idiot's question on Xlib programming

[huixf@tonghua.com.cn]

I write a simple program but can't compile it. Below is the program:

#include <Xlib.h>
main()
{Display *d=XOpenDisplay(NULL);
  if(d==NULL) 
   {printf("can't open display\n");exit(0);}  XCloseDisplay(d);
}

I use "gcc -I/usr/X11R6/include/X11 xhi.c" to complie it and get the following message:

/tmp/cca002281.o: In function `main':
/tmp/cca002281.o(.text+0x9): undefined reference to `XOpenDisplay'
/tmp/cca002281.o(.text+0x45): undefined reference to `XCloseDisplay'

What's the problem? Please reply to huixf@tonghua.com.cn because I will leave the list in a minute.