Re: GTK Casting Problem

[Mariano Suárez-Alvarez <msuarezalvarez arnet com ar>]

On Tue, 2006-11-28 at 17:27 -0700, Chris wrote:
> Hello,
> 
> I am new to gtk.
> To learn the language I am writing simple
> programs.
> I want to be able to create 3 buttons, named
> Button 1, Button 2 & Button 3.
> If I explicitly initialize them it works.
> When I pull the Initilize into a for loop
> things start to go wrong.
> I think that I am either not using the proper type
> or not properly casting them.
> The result is a segmentation error when I compile
> which I think means a pointer is not correct.
> If anyone knows how to fix this issue, it would be
> greatly appreciated.
> I have placed in bold the two sections
> that I think apply to the problem.
> 
> My code is as follows:
> 
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
> #include <gtk/gtk.h>
> 
> [... snip ...]
>
>         for (i=1; i<=3; i=i+1)
>           {
> 
>             *label = (gpointer)"Button ";

Here you are assigning to '*label', yet you have not yet initialized
'label'. Hence the crash. (Indeed, gcc does not even compile this code)

>             strcat (label, (gpointer) i);

This does not make much sense. 

>             button = gtk_button_new_with_label ((gchar *)label);
> [... snip ...]
>     }

Here comes code that does a bit better:

/************************** begin example **************************/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <gtk/gtk.h>

static void 
button_cb (GtkWidget *widget, gpointer data)
{
  g_print ("Hello - %s was pressed\n", (gchar *) data);
}

static void 
delete_event_cb (GtkWidget *widget, GdkEvent *event, gpointer data)
{
  g_print ("A delete event has occurred\n");
  gtk_main_quit();
}

int 
main (int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *button;
  GtkWidget *box1;
  gint i;

  gtk_init (&argc, &argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_window_set_title (GTK_WINDOW (window), "Test Program");
  gtk_container_set_border_width (GTK_CONTAINER (window), 50);
  g_signal_connect (G_OBJECT (window), "delete_event", 
		    G_CALLBACK (delete_event_cb), NULL);

  box1 = gtk_hbox_new (FALSE, 0);
  gtk_container_add (GTK_CONTAINER (window), box1);

  for (i=1; i<=3; i=i+1)
    {
      char *label;

      label = g_strdup_printf ("Button %d", i);
      button = gtk_button_new_with_label (label);
      g_free (label);
      g_signal_connect (G_OBJECT (button), "clicked", 
			G_CALLBACK (button_cb), label);
      gtk_box_pack_start (GTK_BOX(box1), button, TRUE, TRUE, 0);
    }

  gtk_widget_show_all (window);

  gtk_main ();

  return 0;
}
/************************** end example **************************/

Hope this helps a bit.

You should probably consult a C book, by the way.

-- m


-- 
Mariano Su�z-Alvarez
http://www.gnome.org/~mariano