Variable Length Arguments without using stdarg.h

Question

I'm trying to understand and implement a way to handle variable length argument function by checking the stack, but I have not idea on where and how to start. I have found the following links helpful but I still don't quite understand how I would be able to something like this in c for something like a print function.

http://www.swig.org/Doc1.3/Varargs.html

Trailing dots (...) within function argument list in C++

Answer 1

Look in the C Reference Manual by Dennis M. Ritchie at the implementation of printf() that existed long before ... and <stdarg.h>:

printf ( fmt, args )
  char fmt [ ] ;
{
  ...
  int *ap, x, c ;
  ap = &args ; /* argument pointer */
  ...
  for ( ; ; ) {
    while ( ( c = *fmt++ ) != ´%´ ) {
      ...
      switch ( c = *fmt++) {
      /* decimal */
      case ´d ´:
        x = *ap++ ;
        if(x < 0) {
          x = -x;
          if ( x<0 ) { /* is - infinity */
            printf ( "-32768" ) ;
            continue ;
          }
          putchar ( ´-´);
        }
        printd(x);
        continue ;
      ...
      }
      ...
    }
    ...
  }
}

printd(n)
{
  int a ;
  if ( a=n/10 )
    printd(a);
  putchar ( n%10 + ´0´ ) ;
}

You can disregard the old (AKA K&R) syntax of function and parameter declaration:

printf ( fmt, args )
  char fmt [ ] ;

and even replace it with a more modern:

int printf ( char fmt[], int args )

but the idea is the same, you get a pointer to the first optional argument (args) directly, as is done in the above code with ap = &args;, and then keep incrementing it to get access to further arguments.

Or you can do it indirectly, using the last non-optional argument as the starting point:

int printf ( char fmt[], ... )
{
  ...
  int *ap, x, c ;
  ap = &fmt ; /* argument pointer */
  ap++; /* ap now points to first optional argument */
  ...
}

This illustrates the mechanism.

Beware, if you write your code like this, it won't be portable and it may even fail work with your compiler.