Passing values to functions

[Shiraz Baig <shiraz_baig yahoo com>]

To List:
I am a newbie. Can anyone tell me, in the following
elementary program, why doesn't the function "hello"
that it has not been passed any data. Because in
gtk-signal-connect, we are passing NULL data.
Wherefrom is this data coming. In another case, I have
seen drawing_area being passed like this. How does it
get to the function "hello".
Thanks.
bye.

#include <gtk/gtk.h>
void hello( GtkWidget *widget,
            gpointer   data )
{
    g_print ("Hello World\n");
}

void destroy( GtkWidget *widget,
              gpointer   data )
{
    gtk_main_quit();
}

int main( int   argc,
          char *argv[] )
{
    GtkWidget *window;
    GtkWidget *button;
    gtk_init(&argc, &argv);
    
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    
    gtk_signal_connect (GTK_OBJECT (window),
"destroy",
                        GTK_SIGNAL_FUNC (destroy),
NULL);
    
    button = gtk_button_new_with_label ("Hello
World");
    
    gtk_signal_connect (GTK_OBJECT (button),
"clicked",
                        GTK_SIGNAL_FUNC (hello),
NULL);
    
    gtk_container_add (GTK_CONTAINER (window),
button);
    
    gtk_widget_show (button);
    gtk_widget_show (window);
    gtk_main ();
    return(0);
}


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