Signals

Next, you'll want to arrange to respond when users manipulate the widgets. In this simple application, there are two interesting things that can happen: the user can click the button, or close the window using a window manager decoration. Widgets (actually, all GtkObjects) emit signals when something interesting happens a program might want to respond to. To respond to a signal, you "connect a callback" to it---i.e., register a function to be called when the signal is emitted. Here's that code again:

  gtk_signal_connect(GTK_OBJECT(window),
                     "delete_event",
                     GTK_SIGNAL_FUNC(delete_event_cb),
                     NULL);

  gtk_signal_connect(GTK_OBJECT(button),
                     "clicked",
                     GTK_SIGNAL_FUNC(button_click_cb),
                     label);

gtk_signal_connect() specifies the GtkObject to monitor, which signal to connect to, the callback to connect, and finally a user_data argument---this is an arbitrary gpointer which will be passed to the callback. The macro GTK_SIGNAL_FUNC() casts the callback to a standard function signature; since callbacks have a variety of type signatures, the alternative would be dozens of gtk_signal_connect() variants.

GTK+ performs copious runtime sanity checking; the GTK_OBJECT() macro includes a runtime type check in addition to a C cast, and gtk_signal_connect() will verify that the object can actually emit the signal you've specified.