gtk beginner app won't work with assertion fail

Question

I've done this simple app in Gtk, just to test things out... I come from swing so redefining a draw event function is normal for me... Anyway seems not to work:

#include <gtk-2.0/gtk/gtk.h>
#include <gtk-2.0/gdk-pixbuf/gdk-pixbuf.h>
#include <stdlib.h>
#include<string.h>
#include<stdio.h>
#include <iostream>

GdkPixbuf *imm;

void destroy(GtkWidget *widget, gpointer data) {
    gtk_main_quit();
}

gboolean on_expose_event(GtkWidget *widget, GdkEventExpose *event,
     gpointer data) {
    gdk_draw_pixbuf((GdkDrawable*) widget, widget->style->white_gc, imm, 0, 0,
      0, 0, -1, -1, GDK_RGB_DITHER_NONE, 0, 0);
    return FALSE;
}

int main(int argc, char** argv) {

    char* filename = new char[1000];
    GError *error = NULL;
    GtkWidget *window;

    gtk_set_locale();
    gtk_init(&argc, &argv);

    if (argv[1] == NULL) {
     std::cout << "Err.";
     return -1;
    }

    strcpy(filename, argv[1]);

    imm = gdk_pixbuf_new_from_file(filename, &error);

    if (!imm) {
     std::cout << "err closing";
     return 0;
    }

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_widget_set_size_request((GtkWidget*) window, 500, 350);
    g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(destroy), NULL);
    g_signal_connect(window, "expose-event",
      G_CALLBACK(on_expose_event), NULL);

    gtk_container_set_border_width(GTK_CONTAINER (window), 10);
    gtk_widget_show(window);
    gtk_main();

    return 0;
}

..in fact at runtinme says (on line gdk_draw_pixbuf(....)):

(cConvolve:5011): Gdk-CRITICAL **: gdk_draw_pixbuf: assertion `GDK_IS_DRAWABLE (drawable)' failed

is it because the pixbuf is not good??? Or is it because I can't draw to the window like this?

Answer 1

It's because you cast GtkWidget to GdkDrawable, whereas GtkWidget doesn't inherit from GdkDrawable. Use

gdk_draw_pixbuf(GDK_DRAWABLE(gtk_widget_get_window(widget)), blah blah...);

Anyway in normal GTK use you don't have to do any drawing in expose handlers. To display an image, just use the GtkImage widget:

GtkImage *image = gtk_image_new_from_file(filename);
gtk_container_add(GTK_CONTAINER(window), image);

Answer 2

Widgets don't inherit from GdkDrawable, you need to get the drawable from the widget's window.

Answer 3

Thanks. But my purpose was to try drawing on the window bg... anyway, even if I dont' get anymore asserts, it doesn't work:

gboolean on_expose_event(GtkWidget *widget, GdkEventExpose *event,
     gpointer data) {
    int w, h;
    w = gdk_pixbuf_get_width(imm);
    h = gdk_pixbuf_get_height(imm);

    gdk_draw_pixbuf((GdkDrawable*) widget->window, widget->style->fg_gc[ GTK_STATE_NORMAL ], imm, 0, 0,
      0, 0, w, h, GDK_RGB_DITHER_NORMAL, 0, 0);


    return FALSE;
}