call through pointer of function with variable arguments

[Stephen Biggs <xyzzy at hotpop dot com>]

Given this code:
#include <stdio.h>
char buf[2];

int (*gfp)(char *, const char *, ...);
int (*gfp1)(const char *, ...);
     
void f (fp,fp1)
     int (*fp)(char *, const char *, ...);
     int (*fp1)(const char *, ...);
{
  (*fp1)("%.0f\n",5.0);
  (*fp)(buf, "%.0f", 5.0);
}

void h(void)
{
  gfp = sprintf;
  gfp1 = printf;

  (*gfp1)("%.0f\n",5.0);
  (*gfp)(buf, "%.0f", 5.0);
}

int main()
{
  f (&sprintf,&printf);
  h ();
  return 0;
}

... in both the cases, by the time I get to the point where I receive an
expression for the call (in either f or g), the function pointer value
has already been moved to a register and no prototype information is
available, since all I have is the function address (in my port, the
arguments for the current call are already saved aside).  The
proprietary machine I am porting to is sensitive to whether a call has
variable arguments or not and has a different calling sequence based on
that, but how can I know?

Are there any other machines that already have been ported that have
this sensitivity?