can't open XDisplay

[arun]

Hello,

I am compiling  a linux XWindows application in win98 using cygwin/xfree86.
But when I execute the program, opening XDisplay fails, due to the following
code fragment

 dpy = XOpenDisplay(getenv("DISPLAY"));
 if (dpy == 0)
 {
      fprintf(stderr,"Can't open X display\n");
  exit(1);
}


what could be the problem???

thanx....