Associate Data with Object or Widget

[author <millward Ms UManitoba CA>]

I need some constructive critcism. 
The subject is moving data from main() into a callback.
In this case, when a button is pressed, the contents
of a label changes and is also displayed on the command line.
This code works, but
I doubt that its an 'elegent solution'. For one thing, I
have to duplicate the data structure inside the callback.
And for another, all the data I want the callback to get
has to fit into a structure. Suppose there's something
that the callback needs that I can't stuff into the structure?
 I never use global variables unless there is no choice.
Anyway, could someone take a look at this and tell
me a better way to write it? I'm just learnig GTK and I
don't want to get into bad habits.
*/
Associating Client Data with an Object or Widget
*/

#include <stdlib.h>  /* malloc */
#include <gtk/gtk.h>

void callback(GtkWidget * widget, gpointer  data);

int main(int argc, char * argv[] )
{
  GtkWidget * window;
  GtkWidget * button1;
  GtkWidget * vbox;

   typedef  struct paxx
  {
    char * arry[6][20];
    int number_of_strings;      /* number of strings */
    GtkWidget * labelx; 

   } ImageData;

   ImageData *  mydata = (ImageData *)malloc( sizeof(ImageData) ); 

   mydata->number_of_strings = 6;
   mydata->arry[0][0] = "Yesterday";
   mydata->arry[1][0] = "Today";
   mydata->arry[2][0] = "Tomorrow";
   mydata->arry[3][0] = "Today + 2";
   mydata->arry[4][0] = "Today + 3";
   mydata->arry[5][0] = "Today + 4";

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  
  gtk_signal_connect(GTK_OBJECT(window), "delete_event",
                     GTK_SIGNAL_FUNC(gtk_main_quit),
                     NULL );
  vbox = gtk_vbox_new(FALSE, 0);
  mydata->labelx = gtk_label_new("New Label");
  gtk_container_add(GTK_CONTAINER(vbox), mydata->labelx );                   

  button1 = gtk_button_new_with_label("Change Label");

  gtk_signal_connect(GTK_OBJECT(button1), "clicked",
                     GTK_SIGNAL_FUNC(callback),
                      (gpointer) mydata );

  gtk_container_add(GTK_CONTAINER(vbox), button1);                   

  gtk_container_add(GTK_CONTAINER(window), vbox );

  gtk_widget_show_all(window);

  gtk_object_set_data(GTK_OBJECT(button1), "image_data",
                       (gpointer) mydata);
  
  gtk_main();
  
  return 0;
}  

void callback(GtkWidget * button1, gpointer  data)
{
  typedef struct Thiss
  {
      char * arry[6][20];
      int buf_size;
      GtkWidget * label;
  } ARP;
  ARP * zuma;

  static int i = 0; 

  zuma  = gtk_object_get_data(GTK_OBJECT(button1),
                              "image_data");

  g_print("\n buf_size = %d\n", zuma->buf_size );

  if( i >= (zuma->buf_size) )
                 i = 0;
  g_print("\n arry[%d] =  %s \n", i, zuma->arry[i][0] );
  gtk_label_set_text(GTK_LABEL(zuma->label), zuma->arry[i++][0] );
}