The first parameter is the widget that you want to monitor for the "show" signal. When the window is shown to the user, window_is_visible() will be called. The fourth parameter allows you to send one pointer to the callback function, or NULL to ignore it. The callback function should look like this:
Code: (Plaintext)
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void window_is_visible (GtkWindow *window, gpointer data)
{
/* Do something now that the window is shown. */
}