Ctrl-Click keyboard mouse event

[Carlos Pereira <jose carlos pereira ist utl pt>]

Hi list,
When a button press event is triggered, what is the proper way to know 
if the user is pressing down the Ctrl key?

Tipically I would want to know this inside a button press event callback:

int handle_button_press (GtkWidget *widget,
GdkEventButton *event, void *data)
{
mouse button has been clicked: is Ctrl key down???
...
}

(I need this to implement multiple selections from my OpenGL code that 
are consistent with multiple selections in GtkTreeView)

Thanks,
Carlos