp);
}
int main(int argc, char *argv[]){
strtest("s");
printf("\n");
return 0;
}

Answer 1

There's no standard mechanism that will tell you the number of arguments passed to a varargs function. Functions like printf() work because they can determine the number of arguments by examining the format string.

Answer 2

Here is an example showing one way to pass an unknown number of arguments.
#include <stdio.h>
#include <stdarg.h>
void print (char *first, ...)
{
va_list argptr;
char *next;
va_start (argptr, first);
next = first;
while (next) {
printf ("%s\n", next);
next = va_arg(argptr, char*);
}
va_end (argptr);
}
int main(void)
{
print("hello","world", NULL); // NULL as sentinel
return 0;
}
Program output
hello
world
Perhaps you can adapt this to your needs using int arguments.

Answer 3

This is the definition of stdarg.h in the ISO 9899 WG14 n1256
The header <stdarg.h> declares a type and defines four macros, for advancing
through a list of arguments whose number and types are not known to the called function
when it is translated
You have to pass the number of arguments, and possibly the types as well, to the caller. This doesn't have to be done by directly passing the number of arguments, there are other methods such as the one used in printf.

Answer 4

You could pass a sentinel to the function like this
strtest("s", (char*)0);
such that the function can notice that it is at the end of the argument list.

Answer 5

If you look at the man page for stdarg, va_arg includes this text
If there is no next argument, or if type is not compatible with the
type of the actual next argument (as promoted according to the
default argument promotions), random errors will occur.
Unless you call strtest with a NULL as the last argument, va_arg will just keep reading until it hits something that makes it stop. Think of what you are doing right now as equivalent to reading an array outside its bounds.
I'm surprised it was running 4 times no matter what though. I would have expected the count to be equal to the number of args you passed to strtest plus 2 or more.