button number incorrect

[Brian Clark <bc viz tamu edu>]

I'm in GTK 2, trying to get a mouse motion event to fire properly.  
Specifically, I want the mouse motion event to fire only when the left 
mouse button  is pressed and the mouse is moved.  The object I want 
this to work over is an image, and I have put the image inside an event 
box.  But the button number is always 0, no matter what I do.

I have the following attached to the event box:

    gtk_widget_add_events(posteventbox, GDK_BUTTON_RELEASE_MASK);
    gtk_widget_add_events(posteventbox, GDK_BUTTON_PRESS_MASK);
    gtk_widget_add_events(posteventbox, GDK_BUTTON1_MOTION_MASK);
    //gtk_widget_add_events(posteventbox, GDK_BUTTON_MOTION_MASK);

I've tried it with the GDK_BUTTON_MOTION_MASK both commented out and 
inserted.

The signal connect function is:
g_signal_connect ((gpointer) posteventbox, "motion_notify_event",
                      G_CALLBACK (on_histogram_button1_motion),
                      &original);


And the callback:
void on_histogram_button1_motion(GtkWidget *img, GdkEventButton *event, 
gpointer user_data)
{
    Image *theImage;
    theImage = (Image*)user_data;

        cout<< "Motion with button in histogram" << endl;
        cout << "now at " << event->x << ", " << event->y << endl;
        cout << "on button " << event->button << endl;
}

It works, in terms of tracking motion.  When the mouse moves, it spews 
the text, but the button is always 0, no matter what I do.

Any suggestions?

Thanks,
bc