Official GTK4 example 1 (https://docs.gtk.org/gtk4/getting_started.html)

example-1.c

/** example-1.c	
 *
 * COMPILE
 *
 * gcc `pkg-config --cflags gtk4` -o example-1 example-1.c `pkg-config --libs gtk4`
 *
 * EXAMPLE
 *
 * ./example-1
 *
 * SOURCE
 *
 * https://docs.gtk.org/gtk4/getting_started.html
 */

#include <gtk/gtk.h>

#if GLIB_CHECK_VERSION(2, 74, 0)
static int app_flags = G_APPLICATION_DEFAULT_FLAGS;
#else
static int app_flags = G_APPLICATION_FLAGS_NONE;
#endif

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

static void activate(GtkApplication *app, gpointer user_data)
{
	GtkWidget *window, *button, *box;

	window = gtk_application_window_new (app);
	gtk_window_set_title (GTK_WINDOW (window), "Window");
	gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);

	box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 0);
	gtk_widget_set_halign (box, GTK_ALIGN_CENTER);
	gtk_widget_set_valign (box, GTK_ALIGN_CENTER);

	gtk_window_set_child (GTK_WINDOW (window), box);

	button = gtk_button_new_with_label ("Hello World");

	g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);
	g_signal_connect_swapped (button, "clicked", G_CALLBACK (gtk_window_destroy), window);

	gtk_box_append (GTK_BOX (box), button);

	gtk_widget_show(window);
}

int main (int argc, char **argv)
{
	GtkApplication *app;
	int status;

	app = gtk_application_new ("org.gtk.example", app_flags);
	g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
	status = g_application_run (G_APPLICATION (app), argc, argv);
	g_object_unref (app);

	return status;
}