Entering the Main Loop

Once everything is set up, two steps remain: you need to show the window on the screen, and wait for user input.

  gtk_widget_show_all(window);

  gtk_main();

  return 0;

gtk_widget_show_all() recursively calls gtk_widget_show() on a container and its children. The following code would have the same effect in this case:

  gtk_widget_show(label);
  gtk_widget_show(button);
  gtk_widget_show(window);

It's necessary to show each and every widget that you want to appear on the screen. The opposite operation is called gtk_widget_hide(); widgets start their life hidden, and can be re-hidden/re-shown any number of times. It's good practice to show all child widgets before showing the outermost container; otherwise, the user will see the container appear first, followed by its children. Widgets are not actually visible on the screen until their parent container is shown---the exception to the rule is GtkWindow, since it has no parent.

Once your widgets have been shown, you want to wait for the user to do something with them. gtk_main() enters the GTK+ main loop; the main loop is event-driven. That is, user actions trigger events which generally result in signals being emitted and your callbacks being called. gtk_main() blocks indefinitely, waiting for and responding to user input. The main loop is described in more detail in the section called The Main Loop. Events and their relation to the main loop are described in the section called Events in the chapter called GDK Basics.