How can I make my TOPLEVEL window always on top?

[Rocky Zhang <rocky userful com>]

Hi,

I'd like my application always on top of other application window (
Similar as I click the left top corner of the window, then select "On
Top" ). How can I realize it? I saw in gtk/gtkwindow.h there is a
function called: 

gboolean   gtk_window_has_toplevel_focus       (GtkWindow          
*window);

Can I use it to determine if the window is on top or not? Another
function is:

void            _gtk_window_set_has_toplevel_focus (GtkWindow *window, 
gboolean   has_toplevel_focus);

I use it in my source code, when compile, it always said link error. 

My compile command is: gcc test_app.c -o test_app  `pkg-config gtk+-2.0
--cflags --libs`

And it says:
/tmp/ccd8wLTs.o(.text+0x8e1): In function `main':
: undefined reference to `_gtk_window_set_has_toplevel_focus'
collect2: ld returned 1 exit status

How can I do with it? Is it the correct function to set the window "On
Top" among all the opening windows?

Thanks.

Rocky