gtk_window_close() problem

#include <gtk/gtk.h>

GtkBuilder* builder;
GtkWidget* mainWindow;
GtkWindow* settingsWindow;

// settings window
extern "C" void saveSettings(GtkWidget* widget, gpointer data){
    gtk_widget_close(settingsWindow);
}

// file menu
extern "C" void openSettings(GtkWidget* widget, gpointer data){
    gtk_window_present(settingsWindow);
}

extern "C" void exit(GtkWidget* widget, gpointer data){
    gtk_main_quit();
}

// main
int main(int argc, char** args){
    gtk_init(&argc, &args);

    builder = gtk_builder_new();
    gtk_builder_add_from_file(builder, "res/UI.glade", 0);

    gtk_builder_connect_signals(builder, 0);

    mainWindow = GTK_WIDGET(gtk_builder_get_object(builder, "mainWindow"));
    settingsWindow = GTK_WINDOW(gtk_builder_get_object(builder, "settingsWindow"));

    gtk_widget_show(mainWindow);

    gtk_main();

    return 0;
}