How can I solve this problem of Gdk

[=?iso-8859-15?b?QW5hIEd1dGnpcnJl?=z Pascual <al004180 alumail uji es>]

hi, 
i'm new with gtk/gdk. i create a program to design some differents objects, in 
this case a simple line. but i supose i don't use correctly the pointers 
'cause i obtain these errors. 
  
Gdk-CRITICAL **: file gdkgc.c: line 51 (gdk_gc_new_with_values): assertion 
`window != NULL' failed. 
 
Gdk-CRITICAL **: file gdkgc.c: line 456 (gdk_gc_set_foreground): assertion `gc 
!= NULL' failed. 
 
Gdk-CRITICAL **: file gdkgc.c: line 766 (gdk_gc_set_line_attributes): 
assertion `gc != NULL' failed. 
 
Gdk-CRITICAL **: file gdkdraw.c: line 65 (gdk_draw_line): assertion `drawable 
!= NULL' failed. 
 
Gdk-CRITICAL **: file gdkdraw.c: line 212 (gdk_draw_string): assertion 
`drawable != NULL' failed. 
 
my code see above. 
 
// -------------------------------------------------------------- 
 
#include <gtk/gtk.h> 
 
typedef struct 
{ 
  GtkWidget *window; 
  gchar *nom; 
  GtkWidget *drawingarea; 
  GdkDrawable *drawable; 
  GdkColormap *gdkcolormap; 
  GdkGC *gdkgc; 
} e_app; 
 
typedef struct { 
  gdouble lx1, lx2, ly1, ly2; 
  double lgrosor; 
  GdkColor lcolor; 
  GdkFont *lfont; 
  gchar *lnombre; 
} e_linea; 
 
void InicializarLinea(e_linea *l); 
void DibujarLinea(e_linea *l, e_app *a); 
 
gboolean EventoExponer(GtkWidget *widget, 
                        GdkEventButton *event, 
                        gpointer client_data); 
gint main (gint argc,gchar *argv[]) 
{ 
  e_app app; 
  e_linea lin; 
  GtkWidget *vbox; 
 
  gtk_init (&argc, &argv); 
  InicializarLinea(&lin); 
  app.window = gtk_window_new(GTK_WINDOW_TOPLEVEL); 
  gtk_widget_set_name(app.window, "Ventanita"); 
  vbox = gtk_vbox_new(FALSE, 0); 
  gtk_container_add(GTK_CONTAINER (app.window), vbox); 
  gtk_widget_show(vbox); 
 
  gtk_signal_connect(GTK_OBJECT (app.window), "destroy", 
		GTK_SIGNAL_FUNC (gtk_main_quit), NULL); 
  app.drawingarea = gtk_drawing_area_new(); 
 
  gtk_drawing_area_size(GTK_DRAWING_AREA(app.drawingarea), 300, 300); 
  gtk_box_pack_start(GTK_BOX(vbox), app.drawingarea, TRUE, TRUE, 0); 
  gtk_widget_show(app.drawingarea); 
  gtk_widget_set_events(app.drawingarea, GDK_EXPOSURE_MASK); 
 
  gtk_signal_connect(GTK_OBJECT(app.drawingarea), 
			"event", 
			GTK_SIGNAL_FUNC(EventoExponer), 
			&app); 
 
  gtk_widget_realize(app.window); 
 
  app.drawable = app.drawingarea -> window; 
  app.gdkgc= gdk_gc_new(app.drawable); 
  app.gdkcolormap= gdk_colormap_get_system(); 
 
  gtk_widget_show(app.window); 
  gtk_main(); 
  return 0; 
} 
 
void InicializarLinea(e_linea *l) 
{ 
  l->lx1= 50; 
  l->lx2= 100; 
  l->ly1= 80; 
  l->ly2= 80; 
 
  l->lgrosor= 1; 
 
  l->lcolor.red= 0xffff; 
  l->lcolor.green= 0; 
  l->lcolor.blue= 0; 
 
  l->lfont= gdk_font_load 
("-Adobe-Helvetica-Bold-R-Normal--*-140-*-*-*-*-*-*"); 
  l->lnombre= "l- 0 0"; 
} 
 
void DibujarLinea(e_linea *l, e_app *a) 
{ 
  gdk_colormap_alloc_color(a->gdkcolormap, &l->lcolor, TRUE, TRUE); 
  gdk_gc_set_foreground(a->gdkgc, &l->lcolor); 
  gdk_gc_set_line_attributes(a->gdkgc, 
			l->lgrosor, 
			GDK_LINE_SOLID, 
			GDK_CAP_BUTT, 
			GDK_JOIN_MITER); 
  gdk_draw_line(a->drawable, a->gdkgc, l->lx1, l->ly1, l->lx2, l->ly2); 
  if (l->lfont == NULL) 
    g_print("font is NULL\n"); 
  else 
    gdk_draw_string (a->drawable, l->lfont, a->gdkgc, l->lx1, l->ly1-10, 
l->lnombre); 
} 
 
gboolean EventoExponer(GtkWidget *widget, 
                        GdkEventButton *event, 
                        gpointer client_data) 
{ 
  if (event->type == GDK_EXPOSE) 
    DibujarLinea((e_linea *)client_data, (e_app *)client_data); 
  return FALSE; 
} 
 
// --------------------------------------------------------------