hello.c

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer user_data) {
    GtkWidget *window;
    GtkWidget *label;
    
    window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Hello, World!");
    gtk_window_set_default_size(GTK_WINDOW(window), 300, 200);
    
    label = gtk_label_new("Hello, World!");
    gtk_widget_set_halign(label, GTK_ALIGN_CENTER);
    gtk_widget_set_valign(label, GTK_ALIGN_CENTER);
    
    gtk_window_set_child(GTK_WINDOW(window), label);
    gtk_widget_show(window);
}

int main(int argc, char **argv) {
    GtkApplication *app;
    int status;
    
    app = gtk_application_new("org.example.HelloWorld", G_APPLICATION_FLAGS_NONE);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    status = g_application_run(G_APPLICATION(app), argc, argv);
    g_object_unref(app);
    
    return status;
}