Create a new file with the following content named example-3.c.

#include <gtk/gtk.h>
#include <glib/gstdio.h>

static void
print_hello (GtkWidget *widget,
             gpointer   data)
{
  g_print ("Hello World\n");
}

static void
quit_cb (GtkWindow *window)
{
  gtk_window_close (window);
}

static void
activate (GtkApplication *app,
          gpointer        user_data)
{
  /* Construct a GtkBuilder instance and load our UI description */
  GtkBuilder *builder = gtk_builder_new();
  gtk_builder_add_from_file (builder, "builder.ui", NULL);

  /* Connect signal handlers to the constructed widgets. */
  GObject *window = gtk_builder_get_object (builder, "window");
  gtk_window_set_application (GTK_WINDOW (window), app);

  GObject *button = gtk_builder_get_object (builder, "button1");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  button = gtk_builder_get_object (builder, "button2");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  button = gtk_builder_get_object (builder, "quit");
  g_signal_connect_swapped (button, "clicked", G_CALLBACK (quit_cb), window);

  gtk_widget_show (GTK_WIDGET (window));

  /* We do not need the builder any more */
  g_object_unref (builder);
}

int
main (int   argc,
      char *argv[])
{
#ifdef GTK_SRCDIR
  g_chdir (GTK_SRCDIR);
#endif

  GtkApplication *app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);

  int status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

Create a new file with the following content named builder.ui.

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <object id="window" class="GtkWindow">
    <property name="title">Grid</property>
    <child>
      <object id="grid" class="GtkGrid">
        <child>
          <object id="button1" class="GtkButton">
            <property name="label">Button 1</property>
            <layout>
              <property name="column">0</property>
              <property name="row">0</property>
            </layout>
          </object>
        </child>
        <child>
          <object id="button2" class="GtkButton">
            <property name="label">Button 2</property>
            <layout>
              <property name="column">1</property>
              <property name="row">0</property>
            </layout>
          </object>
        </child>
        <child>
          <object id="quit" class="GtkButton">
            <property name="label">Quit</property>
            <layout>
              <property name="column">0</property>
              <property name="row">1</property>
              <property name="column-span">2</property>
            </layout>
          </object>
        </child>
      </object>
    </child>
  </object>
</interface>

You can compile the program above with GCC using:

gcc `pkg-config --cflags gtk4` -o example-3 example-3.c `pkg-config --libs gtk4`

Note that GtkBuilder can also be used to construct objects that are not widgets, such as tree models, adjustments, etc. That is the reason the method we use here is called gtk_builder_get_object() and returns a GObject* instead of a GtkWidget*.

Normally, you would pass a full path to gtk_builder_add_from_file() to make the execution of your program independent of the current directory. A common location to install UI descriptions and similar data is /usr/share/appname.

It is also possible to embed the UI description in the source code as a string and use gtk_builder_add_from_string() to load it. But keeping the UI description in a separate file has several advantages: It is then possible to make minor adjustments to the UI without recompiling your program, and, more importantly, graphical UI editors such as glade can load the file and allow you to create and modify your UI by point-and-click.