Add itens in Combo-box

[Jano Crema <jano bhquality com br>]

I have a interface.c where the combo_box was created,
So.. How can I add itens to this same combo_box in the callbacks.c .
I intend doesn't change the interface.c created by Glade.

OTHE QUESTION:

What event I want to create in combo to refer in callbacks file ?
My main question is for example:

I have a windows with entrys and combos and buttons
I put the text on entrys and click and save button,
The button calls on_btn_save function in callbacks.
This function connect to mysql database and save the entrys.

The code below is correct?
It is work very well, but I think is redundance in Objects
Thanks - Jano - Curitiba - Brasil
--------------------------------------------------------------------------
#ifdef HAVE_CONFIG_H
#  include <config.h>
#endif

#include <mysql/mysql.h>
#include <gnome.h>
#include "callbacks_cc.h"
#include "interface.h"
#include "support.h"


MYSQL mysql;
MYSQL_RES *resultado;
MYSQL_ROW linha;
GtkWidget *codigo;
GtkWidget *descricao;
char query[200];
char *cod, *desc;

void
on_btn_gravar_clicked                  (GtkButton       *button,
                                        gpointer         user_data)
{
   mysql_init(&mysql);
   mysql_real_connect(&mysql,"200.xxx.xxx.xxx","user","pass","db",0,NULL,0);
   
   codigo=lookup_widget(GTK_WIDGET(button), "txtcodigo");
   descricao=lookup_widget(GTK_WIDGET(button), "txtdescricao");

   cod=gtk_entry_get_text(GTK_ENTRY(codigo));
   desc=gtk_entry_get_text(GTK_ENTRY(descricao));
   sprintf(query, "insert into centrodecusto values('%s', '%s')",cod, desc);  

   mysql_query(&mysql, query); 
}