A problem with printing files

[Jakub Kucharski <dexcret gmail com>]

I was trying to write a simple application for printing a file. Nothing 
important. Just to learn basics. I've written this short piece of code:

#include <gtk/gtk.h>
#include <gtk/gtkunixprint.h>

static void
print_end(GtkPrintJob *print_job, gpointer user_data, const GError *err)
{
    if (err == NULL)
        return;
    g_assert(err == NULL);
}

static void
print_page(GtkWidget *widget, gpointer user_data)
{
    GtkWindow        *parent = user_data;
    GtkPrintJob        *print_job;
    GtkPrinter        *printer;
    GtkPrintSettings    *settings;
    GtkPageSetup        *page_setup;
    GtkWidget        *dialog;
    GError            *err = NULL;
    gboolean        status;

    dialog = gtk_print_unix_dialog_new(NULL, parent);
    if (gtk_dialog_run(GTK_DIALOG(dialog)) != GTK_RESPONSE_APPLY) {
        gtk_widget_destroy(dialog);
        return;
    }
    printer        = gtk_print_unix_dialog_get_selected_printer(
        GTK_PRINT_UNIX_DIALOG(dialog));
    settings    = gtk_print_unix_dialog_get_settings(
        GTK_PRINT_UNIX_DIALOG(dialog));
    page_setup    = gtk_print_unix_dialog_get_page_setup(
        GTK_PRINT_UNIX_DIALOG(dialog));
    gtk_widget_destroy(dialog);

    print_job    = gtk_print_job_new("Example title", printer, settings,
        page_setup);
    status = gtk_print_job_set_source_file(print_job,
        "/home/dexcret/Dokumenty/napisy.pdf", &err);
    if (err != NULL) {
        g_assert(status != FALSE);
        fprintf(stderr, "Unable to print file: %s\n", err->message);
        g_error_free(err);
        err = NULL;
    }
    gtk_print_job_send(print_job, print_end, NULL, NULL);
    g_object_unref(print_job);
    g_object_unref(page_setup);
    g_object_unref(settings);
    g_object_unref(printer);
}

main(int argc, char **argv)
{
    GtkWidget        *window;
    GtkWidget        *button;

    gtk_init(&argc, &argv);
    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
    gtk_container_set_border_width(GTK_CONTAINER(window), 10);

    button = gtk_button_new_with_label("Print");
    gtk_container_add(GTK_CONTAINER(window), button);
    g_signal_connect(button, "clicked", G_CALLBACK(print_page),
        GTK_WINDOW(window));
    gtk_widget_show_all(window);
    gtk_main();
    return 0;
}

But it doesn't work. What have I done wrong?

--
Regards.
Jakub Kucharski