#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>
main ()
{
  int fd, serial;
  int laststate = -1;
  int state;

  fd = open ("/dev/ttyS1", O_RDONLY);
  while (1)
    {
      usleep(1000);
      ioctl (fd, TIOCMGET, &serial);
      if (serial & TIOCM_CD)
	{
	  state = 0;
	  if (state != laststate)
	    {
	      puts ("TIOCM_CD is not set");
	    }
	  laststate = state;
	}
      else
	{
	  state = 1;
	  if (state != laststate)
	    {
	      puts ("TIOCM_CD is set");
	    }
	  laststate = state;
	}
    }
  close (fd);
}